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


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(The reply address is broken. Put bcnoop in front of the at!)

"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

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

An incorrect model was fed into the program. The feedline actually radiates
like the clappers and the whole thing behaves like an inverted-L with a
small 3 metres spur which does very little.

If the severe off-centre feedpoint MUST be used, probably the best way to
feed the antenna is via an open-wire line and just accept whatever impedance
is presented to the tuner, via a good choke, at the transmitter end.

As the antenna will have the crude general characteristics of a long, random
length, inverted-L, why not go the whole way - erect an inverted-L and feed
it against ground? Easily modelled and tuned. It also becomes multi-band.
You won't need a particularly good ground connection.
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Reg, G4FGQ