Not only is it a good idea to use a BC filter to protect the your preamp but
also use it when making antenna measurements.
I recently put up a two element array on 160. While checking the SWR
matching adjustment using an MFJ-249 the results were wildly off from
predictions. A quick check with an RF millivoltmeter showed 60 mV+ of local
broadcast station signal. The local station is neither very powerful or
very close. Putting in a BC filter in line with the MFJ calmed it right
down. I saw the same problem with more advanced instruments like the HP
4815A and the HP 3577A.
Almost any antenna on 160 will pick up the local stations and so I make
it standard practice now to use the BC filter.
73, Rudy N6LF
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