The AM Forum

It’s almost Spring so it’s time to start experimenting with some new wire antennas. I’ve decided to build a 80 / 40 trapped dipole. While it’s a very fundamental antenna, I have some questions that I want to better understand.

For the design process, start off by creating a 40M dipole. Next, build two traps, probably coaxial, that resonate @~ 6.9-7 mhz (just below the intended band).

Logically, the length of the wire beyond the traps should be the length required to form an 80m dipole. However, from what I’ve read and observed, the traps seem to shorten the overall length of the antenna.

The question I have, how do I calculate that length?

I ran a trap dipole out of the ARRL '65 handbook (iirc) as a novice.

You don't calculate much, beyond what the handbook says... I'd cut long and trim back myself.

Not sure why you want a trap for 40/80 when you can hang the 40 and the 80 on the same feedline
just fine. Did that for many many years. Would still be doing it, IF I had a 40m ant up! (It's on "the list").

Yes, the multi-leg dipole is the better choice.


But if you want to go the trap dipole route:

On 80M the trap's coil inductance will make the antenna shorter. It's just a loaded dipole at that point.  I would model it with NEC 4 as a simple inductor to get everything in the ballpark on 80M first.  Maybe NEC will model the 40M traps, I dunno.

Otherwise, the easiest way would be to just put it up a little longer than estimated and carefully trim the inner and outer legs until the swr is optimum for your use. You will need an electrical 1/2 wave length of coax (or multiple) to get an accurate reading at the transmitter end.  This is about 84' on 3.870 Mhz.  This length is close enough to then check the 7.250 swr accurately too, at one electrical wavelength of coax on 40M.

Remember that a random length of coax will not give an accurate swr reading unless the swr is 1:1.   The coax length formula is 492 / 3.870 * .66 coax velocity factor = 84'.  After you get the swr into 1:1 area, then you can make the coax any length you need to reach the shack.

This procedure also applies to multi-leg multiband dipoles fed with coax.

T

Thanks. The multiple dipoles on one feed is better, but I have a somewhat space situation. Virtually no trees, just the 50 ft tower, so it's going to be more or less an inverted V.

   Do research on the W3DZZ antenna.  I made one using the 40 meter Reyco traps and the wire beyond the traps was 22 feet each side.  Did not have to trim as swr was decent on 80, 40 & 15.

   One trick to lessen strain on those traps.  Install an insulator in place of the traps, then connect (solder) the traps across the insulator.

   Have fun,

   Ed  WA4NJY
   Bradenton, Fl

   

In regard to the traps, I recall reading something by W8JI, who is a pretty good engineer and not prone to giving in to old wives tales, saying that traps using capacitors made from coaxial cable are lossy and low-Q.  Perhaps you can dig it out on his rather extensive site at  w8ji.com  .  The Centralab 850 series capacitors were often specified in the good old days, as well as the CRL line, but they aren't cheap.  An ebay search on "doorknob capacitors" will scare you.
But don't sweat the small stuff if it turns out to be too much agony.  The imperfect antenna that's up in the air is about 49 db better than the one that never got built.

I searching, I found an interesting idea for fabricating the capacitor. Use a short piece of PVC pipe and apply a layer of the aluminum tape used on heating ducts to the inner and outer surface of the pipe. You can start out with a 3" length to determine how much capacitance it produces and create a "pf/in." spec. The unknown is how much voltage can it withstand. The ends of this cap were sealed by dipping it in wax.

The 11 meter amp builders use brass hobby tubing.  Find it at the 'boutique' craft stores.  They have it that is just slightly larger than an interference fit.  Has just enough slip between the sizes that they can then use a Teflon sheet between them.

Makes a great high voltage / high current capacitor. 

I used the same type, although flat brass sheets, with flat Teflon sandwiched before.

--Shane
KD6VXI