On Wed, 24 Dec 2003 18:27:35 +1100, Bob
wrote:
I am having trouble getting a 40 metre dipole going. It is a little unusual
in that due to distance and cable constraints I am feeding it 3 metres from
one end. I have modelled this on Mininec and it says 200-300 ohms.

Hi Bob,

There are any number of problems here. Being fed 3 meters from the
end of 20 meters of wire is a long shot in calling it a dipole. This
has nothing to do with its performance nor the Z that you computed,
but it, and the method of BalUn construction suggests you have every
chance of suffering big time from unbalanced transmission line
currents.

What you describe as a BalUn is in fact a simple Z transformer. The
two are not always the same thing in operation. For one, there is no
sense of the Bal of the BalUn. That is to say, you have no choking
action in your transformer to offer the balance suggested, and the
offset dipole feed drives that necessity with a vengeance. Try
feeding your Z transformer with a 1:1 Current BalUn (buy one or find a
competent construction article, or research google for postings to
this matter).

Another issue is found in the difference between simple Z transformer
operation and that of true BalUns. BalUns work over a considerably
greater span of frequencies. The problems you describe in large
frequency changes match the performance issues of simple Z
transformers. These are tuning issues that BalUns survive through
having less turns for the same Z transformation.

73's
Richard Clark, KB7QHC