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What Went Wrong With My Coax?
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- เผยแพร่เมื่อ 31 ธ.ค. 2022
- Under very high power, I had an Amplifier fault and then high SWR. What caused this? Did something carbonise and then cleared? Would it re-appear with QRO again? Your guess is as good as mine. I would need to strip it all down and check. I would prefer just to remake them all! Please let me know what you think! Callum.
I've heard the best method to check coax is with a Megger and then do a TDR test. General SWR doesn't flush out the issue typically.
We use to use a megger to dry out wire lines, at the lower voltage settings, never tried it on coax cable...
Hi Callum, I worked in an EMI test lab. We operated from 100 Hz to 18GHZ at high power levels up to several KW. Coax is always a problem. It takes a lot of abuse, especially the more it's handled. If the coax has gotten water inside the jacket, it's dead, forever. The water will wick up the braid just like solder wicks up a wire. The tiny little wires in the shield need to touch one another to provide the shielding. The water causes the copper to oxidize, the oxidation is an insulator. Now all those little wires are isolated from one another and there is no way to get them clean again. The shielding is gone.
You can dry it out but it won't get rid of the oxidation. No option but to throw it out.
Almost every time we had issues with a coax or any other electrical cable, it's the connectors. That's where all the wear and tear is. The other issue was people stepping on the cable. We had large shielded rooms with metal floors. A person can easily mash the cable enough to cause an impedance bump in the cable. It was a constant battle keeping folks off the cables.
When you are troubleshooting connectors you need something that responds really fast. Try using your stick to SWEEP the band of interest. Gently (or not!!) flex the coax and watch as it sweeps the band, if you see it jumping around you have your culprit. If the instrument you are using is slow to respond, it won't be fast enough to catch the change as you flex the cable.
To check your method of testing set your stick to a VHF band, put your dummy load on the stick, screw the collar on, but leave it loose. If you can wiggle the connector and see the impedance change then you have a working setup. Using a high frequency just makes the poor connection more apparent. You could try testing your cables at higher frequencies. The losses are higher and easier to measure the higher you go. Losses from water damage might show up more at high frequencies but I have never tested that theory.
Pin holes? They wick up water like a sponge. Carbon tracks will arc over again and again, they get worse the more often they arc over. Again, the cable is dead forever.
You are looking for a high voltage ohm meter to test for carbon. There are inexpensive ones on Amazon. Otherwise known as a "megger".
A TDR might find the location of the fault. I have seen commercial TDR's find the impedance bump caused by a zip tie that was pulled too tight to around the cable. Very sensitive and expensive.
For a cheaper option, check out a video by W2AEW, it's #88 on his channel.
Cheers!
Matt
I recon pin hole, a connector or perhaps a carbon track (arc).
HNY Callum,
In the RAF we had some reels of co-ax that had been on deployments for 20 years and were still good, and some that were virtually brand new that always caused problems. Our "standard" test was to make sure the co-ax was isolated from any kit/antenna, and test it with a megger (insulation breakdown tester) at 1000v plus. Any suspicions we would follow up testing with a TDR test (time domain reflectometer), where you would get a visual representation of the whole cable length, and any irregularities along the length. You could even see small reflections from where a cable had been kinked. We would get a lot of issues where there was small cracks or pin holes in the outer insulation, and water would "wick" into the sheath - with a TDR you could see exactly where these problems were. On your "unexpected" SWR readings on short patch leads, maybe re-test at a lower frequency and see if you get a different result? Again, maybe it's a RAF thing, but we used a "known good" launch lead (about 5m) before the cable (or waveguide) under test. It will be interesting to see if you manage to get to the bottom of this one.
Yes, if I was a coax factory, I would invest in a megger - but I'm an antenna guy. The coax is just a tool and I don't want to spend time faffing with coax.. So I'll just buy some heavier stuff. Probably more Ecoflex 15 that I know is absolutely good for around 5kW..
@@DXCommanderHQ Poor man's low voltage megger - multimeter with a conductivity measurement setting. 1nS = 1000 MΩ
I second the suggestion to use a Megger. And not a multimeter. A Multimeter tests the resistance at a low voltage. A Megger (or more specifically an insulation tester) will test at, 250v, 500v, 1000v. What this does is, if there is a hole in the insulation or poor insulation this would show up as the higher voltage would arc over the gap. A mulimeter can not do this. Why does this matter, well if there is any moisture ingress this will present as a lower impendance at RF frequencies and voltages. Quite probably spoiling your day.
Why does moisture ingress impact Coax. The braid in particular has a great many contact points where copper touches copper. With moisture this corodes and does not conduct as well. There is no way to recitify this other than junk the coax. Make sure if you do use an insulation tester the coax is not connected to anything and you aren't touching it. If you have a TDR fantastic test after this, they can even show up small reflections from connectors and so on.
You can of course cut your Coax down until you get clean copper again and re-terminate the cable. If in doubt, whip it out.
