Topband: BC filter

[Rudy Severns]

For those who asked about ICE I gave them a call this morning.  Their web
page is not active so you will have to contact them directly:

ICE
PO Box 18495
Indianapolis, IN 46218-0495
1-800-423-2666
1-317-5455412

    The 402 is rated to have 20 dB of attenuation at 1.5 MHz and, using my
local villain at 1.4 MHz, I measured the attenuation at about 30 dB.  Enough
to  help.  For those with interference above 1.5 MHz however, the
attenuation drops quickly and the filter won't buy you much.

    It is certainly possible that with some arbitrary load the filter might
interfere with the SWR measurement at 1.83 MHz.  In my case the effect was
insignificant so I was happy. My antenna is about 800' from the shack on
another hilltop so it was much easier to run over with a MFJ or AEA analyzer
with the BC filter on it to tweak the match than to drag a power source,
transmitter and directional wattmeter.  Of course the wattmeter approach has
the advantage that it can be done at a level far above the interfering
signal so that you don't have to worry about it.

    I would not however, try to make impedance measurements through a
filter.  There again the old fashioned instruments like a GR 1606 antenna
bridge or the Boonton/HP 250 RX meter work quite well for direct
measurements since they use relatively large signals for the measurements
and can use frequency selective detectors (at least with the 1606).  Both of
these are occasionally available on ebay or at ham flea markets and a
serious top band antenna builder would do well to pick one up.

    More modern gear such as the MFJ and AEA units or one of the HP network
analyzers (HP 4815A for example) are quite vulnerable to BC signals  and
transients on large antennas.  W7JYJ has warned me about how easy it is to
fry the probe on the 4815.  The cost of fixing the probe is usually more
than what you pay for instrument!  I am very hesitant to connect  more
modern and expensive network analyzers to a large array.  I am paranoid
enough that I seldom connect my receiver to the array directly but always
put in 20 dB or of pad.  Signal levels are so high that the pad doesn't hurt
at all but gives some protection.  Normally I use separated receive antennas
which are also checked for pickup levels from the main antenna while
transmitting.  When I hook a spectrum analyzer to the antenna to see what's
going on I have to use 30 dB or more of pad to keep the overload light dark.

    The point of all this is to be aware that there are serious measurement
and protection issues related to the large antennas we use on 160 which
require some thought.

    73, Rudy N6LF