On Thu, 2 Dec 2010 12:24:04 +0000 (UTC), "Geoffrey S. Mendelson"
wrote:

Hi Geoff,

This is a pretty busy agenda:

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.

This should have been foreboding (resonance so near by?).

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.

These signal reports sound more like issues of propagation than issues
of the antenna. In other words: normal.

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.

You say instructions, but instructions for what (to what end, to what
purpose)? Why trifilar? This is suggestive of a voltage BalUn, which
is not going to give you the choking action you need to reduce noise
problems.

Keep your BalUn as is (I don't think it is going to materially improve
anything) and add a W2DU style to its input.

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.

This still sounds like the vagaries of propagation. Do you keep logs?

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

Your lead out to the antenna could have a short/open in it. I presume
you use your tuner, however of late I have been given the impression
that SWLers of this generation sneer at it in favor of the magic
BalUns they chase on the Internet (given you are a frequent poster
here, I give you the benefit of the doubt).

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?

The electrical properties are unrelated to the pattern properties. We
speak of nulls in a pattern.

If you are referencing a null in tuning, then that would occur at
either resonance, or anti-resonance (broadly speaking). If this is
your intent, then, yes, "nulls" would represent impedance matching
issues which in turn would put a block to signals. A tuner is a
design tool for this situation.

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?

Uhhhh. We need to decouple "null" somehow, from the discussion, or
give it a more descriptive treatment.

You describe a problem that is binary:
I used to have signals
I don't have signals
what you describe in fumbling with turns ratios is a continuum:
I used to have booming signals
I have weak signals

BalUns are not used to offset/fill/sharpen nulls of any type.

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?

Solve your problems one at a time.

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?

Magnetic performance is impacted by air gaps. In some situation this
is an asset where saturation is an issue. Super glue the pieces for
the closest fit.

73's
Richard Clark, KB7QHC