I have also never tested my patch leads like this... getting a bit restless after watching this one... thanks so much! 🤣
I'll say yes coax does lose some of its properties as time goes on you can lose a Db just because of moisture that gets into the line over years. just enough moisture to start to tarnish the braid then dry up in the summer then back to some moisture back in the fall . most of us hams really don't pay too much attention to coax just as long as it gives us a low SWR but as you have said looking at antennas on MMANA we can get a Db here or 2 Db there which in turns you can possibly lose it in the coax before it gets to your radio. Great stuff as usual Callum Cheers from across the pond de W9US
Yes Ed.. Time to swap out everything..
You can tell a great deal about the character of a man by the way he treats his dog. Happy New Year!
Hi Callum. Happy New Year.
I have found that the coax barrel connectors can sometimes have 'spread' centres where the pin of the PL259 goes in. I found that squeezing them together with fine nosed pliers helps. You can tell if this has happened by checking how much force you need to apply to just put the pin in the centre hole of the barrel. If it's too loose, you could have a dodgy connection leading to an intermittently high VSWR.
If water gets into coax around the connectors, it is basically ruined. If you strip back the outer plastic coat, you will see that the braid will have gone dark due to oxidation. If it hasn't gone all the way along the cable, you could cut off the bad bit and salvage the rest that still has shiny braid. It is best to weatherproof PL259/SL239 connectors, or use 'N' Type connectors, which have better weatherproofing capabilities.
Fantastic video Callum - not the first time that all has been “well”in the shack, until suddenly there is a marked increase in the SWR. However, go back into the shack the next day to investigate and all is well!! - Happy and Healthy 2023 to you and the family 😊
Yes, too much power, old coax, on the limit etc.. Easy fix. New heavier coax..! PS - Happy New Year!
VSWR will vary up and down a length of cable or waveguide. I'd measure power in / power out, apply cable loss factor to the given length, and watch for fluctuations on key down. Could have been a droplet of water inside the connector, and not a cable issue? You'll let us know what you find, Callum Happy New Year, everyone!
Well, it shouldn't do assuming 50 ohms everywhere..
@@DXCommanderHQ Here's a hint... Impedance repeats at the end of a 1/2 wavelength.
@@imken2392ut a matched piece of coax will read close to 1:1 everywhere. While it is possible to get a false 1:1 reading by choosing a particular length, you can’t get a false high reading that way into a dummy load.
Somehow I feel better watching someone else go down the rabbit hole. I have experienced coax acting just fine until it is under load (increasing power) to which SWR goes out of whack.
If I may make a suggestion, you could apply heat from your hot air gun to the connectors while the analyser and dummy load are connected, taking a series of measurements (before heat, during heating and after heating). This may answer the question as to whether or not moisture ingress is a contributing factor. By the way, thank you for your hard work, Callum.
Get a proper VNA like the NanoVNA, the way you are doing the measurement makes no sense: with a long enough coax (sufficient loss) you can leave the other end open or shorted and you will still get a perfect SWR reading.
Oh, it makes perfect sense for a quick and dirty analysis. Frankly, its just a pipe for me. I'll change the stuff. I need to be in an antenna field, not faffing with coax :)
@@DXCommanderHQ A multimeter and a short on the other end would do the same job, more or less. Get a NanoVNA, you will be able to measure coax loss, check (over a wide frequency range) if there are any changes in the line, the antenna system etc. A bit of a learning curve since most people are not familiar with VNAs but once you learn, it becomes as useful as a multimeter for general electronics.
What is the blue unit your using
@@Massey_Fferguson Rig Expert Stick-230
bending it more than its minimum bend radius can form points where the impendence is changed by crushing the inner insulation. This can then lead to hot spots at some frequencies, which can then cause the insulation to flow and change the impedance more; on till at some point it can go bang and flash over. At 1600watts a flash over could affect the impedance which movement might remove.
Good point!
You said about CB operators going on about SWR, well one comment from my 11m days I always tried to follow was that of 1/2 wave lengths of coax or odd multiples of for said frequency ( inc calculations for velocity factor). Following that rule I always managed a good SWR sweep for all my setups on 11m, however I do understand it’s not crucial, but at 29MHz is it possible a 1/4 wave of coax could be giving you a false reading, after all some of the shorter coax lengths you tested did look to be possibly somewhere around only 6ft long.
OK Peter.. so impedance repeats every half wavelength - so the ONLY benefit to using exact half wavelengths is that if you have say 60 ohms at one end, you will repeat 60 ohms at the other (you need to work in the velocity factor). With 50 ohm antenna and a 50 ohm radio AND 50 ohm coax, length is TOTALLY immaterial (other than losses). CB guys invented a number of myths around coax lengths and honestly, they ended up making this a key topic - and it's not. But I thank you for your contribution. Happy New Year!
The best length of coax is the distance from your 50 Ohm radio, through your 50 Ohm coax to your 50 Ohm (or near as possible) antenna. It's just physics. So what? So tune the antenna for acceptable impedance and just forget about the length of the coax. Ironically, if you use longer than necessary, you lose RF energy to cable loss. This CB myth needs to be stopped - it was just a way for people to sell you more coax.
Hi Calum
The only thing I can think of is connector cleaning …
Try clean your connectors in alcohol make sure to use cotton swab to clean the inner part of the connector.
