TopBand: high pass filters

Thu, 4 Mar 1999 16:40:37 EST

I have been having trouble with a local broadcast
station on 1550khz. they run 50kw and pretty much
desens my receiver on 160 meters when they come
on the air. only 6 miles away.

Some friends suggested that I use the MFJ 731
filter which is made to keep broadcast interference
out of their analyzers. I borrowed one from a friend
and hooked it up with no apparent change.

Curious, I took it to work and put it on my IFR COM129B.
these are the results:
greatest rejection is 17db at 300khz, the low frequency
limit of the filter.
the variable notch filter has 9db attenuation at the maximum
insertion loss is less than 2db.
filter rejection at the 1550khz bdcst station is 9db max. (the
high pass filter shape factor is such that the actual filter has
no effect and the notch filter shows the most rejection at its
Max notch).

Because this filter was of NO practical use I built the high-
pass filter as shown in the radio amateurs handbook. with
a few changes to the coil windings the following was achieved:
insertion loss is less than 2db.
filter rejection at 300khz is 78db.
filter rejection at 1550khz is 29db.

Now- no more problems. Easy and not expensive

Larry, n7dd

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