Topband: Nested Loops with one feedline results

[Tom W8JI]

> The 18/24 is not a good analogy, because those bands
> are not octave spaced.  My modelling indicated that
> you need to open up the lower frequency antenna at
> two places, 180 degrees opposed, in order for it not
> to affect the higher frequency antenna.  Also, the
> openings should be such that the remaining wires
> are cross polarized to the higher frequency antenna.

Rick and all,

There is often some combination of open feedline length and loop 
circumference that tunes other bands. The unused feedline isn't an open or 
short unless it is a certain length. Depending on length and termination, 
the unused feeder has a range of complex impedances that can tune the unused 
antenna to any band that is either resonably close or higher in frequency.

There are many more length ranges that do not tune it to a band, so it more 
often than not can work, but working is a statistical result more than any 
sort of open or short.

For example, a 14 MHz delta has a feed impedance of about 120 -j1100 on 10.2 
MHZ. If the unused feeder length presents around +j1000, the loop will be 
tuned around 30 meters even though it is not resonant by itself.

The same loop is around +j1200 on 17 meters, so it could be tuned there also 
with an unlucky unused feeder length. Of course the issue is worse on bands 
where the unused loop is a sub-harmonic of the operating band, like a 14 MHz 
loop near an antenna operated on 28 MHz, but the problem can actually move 
almost anywhere depending on luck.

The more loops that are randomly nested with random feeders, the greater the 
odds some bands will "mess up".

It is a myth to think loops will not interact, or that some random unplanned 
thing that works for one guy will always work for the rest of the world. 
They are really no different than any other antenna.

73 Tom