Then re-measure…
Measuring on a higher frequency than 10 meters like 432 MHz would help show if the coax has issues .
I would also measure s21 transmission loss, instead of “just” vswr
Good luck Niels
OZ1CGQ
Wow, Cal...Can you, ever so gracefully, go down rabbit holes...All the best for a wonderful New Year...KN4VUP
Happy new year!
What’s funny, I was sitting here saying, “better take the hat off or the magical SWRs will be all off....” ......and you took your hat off. Ha! Perfect.
HAHA
I've seen coax get problematic where it gets bends as the inner coatings move about. I've also seen hardline melt or blow out the side from minor dents and such on commercial broadcast lines. I'm always excessively delicate with my feedlines. (my shack runs heliax :) )
Heliax is great if it's properly and meticulously installed. Improperly installed by inexperienced people, it's ugly.
for 1300 watts a 50 ohm load with an SWR of 1.5:1 your peak voltage would be abt 440 volts. could have been a high voltage breakdown somewhere that you will never see with a little analyzer on an ohm meter unless it's a complete short. I built up a little HiPot tester for coax/pl-259 cables, pretty simple and safe unless you do something stupid like put the open end of the cable in your pocket! cheers and happy new year to all, Mike, K1FNX near Boston
That was going to be my question, if you "load" the wire/coax will it show the problem. I've seen this occur in the automotive world where a wire will look good with a dvom but once it's loaded with something like a headlight it can't carry the load.
KE0VNY
@@adamke0vny295 I'm thinking more of a voltage breakdown due to either water or dirt in the connectors which shows up running 1500 watts into the system. Connectors being the likely suspects not the cable itself. Even a whisker of braid could be a problem with 440 volts you would never see with a battery powered analyzer. I would HiPot the cable for starters
73 Mike K1FNX near Boston
@@WECB640 I have been fooled checking cables with ohm meters once too often which is why I built the SO-239 terminated HiPot...... F/B on WBCN they are still broadcasting although all the old on air talent is long gone...the call has been mine since 1963 as an original issue......never thought of it as great...........just what they issued me at the time....... 73 Mike K1FNX near Boston
Good point Michael!
Coaxial transmission line definitely deteriorates with time. Some more than others. Moisture is really bad and generally ruins the cable if it gets inside. Happy New Year.
Tnx Jim!
One of those mysteries. I had a short run of RG8X running portable HF and I always put the analyzer and dummy load on before connecting to the radio. I never could get it below 2:1 on any frequency so I labeled it as bad. Just before I tossed it I checked it again and it was fine. I even used a Nano VNA along with my RigExpert and both showed around 1.3:1. No evidence of water or anything that looked funny. I put it back in the portable box with a couple of other lengths so we'll see if there are issues later. Good luck finding yours. Jack K5FIT
Yep!
Coax can test good with an antenna analyzer due to the low power of the test, but when you apply higher power, the dielectric can leak or breakdown, and that can be tested with a "megger". The reading with the patch might be with the wavelength of the short cable, or the connector has a bad connection to the coax.
Indeed Paul. Pin hole - possibly a carbon track too.
I had the same problem at POTA this week... I tested my Wolf River 1:1 on the analyzer, then coupled it to the radio with a barrel and SWR went overload... Next morning everything was fine at home. ... When I added in the barrel connector, it was snowing ... I suspect a few flakes stuck in the connector dropping Z to zero point crap.
Yep.. Thing is, we just want a reliable "pipe" for the wigglies rather than spend time sorting out coax..!
I had a problem once with a foam core coax where excessive current in the center conductor raised the heat enough to melt the foam and allowed the center conductor to migrate to where it contacted the shield. This occurred at a bend, and when i straightened the coax, the short went away. I ended up using a larger coax.
Amazing.
Well, at least I am not the only one that gets stumped with coax sometimes. The CB band (or rather it's operators) sure has it's own special rules on coax, lengths, that defy all the other radio principles. I have had coax age but it was mostly sun damage to the outer layer. I hope you get it sorted out and share what happened ! 73
Oh yes, the special rules!! If you have read the comments here, you see all the myths perculating through!
There are several issues to consider.
The first is the excessive power 1600W is not that common. What do you use to generate this amount power?
If it uses valve tech then there will be many hundreds of volts on the anodes, and this can get out onto the cable.
Also as pointed out you get a peak voltage depending on the VSWR
The coax itself will be fairly good in it's voltage rating, so it will be the connectors that are the weak point.
Any PL259 connector is suspect they really are not that good. (even N types are normally only specified up to 500V) Those with compression fittings are better than the all-solder type. If you are lucky you will get a breakdown rating for the connectors, If some muck or water gets across the insulation on the inside, also solder flux can be a problem, Any contamination may cause problems. Think of when you solder on the centre pin, this is held in place by a plastic washer. Often this is PTFE. If you get a load of flux across that, then the breakdown voltage between the pin and the body of the plug will be reduced. With higher power then this more likely it is to happen. Using a low power analyser you may not see any issues at all.
Coax does not really age as such, but it is suspectable to damage when you pull it about, that can make the connections move, and if it gets trapped or trodden on then that can deform the cable and cause a mis-match. On HF this will not matter that much AT LOW POWER but when you use high power, then it MIGHT show up.
