200W FM Low Pass Filter with DC

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Ryan Toomey
Mar 3, 2019
This is a nice, low cost low-pass filter that works great. My signal is VERY clean with this hooked up. No detectable harmonics, spurs, or other interference. I absolutely love the fact that you can hook a meter up to it so your power can be monitored easily on the fly. Accuracy seems good enough (please read below).

I have this mounted in a metal case on standoffs. RF Connections are made with RG316 coax to SO-239 connectors. I have the directional coupler hooked to a 150-watt panel meter that I found on rfparts.com

This was relatively easy for me to setup.

Advantages: Low cost, does the job, integrated directional coupler for easy power monitoring. Does NOT heat up with continuous power (I ran 150 watts through this for hours and coils were only barely warm).

Disadvantages: Insertion loss seems a little high. I measured the input power to the filter at 150 watts; However, the measured output power after the filter is only 125 watts. That is almost 1db of insertion loss!
Also, this filter also increases SWR slightly. Without the filter, my SWR is 1.2:1, with the filter in-line my SWR goes up to 1.4:1, so it is not exactly as transparent as I would like. Even with a 50-ohm dummy load hooked up, the SWR is still 1.3:1

Third, the directional coupler is not linear and not exactly consistent - I compared it with a calibrated Telewave 44A and this reads low at the low end and high at the high end. For example, at 50 watts, this shows about 40 watts, and at 100 watts, this shows about 110 watts. Also, when calibrating the directional coupler, I couldn't get the same reading every single time. It seems to vary slightly here and there.

The directional coupler seems good enough to provide you a ballpark reading and know that your transmitter is still working.
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