I’m hoping to put together a SOTA trapped dipole for 40/20/15M that will be center supported by a sturdy 30ft fiberglass pole.
I’d appreciate informed opinions on whether I should go with two (15M, 20M) traps on a side or just one (20M) trap on a side (i.e., go with L/2 or 3L/2 for 15M).
Thanks & 73,
There are pros and cons for both options.
If you use separate 15m and 20m traps on each side, you should be able to optimise the dimensions of the antenna to get a low VSWR on all three bands without the need for an ATU. However, in certain directions, a half wave dipole on 15m offers a little less gain than three half waves on that band.
If you go for just a 20m trap on each side, you will benefit from the slightly higher gain offered by the three half waves on 15m but you will never get better than about 2:1 VSWR on 15m because a dipole cut for 40m is not exactly resonant on the 15m band. Of course you can match it on 15m by using an ATU.
In practice, there is very little difference between the two options. If you have two traps on each leg, this adds to the weight of the antenna so personally I would go with the single trap on each leg and accept the somewhat poorer VSWR on 15m.
For my SOTA activations I use linked dipoles with no traps, and I have always just used the 40m dipole on 15m. I have never bothered with a separate 15m half-wave section.
Good luck! Try both methods and see which works out better in practice!
I prefer resonant dipoles myself… and with the necessity to find a good frequency anywhere in the available bandwidth, having traps could make that a problem.
I would go with a resonant link dipole design using string to support the link portions and 2mm rc gold connectors. 10ft of RG174… and not worry about putting the dipole up higher… it’s already at a compromised height… why mess with the 30ft pole that weighs a ton…
This is what I have been running for a while now.
Thanks for the responses! I’m definitely not interested in a linked dipole.
I think I’ll go with a single trap if the SWR doesn’t look too bad on 15M.
If you want a single trap each side why not go for the W3DZZ design. Ready built lightweight traps available from SOTABeams 1
Don’t you want to include 30m?
Sometimes the bands are busy with contest and 30m is a very good place to escape
Best 73 de Guru
Guru, this antenna is for use only during special events that don’t include WARC.
Thanks for the tip Glyn, but I don’t think the W3DZZ design offers any advantage in this case.
73, Barry N1EU
Barry, would you elaborate on what the intention of the antenna is, and why the trap design is so appealing.
It’s simple - I want the ability to change bands instantly.
Linked dipoles are superior antennas, I just don’t choose to use one.
Is this antenna for portable or home use?
If portable, we are faced by many compromises with antennas but in general they work out better on a mountain top than in a home garden.
I’m no fan of trapped wire dipoles having tried several commercially made ones at my home QTH, when I was in Australia. For portable operations I always used the SOTABeams linked dipole but it is a pain having to lower the mast to change bands. I now use the Spiderbeams/Aerial-51 UL-404 asymetric (OCF) dipole and have not seen any difference between its performance across multiple bands compared to the linked dipole AND I simply change band on the rig and I’m on a different band, no fussing around.
If this is a home QTH wire antenna you are looking for, for multiple bands, I am amazed at how well a 40m full wavelength loop works on multiple bands. Of course you need some space to put one of these in and it doesn’t really lend itself to portable operation (I know, I tried it).
I said in the original post that it was for SOTA. It will be supported by a 30ft pole in the center.
Yes, the UL-404 is a fine antenna. I used one last year for QRPTTF. That is definitely another option for me, although the SWR is a bit high on 20M and a tuner is required.
73, Barry N1EU
Well SOTA can be chasing or activating…
In any case you were asking for opinions so I would recommend the UL-404, I’ve found it a good compromise between performance and convenience. While a little heavier to carry than a linked dipole because of the balun, I think it would be lighter than a trapped wire antenna.
By the way did you mean to say SWR on 30m is a bit high and needs an ATU (not 20m).
It’ll be interesting to see the 80 metre and up version of that 40m and up antenna when it is released (hopefully later this year).
Rick published my review http://www.aerial-51.com/user-testimonials/n1eu-p-qrpttf/ of the UL-404 on their Web site. The review mentions the high SWR on 20M, and I’m sure if that wasn’t normal Rick would’ve commented to me.
But I started the thread actually to ask whether one or two traps per side should be used
73, Barry N1EU
I don’t get a high SWR on 20m, but did on 30m, when I tried it in Australia (where SSB is allowed on 30m), hence my query.
OK, noted your request was related only to how best to trap a dipole, so I’ve gone a bit off-topic (that never happens does it? H.I.).
P.S. here’s my review onRick’s site: http://www.aerial-51.com/user-testimonials/dd5lp-sota/
As was mentioned, I would probably go with the one trap and live with what you end up with on 15m - which may be increasingly less open as we head into the doldrums of the cycle.
Have a look at the endfed dipole antenna EF-MTR von LNR Presicion EndFedZ® Overview – LNR Precision Inc
You get the Manual hr: It is the last one at the end of the page and it works fine for 15M as well.
Manuals – LNR Precision Inc
It works on 40/20 and 30/15 if you remove the Jumper. This is the antenna, I have worked you sevaral times. I use it very often.
73 de HB9BIN, Jürg
I want to keep it center fed. I’ll probably just go with the one trap per side dipole unless the SWR is way too high on 15M.
Your “sturdy” 30ft pole would also be well-suited to carry a remote controlled T1, mounted as high as practical.
This solution along with a 2m length “feedline” up to the pole top (RG-316, no balun) and a 2x6.25m doublet works fine for me on 40-10m.
The remote bias tee fits just inside the T1 (this simple circuit works fine: http://www.jtmiller.com/T1%20tuner%20control.BMP).
Yes Heinz, that’s similar to a NorCal or 44ft doublet but slightly shorter. The NorCal is 6.7m per side. It’s one of my favorite antennas. I normally use balanced feedline with the tuner by the rig. That will be my second choice if the trapped doublet doesn’t work out to my liking.
73, Barry N1EU
A 40m dipole will work on 15m. Each side: 75/7.1 MHz = 10.56m, then 3/4 wavelength: 225/10.56= 21.3MHz.