Another thing to consider is the type of solder. As you are a commercial business you will be using Lead Free solder. That is well known for making bad joints. Also it can grow "wiskers" of solder which cause shorts. Not common in normal use, but it is the main reason why military, aviation, and space system do not accept its use. In this case it's tendency to produce dry joints will be the main issue.
The first test to do is a high voltage insulation test - a old fashioned electricians megger with at least 1000V output if there are any points with contamination that may show them up.
After that you can go onto RF testing.
All leads need to be tested regardless. Any problem will generally be with the connectors and not the cable.
Andy
All good stuff. Tnx.
Hi Cal,
I've always replaced coax, if outdoors, after 7 to 8 years. You and your family have a safe and Happy New Year. 73 WJ3U
Good call Don!
In case it's not been mentioned a mega is really good for testing leakage in coax that tests OK on a SWR meter but fails on RF power 73 Happy New Year John_K2BAG
I agree. Get an insulation resistance meter on it also. You must have a sparky near you. Used to have fun testing the resistances of my apprentices lol.
Just be aware that a long run of coax is a nice capacitor - charge it up with a Megger? remember to discharge it immediately afterwards lol
It's that time of year it seems Cal. I had a section of my RG213 short out. I was out in the snow and dark troubleshooting. I will not use heat shrink over connections again, but will use the mastic self annealing tape like I did after the repair. Oddly, the connections didn't seem bad for ice, but the coax is still shorted after a day or two, so I suspect it is toast.
I have my M&P one piece (so no joins) lined up to order when they get off holidays.
Ah!
depending on the length of the coax you may get a variety of readings at different frequencies because you may be at a voltage null on the cable . The only way to get a proper reading is to use an electrical half wave coax (length x vf) which will reflect the load impedance at the meter end .
Spot on 👍 73 Joe
Indeed, that repeat every half wave.. However assuming 50 ohms everywhere, it shouldn't matter where it's tested.. But I hear you.
@@DXCommanderHQ I think things may be more complicated than first meets the eye. Coax may be be very close to 50 OHM impedance but may not be exactly 50 OHM. The dummy load may not be a perfect resistive load @50 OHM and will have some reactance. SWR's of 1.1 to 1 and 1.2 to one correspond to less than one percent of reflected values. Also your measuring instrument is not perfect and the readouts may include rounding error or truncation error. Different lengths of coax may have different losses. With time and handling two identical piece of the same type of coax may come to have slightly different measurements. A long length of lossy coax will give a good SWR.
Adding to the potential for confusion here is the fact that you are only making one port measurements with the antenna analyzer.
However, I am not sure how all of this relates to main issue of a sudden fluctuation in SWR. I would guess that some kind of mechanical factor such as loose connection would be more likely than an intermittent coax failure.
@DonGilroy thank-you Don. I've been thinking about things and my preference was to use the viaecho VNA as I was like Callum not sure of changing readings when nothing changed.
Great video! Cheers from Georgia, USA.
Cal, i have old multiple coax sections with barrel connectors run out to my back field connected to DXC classic antenna. I'll check the SWR and get back .
I'm back, I have 4 sections of old coax different lengths and 3 barrel connectors the SWR 1.06 . 166.81 feet .
Nice work!!
It’s most likely the cable has been damaged during installation, for example exceeding the bend radius, then couple this with some water ingress and maybe corrosion and the result is a cable that has weak spots along its length. For most coaxial cable 1300W is well within a normal operating range. For example the current flowing would be ~5A and voltage ~250v so quite modest really. A rise of VSWR to 2:1 if the load impedance has not changed implies a impedance change in the transmission path to either 25R or 100R, created either by a dielectric failure from cable damage or melting and so on. The failure could be in two ways that the cable plus load impedance dropped to 25R that would imply a low impedance path between inner and outer elements of the coaxial cable now itself presenting a much lower impedance towards the measured 2:1 ratio, or conversely the cable introduced a series impedance again to achieve the other possibility of a 100R impedance signified by the 2:1 measurement. If you need to test the cable you could achieve the same results on the bench using a drum of cable and a dummy load. Good luck with a resolution to your problem. 73
Yep.. Pin hole, carbon track too..
Ian no expert Callum I had a 7610 into a acom 1010 amp then into a three element Yagi. I would often get high swr. After talking to an engineer at icom thinking it was a radio problem. He mentioned the coax was charging and holding a charge like a capacitor. After doing some better work on improving my grounding it stopped the problem. To be honest the engineer did explain but it was over my head a bit.
That has happened to me as well.. Not this time, but many moons ago!
i bet you have burnt a small hole in the dielectric which when testing with a low voltage meter won't show a fault. but when a higher voltage is a applied may be arcing through the dielectric giving you the problem.@@DXCommanderHQ
The length of the coax does not change the swr of the coax. The impedance mismatch of the coax determines the swr you see with a dummy load. Now if you have a mismatched antenna at the end of a length of a matched piece of coax, the length of the coax will reduce the antenna swr you measure at the other end of the coax due to the loss in the coax. Coax does age. Abusing coax by bending it sharply will also change its impedance slightly, as will over loading/over heating it.
