Multi-Band Trap QRP Antennas

Started by cockpitbob, May 08, 2015, 05:01:56 PM

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cockpitbob

Who's got experience with QRP traped wire antennas?

Now that I have a MTR_V2 and am going backpacking this summer, I want a small, light 40/30/20 end fed antenna for SOAT work.   A 63' end-fed wire with a pluggable link at the 30meter point would work 40/30/20, but it's long and I would have to lower it to change the pluggable link.   I'm thinking what I really want is traps as 20M and 30M.   Traps will make the overall length shorter and I can jump between bands without fussing with the antenna.   The traps need to be small and tapered at the ends in case all I can do is an inverted-V and need to pull the traps over a tree limb.

All I've found on the web is a design using T50-2 toroids and 1,000V mica caps (27pf-47pf range).   Sounds good, but I wonder if the Radio Preppers brain trust has some advice on the subject.

gil

Great idea Bob, I want one too for my MTR! It might even work for my K1, which also has 15m.

I saw traps here: http://www.sotabeams.co.uk/hf-antenna-traps-pair/

A place to start maybe.

Gil.

cockpitbob

Gil, great information there!  Thanks.
Now I guess I have to get technical and figure out why they use toroids with -6 material instead of -2.  There's tradeoffs between various core materials for loss vs. frequency vs. size of toroid vs. number of turns.  I'll have to run some numbers to decide what I'm going to use.

Their price is really hard to beat ($11.60 for 2), but I'm going to do my own since their circuit board is big enough for a 100W trap and all I plan to make is 10W traps.  I want my traps as small as possible.

vwflyer

If you get it figured out I'd love to hear how you do it. Or maybe, you might even consider making a few extra  ::) At the risk of getting in trouble with Gil by turning this thread into something Entrepreneurial I'll wager I'm not the only one who would be interested. Now that I have my MTR and am hoping to do a little summitting myself this summer it would be just the thing. I built a linked EFHW like you have but agree that a trapped antenna would be more convenient for working all three bands. Speaking of the linked EFHW, how did you come up with six feet for the counterpoise? Trial and error? The chart that came with the sota tuner seems to think the counterpoise has to be about half as long as the radiator.

cockpitbob

VWFlyer, once I'm done I'll certainly publish how I made the traps.  If I have to order any parts I might consider getting extra and putting together some kits, but I can't promise anything.
WRT the counterpoise, the end fed half wave antennas all seem to recommend about 6' of counterpoise, though the feedline also acts as something of a counterpoise and I've used them with no counterpoise.  The random length end feds that usually use a 9:1 UNUN and tuner want a much longer counterpoise at the UNUN coupler.

cockpitbob

#5
I ran some numbers on little traps this weekend and did more than a little research and now I'm more confused than when I started. 

My numbers say that using toroids won't work because the voltage across, and hence flux through, the core is high enough to make them very, very, very lossy.  This is all based on two assumptions:
1) Since the wire between the SOTA tuner and trap is 1/2 wave, the voltage at each end is the same.  At 10W I calculate that voltage to be about 180V.  I'm pretty confident this is right.
2) The wire on the far side of the trap acts as a ground, so all 180V appear across the trap.  I have doubts this is a valid assumption.  But if it's not valid, the trap isn't doing a good job of looking like an open switch.

On the other hand, there is "an existence proof" (proved because they exist) that toroids in traps work because Steve Weber, SOTABeams and a few others have made and used them.  SOTAbeams sells a 100W version that according to my numbers should smoke ??? .

So I went to the web to do some research.  After an hour of exercising my best Google-fu, all I learned is that no one knows how much voltage is across an antenna trap.  But, they all believe it is high because they all say to use HV capacitors.

What's next:
I'm going to build a toroid trap and test it, probably to destruction.  I have a 100W EFHW coupler, so I'll make a 20/40 end fed antenna intended for 10W.  I'll run it at 20meters and keep turning up the power until something observable (bad) happens.
I'll also make my version of VK3IL's trap.  He's the guy that does the code for Steve Weber's MTR rigs.  His traps use air core inductors (no ferrite core, so zero core loss).  I really like his design.  It's clever yet simple.  I do feel he used too few turns and too big a capacitor, so mine will be different.  I'm leaning towards this approach.

I'll keep you posted.

cockpitbob

#6
I'm almost done with my 40/30/20 trap end fed wire.  I've learned a fair amount along the way.

I was wrong about the voltages across the traps being too high for toroids.  I've made 2 versions.  One withn air an core inductor (wire wound on a ball point pen's tube) and one using a T50-6 toroid.  To test their efficiency (self-heating) I put shavings of candle wax on them, hoisted them up a tree and cranked up the power.  If the wax melts I know it got above 130F.  At 35W the toroid traps got barely warm enough to soften the wax.  The air core traps didn't warm-up until 50W.  I figure it takes 3-5W to heat the trap enough to melt the wax, so the losses were only about 10% :) .  So they'll be rock solid at 5W - 10W.

For the trail version I'm going with the toroids because the traps are a bit shorter and more tapered and so should be less likely to hang up on a tree branch.

I'll post final results and pictures in a few days.

gil

Very interesting Bob, I am following your progress with interest ;D

Thank you! Can you post photos?

Gil.

gil

#8
You know, I vaguely remember asking Larry at LNR Precision about a 40/30/20m end-fed and why he wasn't offering one. It was a while ago but if my memory isn't failing me, he emauled back saying it wasn't so easy... Why, either I can't remember what he said or he didn't elaborate... I guess we'll find out. I sent him an email inviting him here...

Gil.

gil

BTW, are you using parallel LC traps or series? I'd think that for more than one trap you'd have to use a serial LC circuit...

Gil.

cockpitbob

Just a simple parallel L-C for each trap.  When operating at 10MHz, the current passes through the 14MHz trap's inductor and on to the next section of wire.  That inductance acts as a coil, shortening the needed length of the next wire.  My 40/30/20 EFHW antenna is 54.5' long, so the inductors of the 2 traps shorten it by about 9' compared to my normal 7MHz EFHW wire.

I made more progress this weekend, but I'm not quite ready to sit down and post it all.  The air core and toroid traps work equally well, so for trail use I'm going with the toroid ones since they are shorter and can be made tapered.  They'll pull over a tree branch easier.  I just ordered some of TheWireman's 26AWG stealth steel core wire for my trail antenna.

Since I made 2 air core traps I make a 40/30/20 antenna with some insulated, stranded 22AWG wire I had.  I did 1 minute of key down 35W at each trap's frequency and again the heating tells me they are loosing <20% (<1dB).  So they will be very robust at 10W CW and loose very little power.