On Sat, 26 Jul 1997, Charlie Summers wrote:
> Hi again!
>
> Well, the problem turned out to be a commercial broadcast station 6
> miles away. This 50,000 watt station overloaded my Autek RF analyzer
> and caused a faulty reading.
(snip)
Living within a few wavelengths of a handful of AM BC stations (good
thing I have few fillings...) I have observed similar problems with the
MFJ-259. I use the following techniques to keep the RF out:
1) Use a BC filter at the analyzer output - I have a Kiwa BC-1 that works
fine at HF. There are several BC-reject filters on the market that will
also work. Put a 50-ohm load at the filter output and sweep the analyzer
through its range to find out what effects the filter may have on true
impedance measurements. Avoid using the filter in ranges where the
analyzer shows significant deviations from 1:1. Be aware that the
BC-filter will not act like it's supposed to if terminated with impedances
far from 50-ohms.
2) Use a band-pass filter for mono-band antennas. Same caveats as for #1.
3) If the problem is RF on the coax shield (also a problem here), try
using the analyzer *inside* the tower. The tower will act as enough of a
shield (keep the coax in there, too) to limit the amount of BC RF that
gets onto the outside of the coax.
4) Always double-check your analyzer readings with a Bird wattmeter or
transceiver SWR bridge using forward & reflected power to calculate VSWR.
Use at least 25W to make the measurements. If there is significant
disagreement between the analyzer and power meter VSWR numbers, you have a
problem. I would favor the power meter readings over the analyzer
readings. I have several antennas that can not be checked with the
analyzer due to excessive BC RF.
5) Keep notes of what worked and what didn't so you don't have to waste
time repeating the process on your next project.
73, Ward N0AX
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