You tell 'em!
11 mins in, asking what went wrong. I think this is quite obviously a fluffle valve failure in the encabulator coupling.
The do-dad was not activating the thing-a-majig.
Absolutely one of the flux variables inside the encabulator.
It is normal to have a good swr measure on a lossly cable. Use instead a VNA to test the return loss and impedance. And for high power reliability, test the insulation resistance, best using high frequency voltage.
Of course ideally you’d have a big dummy load and run a kW into it and see what happens.
However, if you’re sure that run is faulty, and it’s not a connector, bin it.
If VSWR isn’t perfect there will be high voltage nodes along the length and I’ve known coax to arc through the inner insulation.
Very hard to find the point - I did once, it looked like a fine, hot needle had been pushed through it.
Bin! :)
A lot of rain lately, water ingress perhaps? but I've seen that you normally seal and protect thoroughly including the vaseline. A multimeter check for DC resistance might pick up water ingress if it's significant amount.
Richard DePas' recommendation below to try a Megger test sounds a great idea if corrosion or water ingress has potentially led to poor isolation, it works at the same order of voltage as seen at 1300 Watts after all.
when i used to work in the cable industry, we had an issue where slightly bad coax would "load up" where the impedence changed usually over time and as soon as you pull the connector off the thing it was connected to it would dump the load and test perfectly fine. usually the culprit is either water ingress or a bad connector. especially if the readings vary...
Also, you have a bunch of coax running under the road... have you tested it with a TDR lately? if that conduit is full of moisture that can affect it especially if there's a little pin hole in the jacket
Good idea but the road coax is fine.. Squirting 1.5kW down there more than once a week..
@@DXCommanderHQ still a good idea to TDR it every so often... but the easiest way to see if the coax is loading up, just put a 500mw transmitter through it into a dummy load for about an hour and watch the SWR.
Time domain reflectrometry is best to sort out breaks in situ and some VNAs do this too. Peak volts < 300 ... dielectric non-linearity?
To complete the investigation I would test the same long underground run of Coax and long field patch lead cold and then run the same high power test again and re test the coax from Shack to antenna to see if you have the variation of increase in VSWR , I would suspect that its a simple plug replace and re fit from condensation or worse condensation has found its way into the underground run. More likely that the field run has got damp into the braid . Do the test again and see if a variation occurs , but then as a final check strip back the plastic outer coating of the field length of coax and the braid will have gone from Bronze to a dull grey appearance. We have had a lot of rain and the cold conditions for ten days and I have just had similar on a 50 m run of very high quality almost new coax . it was certainly condensation .
Regards Clive and a happy new year .
Well CLive.. Effectively I did that because test #2 didn't fail with more power - and for longer (different patch lead) - sorry if I didn't explain, the video became a little strange sorry!
I've had a very similar issue when running high power on AM/FM or SSB (>800w 11m band) and thought it was my coax breaking down in some way.
Turns out it was a coil in the base of my 5/8 wave silver rod. The SWR would only go wonky at highish power in wet weather. I've since upgrading the coil to thicker copper wire and the SWR issue has gone away.
I had no issues when using a dipole with the same coax, same conditions.
Disclaimer: I don't generally use my amp hardly at all, only when German 11m FM CB bands are open!
Nice!
Great now I have to go test all my coax too.
Woops!
Real short bits of coax likely are showing minor or weak reflections where as longer lengths normalize those reflections, so exacting wave lengths become irrevelivant. From my CB days we were to use exact wave lengths of coax feeding the antenna to obtain a match, being an inch off according to Cobra was a warenity cancelation action, my feeling is that the short runs were part of the second element (the reflector)!
The problem with wet coax is oxidation reactions with the copper shield and if the core is braded the core. You can dry it, but the oxidation remains!
Old coax can be OK, but is the jacket intact, or is it cracked. I have seen aluminum and stainless coax, those may function better after wet and I have seen steel braid, that wet is evil. Another issue is the dielectric cracked. If it is thin solid cores may short or just change, impedence. The cores also can break if it was not supported or rolled too tight, especially if it is solid. My 0.25" LMR-240 is not to be curved tighter than about a 4" radius or the core may break. I have a piece of cheap clothes line running from my shed to the house so I can keep my coax off the ground, and away from copper theives.
>>> but the oxidation remains
The thing to do with antenna VSWR problems is to use a NavoVNA as a Time Domain Reflectometer in order to identify the distance the problem is away from the transmitter. Then you can find it before it magically disappears when you wiggle the cable and becomes an intermittent problem.
Thing is John, it went away.. So after some careful thinking, I recon it's a carbon track..
Is Catapulty a nice word for slingshot? Very interesting video Callum!
Yes, that's it..
I use a slingshot to put all my wire antennas up. Happy New Year Callum. 73 Joe
Hi Callum,Happy New Year, INSULATION TEST: One piece of insulation material and a cup of water spaced apart inside the microwave oven.Give it short bursts and check insulation for temperature change.Only do the test when the Mrs. is out,,,73 de John ZL1PO.
Great tip!
