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?
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
