Homemade Magnetic Loop antenna -- will it work?
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- เผยแพร่เมื่อ 16 ต.ค. 2024
- My Magnetic Loop antenna design and initial tests. Ham radio folks: please comment!
Base is a (sadly now discontinued) speaker tripod stand. It fits 1-inch PVC perfectly.
www.amazon.com...
The driven loop is a 7-inch brass hoop from Amazon, cut in one place and with a BNC connecter soldered on. www.amazon.com...
The main loop is 10 feet of 1/4 inch copper tubing from Lowes. There are banana jacks on each end where in connects into the tuning box. This will make it easy to replace the loop with something better, possibly multiple turns.
Motor: 3.5RPM (at 12 volts, but I'm running it at 3 volts)
www.amazon.com...
Motor bracket: www.amazon.com...
Capacitor is from Halted. :( (Why a sad face? see • Goodbye, Halted (Silic... )
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1. Vary distance between Outer and inner loop + vary shape of inner loop
2. Coupling with motor is probably adding some inductance -> make it an isolated coupling
Adjust the capacitor for the best minimum value you can then, adjust the shape of the coupling ring so that it is closer to the main large ring and watch for an even better SWR value. The coupling between the two coils is very sensitive.
Yup!😉
I use magnetic loops. Let me suggest using a "Bias-Tee" circuit to feed the DC voltage (for the gearhead motor) down the coax you're using for convection to your radio. One circuit injects it at the radio end, one circuit extracts it at the antenna. Both can be built for about $20. Works beautifully!
Don’t know if you’re still trying to get low swr, but I’m putting a magloop together myself... the large loop benefits from larger diameter pipe. As previously mentioned, small loop should be 1/5th of large loop. And something that’s often not mentioned, is that losses in the system greatly affect performance. Capacitor tuning plates should be welded to each other, not just relying on mechanical connections; This is key. Hope that helps.
Push the transmission loop So that it flattens out against the larger coil so that there is more close area and proximity between the two loops it will no longer be a circle but will be more of an oval shape that will help to get your SWR lower
Hey man did you followed the 1/5 rule?
The inner loop must be 1/5 the size of the outer. Then the smaller loop you can just move it up and down to get the best SWR. I do tune my mag loop manually would be very tricky trying to do it with a motor.
Did you insulate the shaft of the capacitor and that of the motor? I had a similar swr problem and solved it by putting a short plastic stick between them.
Unless something is constantly moving near the loop the SWR should not vary the way it is. You have a connection problem somewhere. The magloop is a very hi Q antenna and sensitive to less than perfect connections. Also, always test outdoors in an area where the risk of mutual coupling is minimized. Antenna formulas are merely starting points to get you in the ballpark. Factors such as ground conductivity, proximity to the ground and mutual coupling may significantly affect your results.
can you please share the schematic of connecting the loop to the tuning capacitor?
Try making the coupling ring a 1/5 of main loop circumference and couple it close as you dare without touching. You will then find the notch widen slightly.
Rotate the coupling loop along its vertical axis. If the SWR improves, you can shorten the coupling loop incrementally to bring it within the 1/5 size ratio.
hi Mica, to adjust the SWR, raise or lower the coupling loop
Try making the small coil more oval shaped, it may be the wrong size.
i haven't seen any math as to find the 1/10 of the wave length for the band you plan to use. after fiddling with some basic equations will offer this result for comment.
300 / f in mhz = wave length
300÷7.074= 42.41 i. e., 40meters
42.41 x . 1 = 4.24 (one tenth)
4.24meters x 36=152.64
152.64÷12=12.72 ft
12.72 ft is length of material of
"ideal" for 7mhz.
fortunately, most of these magloop
will cover higher freqs.
i hope this info is useful
my loop calculator numbers seem to work with most configurations,
i. e circle, diamond, square and probably stop sign shape.
one thing i noticed with my 3/4 Od copper diamond loop was i needed additional capacity to bring tuning range to 7mhz.
the diamond made if 1/2inch material allowed tuning 7mhz without any extra capacity. i checked this with the software and it showed there was more inductance. further, i made circular loop with 1/4 in dia copper tubing and it suprised me. i could cover 7mhz to 24mhz. wow, that was cool. i made ONE contact SSB 7mhz with sta several states away to my lexington ky QTH, he gave me a 20db over 9 report. that was unusual. hv not had any ssb reports like that again. FYI.