Hi Callum. I have worked in the RF electronics field for over 50 years and the short answer is yes, coax ages. VSWR does not give you all the answers, you need a megohmmeter to test the insulation breakdown of the coax from the effects of UV radiation, chemical gassing and other factors of aging in the coax. This is why it is recommended in the commercial field that coax be replaced every 10 years. You stated that you were running 1500 to 1600 watts of power and that is a lot of RF heating in the coax which could have caused the problem. What you were using to test the coax was very low power and faults may not show until the coax totally breaks down. Even in your DX Commander antennas the wire should be replaced at least every 10 years by your customers to maintain peak efficiency. I hope this helps. Mike/K1EG.
Oh yes, a high voltage stest might be a good idea. I think I will remake all my patch leads!
I have had troubles with patch cables because they get bent near the connector. And sometimes they get bad connection at the connector. Only found when I pulled on the connector. It was intermittent bad. Do know if that is what you have there. The only way I found my was to test it under stress, pull on connector or bend it next to the connector. Just an idea
Hi Cal, is that short patch cable near to an electrical 1/4 or 1/2 wave at the frequency you tested it at 😁
just a thought.
Cheers
Dave.
But it shouldn't matter. If you have 50 ohm cable and a 50 ohm load, you should see 50 ohms regardless of length
What brand of coax are you using these days?
I main preference for genuine high-power R&D is EcoFlex 15.. However Aircell 7 and the 7mm M&P equiv is easy to work with - but I probably push them to the limits..! EcoFlex 10 is also beautiful - however not sure if it's available in USA..
Pool Noodles at the guy stakes will give your doggo something to dodge, or pee on, or both. 😊
good old questions does coax cable lengt matter... happy new year.
Invest in a good insulation tester easy way to tell if coax is breaking down under load . Then do a TDR test to find the distance to the fault .
Oh yes! Although now I know it's all old, I'll swap it out..!
Still rocking like 30year old coax in deployment in places. Probably older in some areas 😬
the coax can be tuned in length to a frequency- old CB trick was to tune the coax to lowest swr on dummy while building up for patch leads etc- since all you had was 5 watts- So that first coax might just not me right for the frequency which is why it shows different SWR..
OK, so you SHOULD NOT Have to "tune" coax length. It should be constant across the length, assuming a 50 ohm load. However, if you have 100 ohms, then you will find the impedance will repeat every half wavelength - this is where some of the old CB myths come in..
@@DXCommanderHQ Your signal actually moves within the coax in the same wave pattern it moves through he air, just as electricity moves at 50 or 60 hertz in your walls in the cables. This is the magic that lets you actually send multiple signals down one coax for cable TV. So, by using the correct length for the frequency (or harmonic) the SWR actually goes down, allowing best signal strength. But the difference is not noticeable at larger wattages usually, though if the cable is wet, the 50 ohms can be changed but the coax can in effect dry out due to sunlight.
QRP watts usually do not notice the saturation effect which is why QRP ers can use 50 or 75 ohm cables usually without the major losses below 60 Mhz.. as long as the coax is not on a resonance it pretty much gives same DB loss whether 75 ohm or 50 ohm.
Or at least that was my own experience with my usage in USA and UK,, have not been able to continue here in Jamaica due to no equipment set up.
What was the power rating for the COAX. Put a dummy load on one end and antenna analyzer on the other end and test.
Coax will age with exposure the weather and UV light, one of the reasons you could be getting different VSWR readings on different lengths of coax could to due to the frequency you are checking it on and multiples of wavelength according the coax lengths.
Did you measure the SWR in both directions, because if the mismatch is at one end of the cable the reflected power will be attenuated showing a low swr than a shorter cable with a similar mismatch. Better to use a VNA in TDR mode to show where mismatch is in the cable.
Interesting.. But no.. If it's not right, it's old and needs to be replaced..
Love the hat looks great on you happy new years callum
Happy new year!
Strange things can happen when coax goes bad. I have seen this in TV-cable (working from the call-center end of a cable-tv provider.
A break in the coax or water in the coax can cause all kinds of issues but it really depends on what frequency is being used the very moment. Some frequencies might be severely affected but others might be way less ('suck out on frequencies')
In your case I'd suspect it might be water in the coax rather or then water in the cable to be honest. Or it might be on the 'open end' of the connection - for example when it's is pouring down creating a short between the center and ground. Might be unaffected on low power but boiling the water away on 1500W creating arcs/carbon tracking against the open end of the SO239 - (although you've covered that with shrink-wrap so it should not be an issue, but I'd doublecheck that part, water goes virtually everywhere where you don't want it to go :-) )
Anton, I like that idea.. I think I will remake everything..
Love that Blue Sweater!
Hey Callum I really like the look of the hat and my goodness Old Bean, you have SNOW on the ground! WTF?? And near the ground stakes make for a nasty bit to cut the grass I suspect. As far as the SWR, I would (were I you) re terminate both ends of the problematic coax with new connectors and inspect the cut off ends to see if there was water ingress or corrosion? No idea about the patch lead issue other than length perhaps? This is a fine way to start the New Year Callum, a coax issue.... Art W1SWL
Arthur.. It's probably old, maybe a pin hole or a carbon track that ony shows up at high power.
