Other SOTA sites: SOTAwatch | SOTA Home | Database | Summits | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

Trap EFHW and Coupler

The 40/30/20m trap EFHW antenna and the coupler shown below were built to match the Mountain Topper MTR-3B in respect of lightweight and simplicity of use.

The traps, rated for about 12 W, are built from T44 cores and mica caps, they are protected with shrink tubing with internal adhesive.
The values of L and C are chosen along W1FB such that XC=XL is in the range of 200…250 Ohm.

  • 20m: T44-6, 24t #24, mica 50 pF/500 V
  • 30m: T44-2, 26t #26, mica 68 pf/500 V
  • Wire: POLYS #26 (intermediate section 0.25mm2 RADOX-125, for easier tweaking)

Because I opted for 3200 Ohm as target impedance (like hyendfed.nl does), all my EFHW couplers are built for a 1:64 impedance transformation (LNRprecision has choosen for 4050 Ohm and therefore uses 1:81 couplers, other antenna experts recommend 2500 Ohm).

Nothing mystic about the winding technique, it makes no difference if the winding is applied just straight ahead (as usual) or if it is divided up in 2 halfes. The latter is used for practical reasons to become the input and output wires opposite (what may be wishful at QRO levels…).
Some of my couplers are equipped with a simple optical RF power indicator (LED). This feature was very helpful last year when a linked EFHW was used (to alert the operator quickly when the band settings at the antenna and TCVR were different…).

  • Coupler: FT-82-43, 24t #22 tapped at 3t, mica 150 pF/500 V
  • RF indicator: 2 germanium diodes (e.g. 1N34, 1N60), LED red w. transparent body

The couplers are used without a counterpoise wire, either with no feedline or with coax of 0.5, 1.0 or 2m length, depending on the local situation.

BTW, Steve AA5TB modeled a tuned LC circuit (known by the Fuchsantenna) and discussed the findings both from a practical and a more theoretical point of view. Along Steve the modeled 0.05 lambda counterpoise can be substituted in practice by a few pF of capacitance to ground (A/N: that happens „by itself“, without asking the operator, hi). So there is no reason to worry about the "counterpoise question“ of EFHW antennas at lower power levels.

The main features of the 40/30/20m trap EFHW antenna presented above are:

  • Resonance frequencies at 7.025, 10.118 and 14.055 MHz
  • Total length 16.80m
  • Section lengths 9.83, 2.68 and 4.25m
  • Total weight incl. coupler, winder and guy rope 132 g

This is now the dream antenna? The answer may be yes and no.
YES, because of it’s simplicity, good efficiency, very light weight, slightly shorter overall length and low wind resistance. The antenna performs very well and has also been successfully used with the KX3 at 10 watt.
NO, because I always believe that you can do something even better, hi.
Well, the slightly lower efficiency of the trap antenna can be accepted because this is in practice hardly noticeable.

Trap EFHW, why not!? Have fun!

Heinz HB9BCB

Addendum of 15.03.2016:
Trap construction details (the photo below shows prototype traps wit T50 cores).

Windings should be tight, slippery windings will fail.
The hole in the PCB is provided for the 1-turn loop during trap adjustment.
Prevention after adjustment/before heat tube shrinking: any slipping of windings/core on pcb can be prevented with some coats of (transparent) nail polish.


Thanks for sharing Heinz; excellent pictures and full description.
Yours is a nice variant of EFHW I didn’t know.

As for you, I’m also thinking how to get an even better antena, hi…

73 de Ignacio

re K6HPX: Thanks Ken for the “like it” and the personal note! Why not publish a supplement to your QRZ page by listing the key points of your unique 160m EFHW antennas mounted on your airplane? A photo showing the coupler with the vacuum cap would also be of interest! That would undouptedly be also “a breath of fresh air” on this reflector, hi. Ok?

re EA2BD: GL Ignacio for your “VK3IL replica” project!

1 Like

Beautiful work Heinz! Thanks for sharing.

73, Barry N1EU


That’s a very nicely built antenna! One question though, how is the LED indicator wired and why two diodes? I can’t really see from the photo. I like the idea of having an output indicator, which presumably only lights when a high voltage is present meaning a resonant antenna wire is connected?



From what I can see in the pictures it looks like the LED is attached to the small piece of PCB and pokes through a gasket to the outside of the case. The two diodes (with red markings) are oriented to allow current to flow in through one and out through the other (see the reversed polarity bands,) providing an electrical path for the LED and some sort of voltage drop so as not to blow the LED. Since an EFHW has a high voltage at each end at resonance, both the voltage and the brightness of the LED would drop if the applied frequency moves away from the resonance of the antenna. Pretty smart idea, really.


It’s exactly as you said, John.

Have found this idea many years ago when I browsed through QRP homebrewer projects for useful features. Later this feature was taken over by QRPprojects.de for the Multiband Fuchskreis Kit: http://www.qrpproject.de/Media/pdf/MultibandfuchsHandbuch.pdf

I was immediately impressed by the feature because it appeared to me like a perpetuum mobile, hi. This, because there is no connection to ground and the 2 ge diodes are tied together to the same potential.
In my understanding the diode at the input side of the LED acts as HF voltage rectifier and the one at the output side as “reverse polarity protection”. How exactly then the stray capacitance contributes to illuminate the LED is a little less trivial to understand.
Anyway, the feature performs excellent without any unwanted side effect (since there is no coupling to ground). The only drawback is, because perpetuum mobiles doesn’t exist in practice, the feature absorbes some mW of precious HF energy, hi.

