Self-resonant Trap - No Capacitor?
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- เผยแพร่เมื่อ 3 ต.ค. 2024
- I wanted to turn my monoband 20m vertical into a dual band 20/40m antenna by installing a trap. A little bit of theory, a build, and of course some testing. The theory here applies to multiband dipoles as well.
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This channel deserves many more subscribers! Good content for sure. Thanks for walking through this setup.
Thanks Michael. I agree!
Great video, The traps well explained and love the use of the dip meter. :)
I ended up making a coax trap after this and it worked MUCH better.
If you're in the US and you're looking to put up a VHF or UHF antenna, make it double as a TV antenna and the HOA cannot bother you about it because by law, HOAs are not allowed to ban or excessively impede the installation of a TV antenna. I suppose it might also work to have a vertical HF antenna double as the mast for a TV antenna (even if only one can be used at a time) as an interesting workaround.
Very well explained. Thanks for takeing the time.
Good simple presentation, covering precisely everything !!!!! Only the Snake part was bit scary 😬
There is one more thing regarding this approach you should be mindful of. The coil has high losses near its self-resonance frequency. It's generally recommended to use a coil on the frequency which is 4+ times lower than the self-resonant frequency. In this case, 7 Mhz is only 2 times lower than 14 Mhz, which means that a considerable amount of energy goes into heating the coil on 7 Mhz. On the other hand, an 80/20 meters antenna would work OK.
Interesting. I’ll have to look into this. Thank you for the input!
I thought this was going to be a video about di-poles, but then a guy in a white suit and a fish-like head exclaimed "its a trap".
Ha! That would have been a better thumbnail
Yeah, I don't know why hams start scratching their heads when it comes to verticals - just half a trap dipole...
the greatest tool in any electronics workbench, electrical tape
good video
Great channel
Thanks!
You´ll get a better reading when you raise the antenna to about 6feet and slope the radials.
What a few elevated your feed point a few feet and kept your radials off of the ground Then you could get by with just three or four radials and it would work fine but when they're touching the dirt that's when you have to have a lots and lots of radios because of the ground losses
Hi Pat. Unfortunately, I could only get away with a few radials, and tried it for a while. There are a lot of animals where I live and they were constantly ripping the radials out. I'm most likely going to leave the pole, rip out the radials, etc and connect an EFHW to the tip of the pole. Video in the works!
What diameter was your coil? What lengths of wire did you wind up with for the radiating elements?
The coil was 50 turns of #16AWG magnet wire on a 2" OD fiberglass form. The 20m section was approximately 16.5ft and the 40m section at the top was approximately 2ft long. The short section on 40m, I'm not so confident about. I wouldn't recommend building this yourself, unless I have time to improve the SWR and test on the air. This was more of an exercise in tuning traps... a little experimentation if you will.
In modeling that in MMANA, I came up with the 20m section to be 17’ 8.5”, then an 85uh coil, which is a few more turns, maybe 55… then I placed a 100 ohm resistor across the coil to improve the Q some, that made the top section 20.86” long. The bandwidth is slightly improved on 40m with the swamping resistance. You might just try adding the resistance to yours and see what it does. It will have little effect on 20, but I would guess the resonance on 40m will lower. Without cutting, use your analyzer to see if the bandwidth improves enough to warrant retuning the upper element. I model less than 3:1 from ~7.06 to ~7.23. Without the resistor it’s only ~7.11 to 7.185.
Thank you!
I would plan on doing at least 30 radials, 60 would be ideal. At least that is my experience here in Florida.
Thanks for the advice. The plan is to put down as many as I can until the mosquitoes make it unbearable!
Try to use raised radials, 4 raised radials equal 30 in the lawn.
That's what has always frustrated me about RF. You design and fabricate everything carefully, and then you have to trim things with scissors -YOLO style- to get it to actually work. I get that the real world is always more chaotic and the equations are for ideal components that can't actually exist, but it just adds to the whole "dark arts" cachet of RF engineering. :)
Also: BEARS?! Has central Florida always had bears? I was always somewhat mindful of snakes and gators, but I never even considered bears.
Hey Jim. Yeah, antennas especially.. at least at HF "ain't no need for decimal places". I have a funny shot of 3 bears going down my street at 2am knocking over trash cans. Couldn't find it for this video unfortunately.
Those that have RF experience know there is FM everywhere.
Trap Antenna Tuning & SWR 👌
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click dislike 4 times lol liked once because I didn't think of it doe!
more radials dont always work trust me i should know