Away from amateur radio I had an issue with a 153MHz paging antenna at the top of a 150ft plus tower. The customer insisted that the range was down. Checked at the transmitter and SWR was fine as was forward power. (Did not do a TDR measurement). Paid riggers to go up the tower, and measure at the antenna feed point, again OK. (LDF 4--50 was fine). In the end to shut the customer up we paid for riggers to go up and change the aerial. Everything happy again but I was driven to investigate further. Doing a swept frequency measurement on the white stick antenna it was strange that the SWR was in single figures even at about double the design frequency. Ran a stanley knife along the tail and the braid was black and damp, a hairline crack in the fibreglass of the antenna had let rain in. Waterlogged cable does not always lead to high SWR it becomes an effective attenuator. Happy New Year.
Amazing!
@@DXCommanderHQ you would not believe how much loss was in that short tail. (granted it was VHF). Coax will tend to "breathe" with temperature variations, as it gets hot in the sunshine the air inside will tend to push out from the connectors and as it cools it will draw in air or moisture.
FWIW some types of cable are often pressurised by a small compressor and dessicator to keep them dry internally. No good with solid dielectric cable though...
As a rough test with your "stick" you could try with NO load on the cable and find a frequency where the SWR is 3:1 that corresponds with 25% reflected power, ie 3dB in each direction. Check that against specs for the cable.
I'd use my Megger on any cable that was suss. 500 to 800 volts will soon find dielectric breakdown.
I added bright yellow sleeves to cover the guy wires.
Nice!
I have been fighting with high SWR for a long time now. I've narrowed it down to one bad piece of coax, but now that I've gotten that far, I don't know where to go with that... toss it? Figure it out? Fix it? I just know if I cut one end off, it will be the other end with the problem.
Yep Steve, I'm goiong to throw mine away - just keep the new stuff..
I haven’t watched to the end yet … but my bet is water ingress causing dielectric failure at high V point :)
Happy new year Cal!
Hell yeah coax goes bad. 😂 look for tarnish on the shield. Moisture sometimes, other is just high voltage over a long period of time. As the shield heats up it will cause reflections
Good to know!
Enjoyed the video I hope you find out what's up with the coax. Is it possible it's something in your coax box outdoors? Happy New Year and I like the Hat. 73
Can your tester do a TDR trace? I would test from the connector to your radio to the feed point and then the antenna. and see if it shows any spikes. It will give you the length to the problem/s. I used to use one so sensitive it would show if a cable tie was pulled too tight. was an awesome bit of gear! My NanoVNA does a pretty decent job. I have learned so much from that cheap gadget and its PC software.
I wonder how much RF energy your wire fence picks up(or reflects maybe)? Would be cool to measure it somehow. Maybe insulate all of it from the ground and then put a Watt meter on it to a ground stake.
I know this is a wild idea. Equipotential bonding arrangements could be an issue. causing ground loop impedance issues from your shack to the distribution cabinet.
Mate.. this stuff is all so old, I might as well cut the connectors off and re-make them with new coax :)
@@DXCommanderHQ I meant testing your underground run and whatever antenna and patch leads for it. You must have a sparky (Elec-chicken/Angry Pixie Wrangler/Ones and Zeros Plumber) on your estate with a Megger (insulation resistance tester/meter). I insist you get a VNA with TDR and Smith chart mate. Would make some great content with swearing! but once you get what you are seeing then the truth is out there. I would make a visit with mine but I'm out of range of you financially currently :( dang!
I'm that skint that I have had to plugin two mice into my PC as my right button has broken in one and the optical in the other. Referee!
Cal, It tales GUTS to do things on cámara no matter what it is making yourself open to folks that Love to attack?"
We can ALL learn from all of these crazy videos. I invite the hardest core professional to beat me up.. I love it!
Eh up Cal perhaps the magic wasn't working so well when the swr played up 😉all the best for the new year
Happy new year!
Maybe try a TDR measurement to see if you have a fault internal to the aircell 7 perhaps?
I do not like the new coax that's out now that multi flex it just does not feel right and when you bend it a bit it does not keep its shape but I only use max 200 watts most of the time I only use 50 watts and i have use Mr Lynch many times and can not fault him unlike some others ,thanks for the video G0VVT
A long length of coax will have a higher loss and show a lower SWR than a short length for the same slightly mismatched load.
Once water gets into a piece of coax best to replace it and use better sealing techniques to avoid water ingress to begin with!
David, time to replace all my outside stuff then!
Dumb question, but doesn't the RigExpert stick have a cable analyzer function? I remember seeing a video of Mike K8MRD using a stick pro to analyze cables, but it might not be what you're looking for. Also K7AGE has a recent video regarding meggar usage (if u have one). Thanks for sharing!
It has all sorts sure - but I was after a quick fix..
@@DXCommanderHQ gotcha, thanks for the reply!
A sudden rise in swr is more than likely mechanical than the feed line mate...... How's that Top Band antenna coming along ? I'm waiting down here for you 🙂🙂
Mechanical - or a piece of grass maybe.. PS Happy New Year!
