A BCI Filter for Your Antenna Analyzer

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I was thrilled with my new MFJ-259B Antenna Analyzer, but the 50kw AM broadcast station a mile away made it impossible to use my new toy.  I wound up designing and building an AM Broadcast Sucker-outer.

The undesired AM broadcast signal is shunted to ground with a series resonant circuit consisting of  L1 and C1, C2, and C3.  At the HF frequencies, this series circuit will look like an inductor from Input to ground, so I put a variable capacitor (various combinations of C4 and C5) across the series resonant circuit to create a parallel resonant circuit at the operating frequency of the antenna being designed.

The entire circuit is built in a large outlet box, but I'd recommend using an aluminum enclosure from Radio Shack because it's much easier to drill holes in.  L1 is about 50 microhenries (not critical), which is 60 turns of #30 wire on a piece of PVC tubing that's 1 inch OD, by 2 inches long.  C1 and C5 are 15 - 365pf air variables.  C2 and C3 can be combined for a total of 800pf.  C4 is switched in to cover 15m and 10m.  Layout is not critical, but short lead lengths are always a good idea.  This was a prototype that worked, so I never bothered to clean up the design.  I think replacing S1 with a pair of toggle switches might be a good improvement.

Connect the tunable filter Input to your Antenna Analyzer, and the Output of the filter to the antenna.  Tune the analyzer to the operating frequency of the antenna, and adjust C1 for a minimum "SWR" reading.  In doing this, you are nulling the AM broadcast signal.  Now put a 50 ohm load on the Output connector of the filter, and adjust C5 for 50 ohms with zero reactance.  You are now ready to make antenna measurements without that pesky AM Broadcast signal driving your Antenna Analyzer crazy.

If you have any questions or suggestions for improvement of this circuit, please let me know.  This was a project that wasn't on the list when I wanted to tune my 40m Inverted Vee, and I didn't stop to make improvements.

Ed - k9ew