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Re: [TowerTalk] Low Pass Filters: Are They Of Any Use Today ?

Subject: Re: [TowerTalk] Low Pass Filters: Are They Of Any Use Today ?
From: Jim Lux <>
Date: Thu, 19 Apr 2012 06:26:17 -0700
List-post: <">>
On 4/17/12 4:38 PM, Jim Brown wrote:
> On 4/17/2012 2:50 PM, David Thompson wrote:
>> This topic is far a field from towers and antennas
>> except a low pass filter still controls fundamental overload of TV sets on
>> an antenna/digital converter.
> That is absolutely false.  A low pass filter ONLY suppresses harmonics
> above its cutoff frequency, usually 30 MHz for traditional products.
> ARRL's definition of "fundamental overload" is that the fundamental
> transmitted frequency is overloading the victim.  A low pass filter does
> NOTHING to help that.  The only thing a low pass filter does is kill any
> trash that a transmitter is putting out on the TV channel.
> Interference to TV receivers from HF transmitters is generally best
> cured with common mode chokes on antenna leads and other interconnecting
> cables.

Or buying a new TV?

This is similar to the problem with cosite interference, e.g. at Field day.

if the receiver has insufficient rejection of the fundamental of the 
interfering signal, no amount of transmitter filtering will help it. 
You need to suppress the signal at the input of the receiver, either by 
using a bandpass filter that is tailored to the receive band, or by 
somehow nulling/removing the transmit signal.

There's a big business in adaptive interference cancellers for systems 
where you can't just narrow the filter on the receiver (e.g. cosite 
frequency hopping radios in the same band)


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