Magnetic Loop Issues (
ฝัง
- เผยแพร่เมื่อ 16 ก.ค. 2024
- Domenico, IU0PWE, live in Italy! And is having some ROS issues with his loop antenna, let's see what we can do to help him.
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You are SUCH a Help and SUCH a Resource, Dave! Thanks for all You Do!
I never realised that mag loops were so sensitive to temperature changes, good to know Dave.👍
You guessed right Dave! ROS = Rapporto Onde Stazionarie which is the same as SWR. I love your channel and considering this antenna!
Thank you, Dave.
I learned a few things.
From operating and repairing mass spectrometers, I learned that the detector head preamps are sensitive to both temperature and humidity changes.
Many other circuits also have problems. A shift from shade to sunlight will shut down a computer - about 10 degrees centigrade.
N0QFT
MFJ, yeah I had an interesting experience with an FMJ tuner. Because of the poor reputation, I STARTED by removing the cover to inspect the components. I was shocked to find on of the wires going to a center pin on one of the PL coax connectors loose and unsoldered. I could have damaged my radio by transmitting into that.
MFJ makes some nice items, but they have terrible quality control issues.
Suggestion: Make your MFJ loop antenna one of the giveaways in an upcoming month. Surely, as you have so many antennas, it must be redundant in your collection. wink, wink.
I'm wondering how much bandwidth you get on 20M or 40M before you have to retune?
At the current price of $650 (USD), it is a rather pricey antenna.
Dave really enjoy your work your a great presenter very pleasant a joy to watch 🙏
Thanks for the video, I wondered how well small loop antennas worked.
Those MFJ loops have a bad record of manufacturing defects. The rule is to open them up and check for shorts, missing screws, etc before powering up. Used to be plenty out there on the web about the problems, especially in Yahoo Groups.
Thanks Dave
I purchased a used MFJ magloop 1786 a couple years ago, opened it up and saw the feedpoint / driven element assembly was Heliarc-welded in the wrong place on the loop. This caused the butterfly capacitor to flex, and the plates to become closer and farther away depending on the plates' position. No way to fix this without proper Heliarc equipment! Needless to say, I had to avoid running high power to this antenna. I agree with Dave and the other comments that say it's useful to take the case off and do a visual inspection. Insects have caused issues for some users, as well illustrated in one TH-cam video that can be found. 73, K7KS
The description of the problem sounds like what I've experienced with older equipment and coax. My theory is bad coax connection someplace where parts and adapters fit together. I have also had this problem with inexpensive RG8x with factory molded connectors. The insides of those molded connectors looks like cheap junk done by someone who doesn't care how careless and sloppy their work is with a cable someone is going to put some power through. I don't buy those anymore. Dang I hate intermittent connection problems that change with power changes too.
Thank You.
As well as what Dave says, I would advise checking all connections and cables and try swapping them temporarily if you can. A bad cable or connector will also give intermittent and varying performance.
Compared to a dipole antenna the small transmitting loop compromises efficiency, especially at lower frequencies.
Varying SWR readings may occur with wind moving the antenna or the attached coaxial cable, proximity to moving tree branches, and loose coaxial connectors.
Great video. I'd like to see comparisons on transmit.
Humidity, raindrops, snow, and temps affect my 40m dipole. Yes, we do get snow and ice in Houston....
I really enjoy your shows David as I always learn from them please keep on what antenna can I use just for listening so any help or idea 💡 would be grate I love shortwave uper and lower am and mw too but I live in a flat where its not aloud to have dipole or wire Help 73s Ian
R.O.S. is the Italian for S.W.R.
ROS (Rapporto di Onda Stazionaria) = SWR
I wonder if anyone modified the plastic case by installing hinges and a couple of latches. It seems like the innards should be inspected fairly often and with a gazillion screws - not fun.
You still have it but it does not look like you have it connected and using it.
I currently have 3 magnetic loops deployed and a fourth one that I have to put into service.
2 of these are entirely homebrew and one partially homebrew.
My sense is that what the person is describing is outside of what you would normally expect in terms of drift of a magnetic loop. I agree with Dave that the loop needs to be opened and the interior inspected. I suggest there could be a loose wire somewhere, some play or backlash in the tuning mechanism or some mechanical instability in the butterfly capacitor.
There could even be mechanical instability where the feedline couples to the antenna.
Under normal circumstances, the loop is not only sensitive to temperature but very much so to humidity. Humidity affects the tuning of the loop to a greater extent than does temperature.
I have a 1788 on order - might arrive in January of 23.
Would it be helpful to seal up the plastic case to prevent critter access or is there a good reason why it is not air tight?
BTW, I receive a lot of flack on the eHam forum for mentioning I'm considering a small transmit loop. They all believe at age 76 I should be hanging from a ladder stringing up 80 feet of multi-band tuned dipole wire. Not going to happen.
I appreciate your several videos on these antennas and your mention of their popularity in Europe. Those critics in the US are apparently not as adventurous or open-minded.
Sounds like the case should be sprayed white.
Why?
@@sammiller6631 I think the assumption is that temperature change is the culprit. Personally, I think something else is happening. Something loose, warped or inside the case is more likely. With certain ham products, it is always a good idea to open the case and have a good look around.
Dave's earlier videos and maybe some others will give you a good idea what is inside this loop. As I recall at the top is the butterfly capacitor and the mechanicals for rotating it and bringing it into resonance.
At the bottom is the coupling loop which is small and enclosed entirely within the black housing. You would not know it was there unless you looked.
The coupling loop simply connects the center conductor to the outer conductor. Presumably at RF it forms a mainly resistive 50 Ohm load.
@@sammiller6631 To reflect the sunlight that is causing the heat problems Dave mentioned.
I had the same thoughts. So I tried for a test to wrap the black plastic housing of the MFJ-1786 with thin Aluminum Foil. It did not solve the problem. I still had an enormous drift on 20m (As I found after searching the internet, the problem seems to be worst on 20m... there is almost no drift on 10 and 11m)
Mag loops are also very sensitive to capacitive changes in their environment. If it is hanging from a cord and turns or sways it will change the tuning and swr. If a person is near it and moves swr and tuning will change. somethig to be aware of.
Non of this 'advice' answers any of Domenico, IU0PWE's questions. Have you seen his QRZ page. He is not a 'beginner'.