Re: RF Clipper Diode Design

Tom Crosbie G6PZZ

A long time ago when I was first licenced, one of the facts of life I learned was that one day, I might interfere with someone's TV reception, very possibly my own. We learned how to make a very simple notch filter using nothing more than a T-piece and a length of coax approx. quarter of a wavelength long. At 88MHz, this would be approx. 0.9m long. If we consider the T configuration, the top bar of the T represents the feedline between antenna and receiver, the upright bar is our 0.9m of coax, or stub. These were often referred to as stub filters. The receiver is tuned to the offending frequency and the stub is then trimmed gradually 5mm at a time until there is a reduction in signal strength. You'll know when you've cut off too much when the signal strength goes back up!

Given Michael's extreme challenge, it might be worth trying as it only costs a few connectors and a length of coax. If this works on one of the frequencies, it will work on the other, make another stub and insert it in the feedline in series with the other. At the other end of the band, the quarter wavelength is just 0.75m so there isn't a huge difference across the whole of Band II. Start a little longer with the stubs than theory dictates, perhaps 1m. As to the theory behind this, the stub will be resonant at a frequency relative to its length. This will have the effect of filtering out some of the signal. There are more learned people around here that could explain this more eloquently and mathematically than I can!

Martin's solution is a great option and is what the professionals would use. But it's quite expensive, even buying surplus. I recall similar home brew projects literally featuring beer barrels. I offer this from a dim and distant memory in the hope that a simple, low-cost option just might help.


-----Original Message-----
From: <> On Behalf Of Michael.2E0IHW via Groups.Io
Sent: 03 February 2020 14:11
Subject: Re: [SDR-Radio] RF Clipper Diode Design

That was my concern, Martin.

Even though challenging for VHF diy, further details of the theory and practice would be appreciated.

Michael 2E0IHW

On 03/02/2020 13:38, Martin via Groups.Io wrote:

My understanding of the problem is that he is trying to receive weak
FM Band II broadcasts signals in the presence of strong local transmissions.
The filters that we used were often a 1/4 wave loaded line or helical,
enclosed within a metal tube about the size of a domestic fire
extinguisher, and each one was tuned to a specific transmission
frequency, so unfortunately it's not really a practical solution for
home use.
Martin - G8JNJ

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