How they work

RF Filters
How they work
In Use






Here is a typical characteristic of my 2 metre bandpass filter.

The centre frequency of the graph is 145 MHz and each square horizontally represents 10 MHz. Therefore, the left hand side of the graph is 95MHz and the right hand side is 195 MHz.

The vertical scale is 10dB per square, we start at the top of the graph at 0dB ( ie no loss of signal) to the bottom of the graph at 80dB.

You can see therefore that at 145MHz the loss of signal through the filter is very low, about -1dB, but at 95MHz we have a loss of 45dB and at 195MHz a loss of 80dB. 

By installing this filter to your 2 meter receiver or transceiver, all the signals on the 2 metre band are received as normal, but signals out of band are attenuated as per the graph

Received spectrum without filter

Received spectrum with filter

Above is a copy of the spectrum as received at my location; the centre frequency is set to 145.700 MHz (my local repeater GB3AR). Notice that the signal received at 145.700 MHz is as strong in both cases, however even the signals at approximately 10MHz both sides of the centre frequency are attenuated, with the signals in the region of 100MHz (FM Broadcast) are very low, as are the signals at 155Mhz and above. 

The strong signals at approximately 135 MHz and 153 MHz have been attenuated by approximately 20dB and 35dB respectively. These strong signals, being close to the 2 meter band, can cause interference.

 For reference, if 1 watt of power is reduced by 20dB the result is 10 millwatts (0.01W).

                                                             or by 35dB the result is 0.316 millwatt (0.000316W).

 On transmit; any spurious emissions outside the pass band would also be reduced by the above amounts, shown in the graph, thus keeping the transmitted signal clean.

For Amateur TV working, the pass band of the 23 cms receive filter should give sufficient attenuation of your transmitted signal, so that the received signal from the repeater can be seen whilst you are transmitting. (A look through filter) Look at ATV Filter page.

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