30m/15m Fan-Dipole?

[ckmcdonald]

I need an antenna for 30m and 15m and I have an attic just itching to have a little copper in it. A simple dipole would work fine for my purposes.

If my thinking is good (probably a bad assumption) it wouldn't be a good idea to hang both a 30m and 15m dipole up there as separate antennas (separate feed lines) - wouldn't that create issues with RF feeding back into the shack on the antenna not in use? They would have to be parallel to each other and very close together (2' - 3').

Ideally, for several reasons, I'd prefer both bands and a single feed line. So I've been looking for a 30m/15m OCF or a Fan dipole. Looking around the net I'm not finding any evidence that such antennas exists.

I think I'd prefer a Fan Dipole. Can someone give me some pointers on this. Is it possible? Purchasable? Design guidelines?

Thanks
N7AY

 

[wyShack]

Just run the two with a 'common' feed point and run your coax from there to your shack. Trim the 15 meter antenna first, then the 30 meter. I would just 'hang' the 15 meter under the 30 meter by a couple of inches. As the antennas are 'tuned', each one will 'reject' signals not in band. If you are using a tuner, both elements may radiate.

73

 

[prcguy]

I agree a fan dipole sharing a common feedpoint would be easy to make and its effective. I would keep the ends of the antenna as far apart as possible for best performance and least interaction.

If you make the longer dipole first and tune it, the addition of a shorter dipole will detune the first slightly causing it to be shorter. You would tweak the second one close to your freq, then tweak the longer one and walk them both in together by checking each one after tuning.

Another possibility is a trap dipole. If you can find or make a pair of 15m traps you would have one dipole that would be slightly shorter than a full size 30m dipole due to the loading effect of the traps.

You could also go the resonant end fed half wave route using a 64:1 transformer and a single 15m trap. You would first tune a half wavelength of wire to resonant slightly lower than 15m, install the trap, then add and tune a wire to resonate slightly lower than 30m. Then fine tune the 15m length to resonant on your freq, then the final 30m freq. The overall length of this antenna will be slightly shorter than a full size 30m dipole but the advantage is you feed it on one end and not the center. This may be beneficial compared to a center fed where the coax exiting the antenna may have to run parallel with it and disturb the pattern.

I have several resonant half wave end feds with traps, a few that are 80 through 10m using a 40m trap to isolate the 80m section, which allows tuning it anywhere I want in the 75/80m band instead of living with it resonating around 3.6MHz like most 80m versions. I also have a 20/17/10m version using a 17m trap thats about 27ft long.
prcguy