I have been trying to understand the characteristics and anomalies in a short
loaded dipole mounted in a permanent location for some time. The antenna is a
'large' screwdriver design, capable of over a KW. Models predict a low feed
resistance for this antenna, yet shack SWR readings are better then 1.1. A feed
resistance of 20 Ohms or less should not result in that low of an SWR.

In pursuing this I purchased, repaired, and calibrated an HP 8405a Vector
Voltmeter. This is an amazing instrument. It takes a carefully setup, however it
is very sensitive. By tuning the antenna the 8405a showed two zero phase nulls
very close together at 3970 KHz. One has a low SWR, the other has an SWR over 3.

The next step is to actually measure the antenna input resistance at this second
higher resonance point. I then plan to wind a custom balun to match the
antenna feedpoint to 50 Ohms. This should improve the performance by close to
the balun ratio.

As an observation this is an example where an antenna being operated with low
SWR may not optimum. I will also figure out what is happening to cause the low
SWR at the first resonance point. That is still a mystery.

Dan - kb0qil

On Wed, 21 Dec 2005 07:20:24 -0800, dansawyeror
wrote:

Models predict a low feed
resistance for this antenna, yet shack SWR readings are better then 1.1. A feed
resistance of 20 Ohms or less should not result in that low of an SWR.

Hi Dan,

Did you include the 20 - 40 Ohms of Earth/Ohmic Loss in your Model?

73's
Richard Clark, KB7QHC

Oh yes. The antenna is 5 to 7 and ground is 15.

Richard Clark wrote:
On Wed, 21 Dec 2005 07:20:24 -0800, dansawyeror
wrote:


Models predict a low feed
resistance for this antenna, yet shack SWR readings are better then 1.1. A feed
resistance of 20 Ohms or less should not result in that low of an SWR.


Hi Dan,

Did you include the 20 - 40 Ohms of Earth/Ohmic Loss in your Model?

73's
Richard Clark, KB7QHC

On Wed, 21 Dec 2005 17:14:37 -0800, dansawyeror
wrote:

Oh yes. The antenna is 5 to 7 and ground is 15.

Richard Clark wrote:
Did you include the 20 - 40 Ohms of Earth/Ohmic Loss in your Model?

Hi Dan,

Then it appears you are not measuring the radiation resistance + loss,
but rather its transform through the tuning of the
'large' screwdriver design

You might want to test your HP8405 against a 50Ohm load at the end of
the drive line, against known mismatched loads, and some measure of
the Q at each these nulls.

Do you choke the measurement drive line?

73's
Richard Clark, KB7QHC

On Wed, 21 Dec 2005 17:14:37 -0800, dansawyeror
wrote:

Oh yes. The antenna is 5 to 7 and ground is 15.

Wait a minute. You said this was a "dipole", right?

If your model is operated over, but not referenced to, ground, then
ground effects are built in.

If there are sliding contacts in this antenna (I assume there are) I
wouldn't trust any rf reading until I used a good ohmmeter first.

Yes, I am measuring at the end of the cable. The 8405a is configured in a
balanced setup between a reference arm and an unknown arm. When the unknown arm,
i.e. the antenna is terminated in 50 Ohms the line length does not matter. That
was a surprise to me. The 8405a then reads the unknown at the end of the cable.

Richard Clark wrote:
On Wed, 21 Dec 2005 17:14:37 -0800, dansawyeror
wrote:


Oh yes. The antenna is 5 to 7 and ground is 15.

Richard Clark wrote:

Did you include the 20 - 40 Ohms of Earth/Ohmic Loss in your Model?


Hi Dan,

Then it appears you are not measuring the radiation resistance + loss,
but rather its transform through the tuning of the

'large' screwdriver design


You might want to test your HP8405 against a 50Ohm load at the end of
the drive line, against known mismatched loads, and some measure of
the Q at each these nulls.

Do you choke the measurement drive line?

73's
Richard Clark, KB7QHC