I was hoping you (or someone else) would tell me if water/moisture can be removed from coax once it innin the line. And explain why it can or can't. The search continues.
Can't.. Causes tarnish and all sorts of problems.. However on 100W, you would probably not see the issues.
what kind of leash is that? I can use a few here in Jamaica..lol..looks like it is not hard on the dogs throat
Just some soft rope we bought in a holiday shop couple years ago. DOgs seem happy with them - and they don't choke - quickly loosening (instantly). I think the big chrome ring helps.
Have run several thousand watts through coax without a hitch 5k plus. Any more goes to hardline
Coax could break down at the peaks of the VSWR using high power if there was a high VSWR over the length of a piece of coaxial cable this could damage the dielectric and what is called bubble it at the peak of each standing wave which will change the characteristics most probably changing the velocity ratio which in turn will effect the match to a transmitter - Fred - G0AOJ
PS: Once coax gets water in it it is basically scrap as water will travel down it with capillary action just like solder will on copper.
It could have been a bad center lead.
Maybe a thin spot.
A lot of things :)
Good one Cal😆
Using coax on my 2/70 vertical, that's probably 20+ years old, it's still fine, and it's just RG213
Try 1,500W :)
Another way to test coax and its conductivity is with a megger. It supplies a larger than normal voltage to the coax and if there is a larger resistance in the coax it will not register a proper high voltage on the meter. K7AGE has a video where he tested his coax just to see if it was working properly. I have a megger I was given and did not know how to actually use it. Now I am testing some of my coax just to see how conductive it really is.
Test your own resistance with it at 1000V ;)
Yeah Roy.. Don't have a megger.. Maybe I just swap out the coax..
@@DXCommanderHQ Megger's are around £75 on Amazon. Send it back when finished with it ;). Works just like the continuity test on your Multimeter but at 1000V. It will tell you if there are any sneaky ways for electrons to take a shortcut. And a TDR will tell you where these places are within a few mm. Jeez just saw how much a Nano VNA has gone up in 3 years my kit was around £35.
Would it be an idea to check continuity of the coax first? Then the connectors? Then the power test into a dummy load or power meter?
Erm.. Pointless if we're actually getting a reading..
Don’t forget that not all PL259s are close to 50 Ohms, even if they have been attached correctly. Just a thought.
Oh yes!
Please be aware that CB operators doesn't have much more than SWR to optimize anything else is quite limited if they want stay legal.
I have customers who WERE CB operators, worried because they see SWR of 1.5:1.. It just doesn't matter, even on a 4W AM/FM radio. The difference in actual power output at the antenna is almost immeasurable from anything better than RG213.
I wonder if the short length of coax that gave a problem was close to 1/2 or 1/4 wavelength on the Freq you had the analyser on (29 MHz). As for the burnout check your connection box in the field it may have got water in it and arced across. By the way the max allowed power in the UK is 400w. Not saying... But. 73 Ed
OK Edward, 1/4 wave or 1/2 wavelength SHOULD make absolutely no difference since impedance repeats half wavlength anyway.. And if we have 50 ohms at the end of the coax then regardless of what else if going on, we should see 50 ohms at the meter.. This is an old myth folks seem to worry about (in my opinion!) :)
@@DXCommanderHQ my experience is that when I see something unexpected or variable on an antenna or cable to an antenna it is often a multiple of 1/4 or 1/2 wavelengths long. visually inspecting the cable and antenna shows no faults. Shorten or extend the cable by a couple of inches and the problem is solved. It could of course be the analyser reacting to the length rather than a 'real' fault. 73 Ed.
I'm probably misremembering what I've learnt but isn't it that the longer the coax, there is more of it to absorb the reflected power and present a lower SWR than a shorter cable?
You are not misremembering. In fact, a famous 43-foot verticals takes advantage of a long coax to make the VSWR at the source end appear closer to 50 ohms than what actually exists at the base of the antenna's fixed impedance transformer.
@@billygamer3941 Thanks for the reassurance, I was beginning to think I had lost the plot! LOL
if its copper / copper coated, its unlikely to be any good if its got wet
Could it be that the antenna wire was heating up, expanding ie. getting longer and changing the tuning and therefore the SWR?
Nope. It was instant. Change coax, all was fine.
That cable is only rated at 2000W at 10MHz. Rating goes down with increasing freq, and that rating is only good with a 1:1 SWR. As SWR goes up, rating goes down to 500W. So most likely, your combination of freq, power and SWR exceeded the max power capability of the coax at 1600W. High swr from a dodgy connector intermittent would have doomed it instantly.
Rating and reality are two different things. Been doing key down on SSB Electronics 7mm stuff for years.. But we've swapped to EcoFlex 15 since then for testing.
Dunno but I think if you can see the braid through the shield, it's probably up the duff and time recycle it.👍
COmes like that out the factory.. Checked.
You looked Van Morrison with the hat on
pro tip : get yourself a proper VNA/network analyser, or even on of those cheap chinese VNA's and forget VSWR and come into the world of RL (return loss) you will get a beter understanding on the workings off you antennas. pse forget those expensive swr hamradio meters great for quick and dirty checks but absolute useless for proper measurments. cheers Joost, pa5nt