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The antenna for the favorite bands - not afraid of traps

I must admit - I am a sloth. At least I don’t feel like screwing around with the antenna during an activation… and I would like to keep the effort during the setup and also the dimensions and the weight during the transport as small as possible.

Of course, even without an antenna tuner, I don’t want to commit to just one band. For a while, I got by with simply stringing several resonant wires and changing them on the TRX. But that was too tedious for me at some point.

For some reason (I can’t really say why) I didn’t like traps. Maybe it was the hassle? That changed when I saw the following video:

The programs he used to calculate the traps can be found here:

I built a lightweight antenna for my current favorite bands.

I used whatever was in the box for the capacitor. The windings can be easily calculated with the programs… and as you can see in the video, a fine tuning via the SWR is also a simple thing.

A few cm after the BNC connector I wrapped a current balun with FT 114-43. Then follow about 3m RG 174 (that’s the 0.5 lambda for 60m) and then the 1:49 balun on a FT 82-43.

The wire itself has 0,14mm² (a bit thicker than 26AWG).
The distance to the first trap (17m) is 7.30m, the distance from the first to the second trap (30m) is 6.05m and the remaining wire for 60m is 11.44m long.
I determined the lengths with the NanoVNA… and they will surely differ a bit for everyone.
To be able to hang up the wire better I have threaded small plastic eyelets between the traps.

Everything is small and light. The SWR is better than 1.3 on all bands.

73 Armin

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Hi Armin,
You might want to post a link to this in the “Show us your antenna” thread.

My go-to for no-tuner / no-links multiband HF portable antenna is an off-centre-fed dipole. OK, it has the heavy 4:1 Balun, but apart from that for 40/20/15/10m it works well enough on even a 6m fibreglass pole.

73 Ed.

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Well, one reason to dislike traps is that they are lossy. I’ve always been told that, running QRP especially, to mind your losses.

73 Eric KG6MZS

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The skill is to design and manufacture the traps/UNUN etc. such that the loss in those assemblies is no more than the loss in the length of coax that would be needed if you used a classic dipole.

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True, and, as you most certainly know, depending on the type and length of coax used, those loses are not inconsiderable – especially for QRP. This is why I favor antenna systems with little or no coax when convenient.

73 Eric KG6MZS

I’ll do that sometime when I get them set up… :wink:

73 Armin

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…in this case the coax is part of the antenna system… about the losses in the traps I can unfortunately say nothing.

73 Armin

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Thats why I don’t use ATUs or traps… :slight_smile:
only an antenna analyser to tune the antenna.

73s Ingo

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Each element in an antenna system causes losses, whether plugs, feeder lines, antenna wire, traps …

But the crucial question is how big are these losses?
I don’t trust opinions or feelings, but verifiable calculations and measurements.

With plugs, cables, baluns, traps with the help of network analyzers, with antennas by current measurement in the antenna wire.In addition, sophisticated simulation programs are now available.

We amateurs rarely have the time to deal with it extensively. Or such tools are only available to few of us. This was especially true in the past.

That is why unprovable prejudices against certain elements keep popping up.

I’ve been using the EFHW with the 1:49 unun since 2013. In the first few years, every third qso partner told me that this antenna principle should have significant losses compared to a classic dipole.

What does significant mean? Nobody could say whether -1db, 3db, 6db.

Today I know it’s - 1-2db, a 0.2 - 0.3 decrease in S value.
By the way, such minimal losses occur in any ordinary antenna tuner.

Go on, Armin!

73 Chris

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Of course, everything has losses and that is a pity especially with QRP, because you are often in the border area of the possibility of a QSO.

In SOTA, however, I have found that the setup can be more decisive.

For example, it may be better to take the snow-free southern area of an AZ than to be directly on the snow-covered summit…or to have more distance to trees with heavy foliage and also buildings…

(But there are often also “tactical” considerations… If I want to reach my colleagues on the summits in HB9, I will not stand on the north side 20m below the summit in the black forest).

Of course, one can also make mistakes when building the antenna. Certainly, a wire that is too thin or a ring core that is too small also brings its losses. (In this antenna I used the same values for the traps as they are in the output stage of the QCX).

During an activation I had set up an endfed for 30m so badly (simply thrown over leafy branches of a bush) that the poor SWR caused the toroidal core of the 1:49 balun to become quite warm. So I had all the faults together at the same time! :woozy_face: However, I quickly noticed this in the performance and built up the antenna better… and the toroidal core later became a little larger and the wire a little thicker. :sweat_smile:

73 Armin

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Armin,

As far as I know, the QCX “output stage” you referenced does not have a parallel resonant circuit, but rather a low-pass filter (LPF). However, an LPF works quite differently than a parallel resonant circuit…

A trap is known to be a parallel resonant circuit whose L and C values can be calculated using Thomson’s oscillation equation.

DK7ZB once published the L and C values he used when building 100 watt traps with toroids (see “Traps with Toroidal Coils”).

In any case, traps must be adjusted to the desired frequency before installation.

73, Heinz

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Hallo Heinz

Yes, of course - my statement referred to the material used (T37-2 and 0.3mm enamelled copper wire).

That is true and is also explained wonderfully in the video above. I also glued the windings after the finetunig. NanoVNA is really a wonderful tool for such actions… also when cutting the wires to length afterwards.

73 Armin

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All right with that Armin, I’ve been building and using (efficient) trapped antennas for decades, not just since SOTA… Have fun with them.

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There is one detail that is often overlooked.

The antenna wires connected to the trap create an additional capacity in the range of +/- 10pF. I could look up the formula for the exact calculation.

On the other hand, it is recommended (source?) to easily detune the trap to reduce the current in the parallel resonant circuit and to reduce the heating of the toroidal core.

Bill Gates: It’s not a bug it’s a feature

73 Chris

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