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Author Topic: Fuchs antenna.. LC end fed antenna.. tapped coil, EFHW resonate  (Read 1084 times)

Offline Kage

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Going a bit nuts here as usual from my manic state of mind.

Been researching to replace my DIY resonant fuchs 1/2 wave end fed antenna for 11 meters. Original design I used was a 1:8 ratio coil with two windings primary and 16 secondary going into around 15 or so pF capacitance using of all stupid things a trimmer cap. Well I learned my lesson with weather and rain using this as a base antenna. Thought I had it waterproofed but every rain or humidity would climb SWR and lower frequency. NP0 trimmers are useless in outdoor weather and moisture.

This time I am going with a straight up lumped LC tuned stage, long story short there are three kinds of these end fed antennas for 1/2 wave end fed feed, the fuchs (better known as EFHW transformer coupled pri:sec transformer, sometimes resonate air core or powdered iron core, or non-resonate on ferrite which is lossy by a dB or more for multiband but gives flat SWR at expense of low efficiency and energy transfer). The single coil tapped version to convert a range of 1:6 to1:9 ratio (50 ohm to 2500-4700 ohm end fed), and my current setup which simplicity wins for efficiency the LC coil, a coil in series with transmitter output to antenna input and a capacitor across antenna to ground to convert current to voltage fed. There has to be very little loss using this method because it's direct and no cores used or lossy link coupling.
I think the drawback is the fact it isn't linked coupled will cause more standing waves on coax back to the shack.

With CB radio and 4 watts of power, got to make every watt count!

My question here before I complicate shit more is how many people have used these? I know the Antron/Solarcon a99 and other antennas use the same exact thing. The 99 uses an LC coil/cap, one coil inside of the other to act as a hairpin impedance match but I'm not even sure it's really that, it just looks like an RF coil to ground so that the antenna is connected to ground for static, otherwise the A99 is a simple LC match network into a 1/2 wave wire.
Here is an article on the A99 deconstructed to show how overpriced hype this thing is, it's literally an ordinary LC coupled half wave end fed...
http://www.radiomanual.info/schemi/ACC_antenna/Solarcon_A-99_review.pdf

I love EFHW antennas because the simplicity of mounting being in a small lot, got to use whatever I can. Wire types allow inverted V and other configs too.

I'd love to know if using an LC with a 130-2 toroid is as effective as an open air coil at 27MHz. I am testing both but can't figure out which is less lossy. The toroid seems to give less SWR across the CB band which leads me to believe it has a bit of loss, so I am currently testing with house copper stripped wire to make my coil, and a coax stub for the high voltage capacitor.

It looks ugly as shit right now but so far it tunes up with ~1.2 SWR over 26.5 to 27.5MHz but the coil is so sensitive to size change, it will take some glue to clamp it and lots of RTV silicone to seal it up before connecting the top bolt to my 18' adjustable 1/2 wave conduit pipe in the air...
https://imgur.com/a/jmUv4GI

Not even sure how well the open air core copper coil will do. Seasonal changes here are extreme and rain got into my last "fuchs" coil match which led me to this simpler design. I want to use my 130-2 toroid donuts for this but I feel they have some sort of loss at CB since they technically are good to 10MHz on the paper, no idea how to calculate loss over air wound when used as purely an inductor like this.

After this coil cures the RTV silicone I plan on making the same coil but using the red 130-2 coiled torroid in place of the air coil design in a different water proof unit with snipped coax capacitor and compare the two. Hard to do at my location because CB is dead right now. Hoping to get some skip in with that short spirt in wintertime. I guess there is always field strength metering. If torroidal wins out or comes in equivalent I think we just found the new best DIY end fed design for goofed setups like I have. Though only a few hundred watts can run through them, but I honestly run barefoot and enjoy the challenge of antenna design over idiotic power into thimble antennas.
« Last Edit: September 23, 2019, 2103 UTC by Kage »
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Offline Kage

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Re: Fuchs antenna.. LC end fed antenna.. tapped coil, EFHW resonate
« Reply #1 on: September 23, 2019, 2107 UTC »
BTW there is one good thing about my playing with this stuff, I learned about toroidal cores and what the numbers mean. Sounds silly to the experts I am sure, and honest to God I have built RF electronics and it took me this long, but now I know the first number means size, the next number means mix. I feel like such an idiot lol. We all learn something new every day. Built radio transmitters for years but never knew that lol, just went by frequency spec, or tested them the hard way  by winding equals and sending RF through to see if they'd pass it, but I was over-complicating things like usual. Though it doesn't hurt to do the wrap and test method to test, never know if these companies sell legit core material until hitting them with RF.
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Offline Josh

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Re: Fuchs antenna.. LC end fed antenna.. tapped coil, EFHW resonate
« Reply #2 on: September 23, 2019, 2246 UTC »
I ran a 2wl horizontal 11m antenna up about 1wl. One guy could hear me at s9 locally, and a guy 4 blocks away from that guy couldn't even tell I was there because there are insane lobes and nulls once one gets above a wl in length. Fun stuff to play with.

On the losses, use a fsm to see who gets more power out and a ir thermometer or your hand to check for heating of components. The fsm should be left in the same spot for testing, as well as antenna orientation, of course.
« Last Edit: September 23, 2019, 2251 UTC by Josh »
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