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Old 17th Feb 2020, 2:18 pm   #6
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Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 6,751
Default Re: Aerial blocking circuit for TRFs anybody any ideas

What you need is what is known as a 'rejector circuit' in the aerial down-lead, fitted close to the radio. That cct. is nothing more than a coil in parallel with a capacitor - ideally a variable one, initially, until the precise value can be found by experiment and consequent measurement and a fixed vale of capacitor fitted instead. Having said that, if the variable is left in place, no harm will be done.

The resonant freq. of the L/C cct. should equal the freq. you wish to reject. Such a parallel tuned cct. presents a high impedance to that freq. It will be desirable to aim at a fairly high Q, so the L/C ratio needs to be at least 100. Use an air-spaced coil with a large diameter to length ratio. A coil diameter of about 3" would be a good start.
If you have access to a Q-meter, then the whole job will be a lot easier.

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