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