One source of changing readings on you instrument is that it is in a plastic box, maybe fabricate a box of PC board material and you should be able to find a fine wire mesh to put over the LCD screen. The more the instrument is isolated from its surroundings the more stable the readings should be. I too would like to point out that an SWR of 2.5 is perfectly acceptable, contacts will be made, I don't understand some hams obsession with a 1:1 SWR, it is like those guys who took their FT-817 and adjusted the o/p power to its maximum from 5 to 7 watts, the extra 2 watts isn't really going to do anything to other guy's receiver's S meter.
the swr jumping around..
recheck your solder joints.
some math for you
10ft copper tubing x 3.14 (pi)
= 31.4 inches the dia of outer loop.
31.4 x . 2 x 3.14=6.28 in dia of coupling loop.
6.28 x 3.14 = length of material to make coupling loop 19.74 inches.
I'm guessing by the size of the air variable that won't handle more than 30 watts without arcing, I bought a 12" to 14" long, roughly 10 kv Caldwell air variable capacitor for $65, very touchy though, compared to my other loop with a vacuum variable. Your issue is your loop material, use 1/2" or larger diameter soft flexible copper for your large loop the small can be 3/8" or 1/4" copper, I have built the large loop using large coax, they don't work as well as copper tubing, typically higher swr, to really tune your loop effectively, I would invest in a vacuum variable for a motor driven loop, the vacuum variables are much more incremental for tuning, I have never seen antennas work so well for their size and the fact that they are portable is far more practical for events, exercises, trips, etc. 73's and good luck with your builds.
I had the same swr meter....adjust the step config to 1khz...then scan...u get the exact resonance of your loop.73
This swr flactuation makes me think i) how good this swr meter is ii) whether there are any common mode currents which are changing with even the tiniest movement. Then we can start thinking about the antenna design itself..just my two cents
I have the materials to revisit this soon. Current WX means my outdoor antenna is down, so maybe that's a good excuse to play with an indoor antenna. Only need more time for tinkering...
Put the small loop near one of your angles, just not touching.
suggest you chk the driven loop - it looks very small to me. should be 1/5 of the large outer loop
Is there any Magnetic Field to a Magnetic rods .
Use different swr meter and get further away and vary small loop spacing and shape.
First I would check the size of the inner loop. It should be about 20% of the size of the outer loop. Did you raise it higher off of the ground? You are tuning on the 20m band. You need to be at least a quarter wave length off of the ground to avoid ground reflection. 73
tuning cap has to be on the top +++@!
coupling loop you have to Experiment +!
in shape and dimention. use solid conductor (well insulated from air, this mean Cover it wih paint or pvc...or whatever).
73
make the small loop oval and get it closer to the big loop to lower the swr
I don't have a background related to ham radios but it was interesting to learn about it.
well did you manage to get it tuned in ?
adjust loop shape and location (distance)
(1) I think I won't buy a crummy antenna analyzer like that one.
(2) And I think you may have to 'think like Tesla" to figure it out.
...I'm not being a troll - I just don't know -- I've decided to build a couple of decent sized mag loop antenna, and am Just collecting information.
They are interesting and both easy and complex at the same time.
I got a better analyzer (RigExpert AA-55) for a really good price, so I'm well set up there. More on this after my current project gets completed...
And actually 1.3 is a very decent SWR ratio. Actually
actually
you need more capacitor
most people i talk to say that loop is better rx only
by meter