I made a 300 ohm twinlead folded dipole that is resonant at around 15mHz.
It was cut because I wanted to try some 20m operation with an indoor antenna
and that fit exactly the space.

The ham I wanted to reach was km away, across a valley. I figured with a
tuner and a few watts, I could reach him.

I never used it for that, he moved before we ever got together.

I put the antenna up outside to use for an SWL antenna. It is fed with
about 6 feet of twinlead and then 75 ohm coax. It worked best at 15mHz,
and slowly degraded as frequencies got lower. It was useable at 7mHz for
reception.

At MW frequencies it's basicly a noise pickup and no signals could be
received.

I made a balun from 9 trifilar turns of wire that I had (.5mm plastic
insulated). The instructions I was following said to use 10 turns of
#14 enamel on a wider core, but 9 were all that fit on the core I had.
The core is 8mmx80mm with a permeability of 400.

When I hooked it up to the antenna, I found that the resonant frequency
shifted to 14mHz. My guess is that the feedline now is part of the antenna.
However at 7Mhz it is totally dead. So dead that a signal that reads S5 on
my receiver using a 2m JPole hung from the near end of the folded dipole
as an antenna does not move the s-meter at all. The null extends from
around 6950 kHz up to around 7300.

The question is why and what can I do to improve it?

Is it because there is null in the system because it is resonant at 14mHz and
it is unresonant at 1/2 of that? Is it because the balun acts as a 7mHz trap?

I now have some cores that are 8x140 and 10x200. If I make a balun using the
same wire (enamled wire is not available to me) with a different number of
turns, will it behave differently? For example, if I make a balun with a few
more turns, will the null move down?

My guestimate is that I can get 15 turns on a 140mm rod, and 22 slightly larger
ones on a 200mm rod.

How far down can I go? Will moving it down affect upper frequency response?
Can I move it down far enough that the balun will do something useful
at AM broadcast frequencies?

Some of the 10mm rods were broken into pieces in shipping. Does that affect
their performance? Can I just use the pieces as they are?

Thanks in advance,

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Those who cannot remember the past are condemned to misquote it.