BTW, I first thought to integrate the indicator into the MTR transceiver but the voltage across 50 Ohm is not sufficient to illuminate the LED properly.

1 Like

I was just looking up OA91 availability and found that there are still surplus stocks available. I suppose there could be someone making them somewhere but I’d guess most will be NOS (new old stock). I found a datasheet from 1968 for the OA91 and sadly some unexpected browsing occurred as indirectly I saw mention of the AY-5-1224 24/12hr clock chip. I had to check it out as I made a sidereal time clock using one and an old LED calculator display when was off school, I should have been revising for exams. They were SOTA (state of the art) in 1974 and my clock was made in 1979. I’m fairly sure I found it or remnants of it when clearing out my late mother’s house. I’ll waste hours tonight in the loft searching for it again.

Anyway that’s 30mins wasted instead investigating a segfault when delete[] is called. So thanks Heinz! :congratulations:

p.s. beautiful construction.

1 Like

Nice work Heinz! I did a double take as I thought it was a professional product like you’d see from SotaBeam. I like how you constructed it and its now on my “to do” list. Great job and thanks for sharing!
Mike W6AH

Have just updated my original post with some details of trap construction (shown are prototype traps with T50 cores).
—> that seems to be too late, hi. So there it is:

Addendum of 15.03.2016:
Trap construction details (the photo below shows prototype traps wit T50 cores).

Windings should be tight, slippery windings will fail.
The hole in the PCB is provided for the 1-turn loop during trap adjustment.
Prevention after adjustment/before heat tube shrinking: any slipping of windings/core on pcb can be prevented with some coats of (transparent) nail polish.


some images are no longer reachable unfortunately …

PS: sorry, i’m late … Everything in my life, I got wrong. I worked too much and used the radio too little


I found out, quite by accident, that the banana socket on the Fuchskreis tuner also works well as a rudimentary RF detector for coax in the shack.
73 Matt

I agree, antenna construction articles such as this should have permanently archived images.

73, Barry N1EU

1 Like

Nice job. Appears to be essentially identical to what I have been using for years. Comments: Use #28 wire and more inductance, less capacitance, to achieve higher impedance on the traps and shorter overall antenna length. NPO SMD caps seem to work fine. Matching unit is a AA5TB tuner built into a dental floss case. Details, including source for the tiny circuit boards, are here: https://www.ke6mt.us/tag/ws0ta/ Since there is no current on the end of a halfwave wire, the wire itself serves as the feedline. In other words, there is no feedline (to carry, coil, uncoil, or contribute loss). 73 fred kt5x (aka WS0TA)

AA5TB tuner in a dental floss case, the SWR indicator is the “tinySWR” by DK3IT:

The traps use the little board from OSH Park, link above:


Beautiful work Fred!

73, Barry N1EU

wow !! icon_pray

Well Andy that is a bit scary, I made a similar clock in 1976, It has run ever since on our bedroom table.
I make that 70,956,000,000 counts. Having seen your comment I looked up the spec sheet, and yes it is still current. So I downloaded it, never know when I may need it.

1 Like

An OT interruption, apologies in advance.

I’m sure that sidereal clock is somewhere in the loft. Back then it was a simple way of getting the sidereal time to use with my star atlas to figure out what would be visible. Though after a few years of watching you know what is where from the date. Nowadays, just use your phone and browse to a website that can tell you the sidereal time and the Julian day and…and… That solutions seems less cool even though the amount of technology needed is massive compared with soldering a “thing” yourself out of assorted CMOS 4000 series chips to muck with the 50Hz signal into the clock chip.

That was the 41 year old solution. Now I’d write a bit of code for a PIC CPU to generate the time and drive a display. Or, as I said, look it up.

I just checked what happened to General Instruments, the AY-5-1224 manufacturer. They made lots chips I used in the 80s apart from clocks, including the AY-3-8912 sound generator in the Atari ST and the SPO256 allaphone speech generator. I have a couple of SPO256s on Veroboard which connect to the parallel printer port of your computer. Then you can “print” the data to them nice and slow and get something that sounds like The Robot from Forbidden Planet. Luckily I no longer have any computers with parallel ports or I’d spend all day getting one of the working to call CQ SOTA !

As for General Insturments… they also made cable/satellite TV gear and the like as well as semiconductors. The TV part has been sold a zillion times to just about everyone in the TV industry and is still going (ARRIS, Zenith, Motorola Mobility, Google, CommScope) whilst GI spun off the semiconductor division into Microchip Semi, the PIC maker (amongst other things). So my idea of making a sidereal clock with a PIC would be a homage to the original GI clocks.

I had another clock that used the older AY-5-1202 driving a VFD that I made from a kit in 1974, don’t know what happened to that.

One question for David, why do you need a sidereal clock in your bedroom? :wink:

I missed that at first reading! Neat, a clock that is right once a year! Combine it with a Julian date calendar and it would have been the ideal nerd toy for an astronomer…

Is there a chance to restore the missing images of this post, so a newbie could follow the build process and avoid mistakes? :grin: