Topband: Antenna analysers in close proximity to BC station.

[WO0W]

Hi, Tom;

Perhaps you can take a portable transmitter and manual tuner to the site.  At the point where you wish to place an L network, attach the manual tuner and transmitter and adjust the tuner for a good match.  Take the tuner away from the source of interference, put a 50 Ohm dummy load on the tuner input, and measure the impedance on the tuner output with the MFJ-259B.  I believe that should be close to the value of the impedance of the antenna that was measured, with little or no affect from the strong broadcast signal.

In event the application you are using doesn't report the voltages on the L network components, the application, Transmission Lines for Windows, found on the CD with the late versions of the Antenna Book will present values for the L components and the voltage applied to each with a given input power.

73 de WOØW

From: "Tom Boucher"<tom at telemetry.demon.co.uk>
To: "160 reflector"<topband at contesting.com>
Subject: Topband: Antenna analysers in close proximity to BC station.


A ham friend asked me to design a matching network for his 160 metre end fed quarter wave, so I asked him to provide an impedance reading using his MFJ-259B. I would then use the Berkley site (http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matching/ImpMatch.html  ) to provide the necessary values for an 'L' network, as I have done many times at my own station.

  

The readings he provided were total nonsense and quite erratic, so we concluded his MFJ-259B was dead. He assured me that he always does a static discharge before connecting the MFJ.

  

So I paid him a visit, taking along my Palstar Antenna analyser thing, which has always performed well at home, and what-do-you-know, the readings on that were also erratic, total nonsense and it behaved in a way I have never seen before.

  

Than someone suggested the problem may be due to a 50Kw BC station on 909 KHz, situated less than 5 miles away, causing both antenna analysers to misbehave.

  

We ended up with a good old-fashioned link coupled parallel tuned circuit with the antenna tapped a few turns up from the ground end. This works fine but he is power limited due to arcing across the tuning capacitor. So we would ideally like to revert to the 'L' network plan, but how to use the antenna analyser in the presence of a high BC station field. Anyone any ideas?

  

73

Tom G3OLB