"Bob" wrote in message
...
Hi all

With the approach of HF priviliges for Australian "limited" amateurs I
have
put up a few antennas for various HF bands.

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.

To get the 50 ohm line impedence I constructed a broadband RF balun on a
Neosid F25 ring. I should point out that I already have one of these
working on a single 20m quad loop, transforming the line to about 100
ohms.
The ring on the 40m antenna has 10 turns on the primary (about 200 ohms
inductive reactance) and 29 on the secondary. It has taps at turns 10, 14,
20, 25 and 29 for 100, 200, 300 amd 400 ohms respectively.

The modelling I did gave me a start point of about 18.8 metres. I guess
this
is because of ground proximity and 4mm thick wire. The problem is that the
thing loads up real good on 11.0MHz and has about 2.5:1 VSWR on 5.9MHz.
The
50 coax feed is an arbitary length of RG58 maybe 15 metres long. On 40m it
is unusable!

The question is, does the balun secondary contribute to the the antenna
resonance sufficiently to shift its operating frequency that much? I'll
admit I am a little in the dark here and any help/thoughts would be
appreciated. I am thinking that the off centre feed is seeing the
reactance
as a significant component of the resonant frequency.

Cheers

Bob VK2YQA

You might try some shunt capacity on the 50 Ohms side- I have seen this on
a broadband 10:1 I built.
I think my first choise however, would be a simple L matching network- very
predictable and easily built. Shunt C on the Hi Z side and a series inductor
to the 50 Ohm center conductor. Then follow it up with a current mode choke.
Coaxial cable makes a fine HV cap.

Dale W4OP