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You Need A Cavity Filter When Site Sharing

You need a cavity filter if you are operating on a shared site and your transmitting antennas are closer than 200 metres from another users transmitting antennas, usually the case if it’s the same tower or even if it’s a different tower, but located on the same site of land.

What happens here is that your transmitting antenna also acts as a receiving antenna and receives the signal from the other user. Lets say the other users signal is 92 MHz and this enters your transmitting antenna, it comes down your antenna cable and enters the output stage of your transmitter. Lets say your signal is 105 MHz. The others users signal of 92 MHz mixes with your signal of 105 MHz in your output stage and creates two new signals that are 105+(105-92) = 118 MHz and 92-(105-92) = 79 MHz. If your antennas are closer than 200 metres to each other then the new 118 MHz and 79 MHz signals will probably be higher than -60dB on your carrier and will go back up to your antenna and be re-radiated. The signal of 79 MHz radiating at -60dB or higher on your carrier will almost certainly cause some interference to other users of that frequency. The signal of 118 MHz is even worse, really BAD, this is right bang in the middle of the aircraft band and at higher than -60dB on your carrier will interfere with control towers for landing and take off of planes, basically REALLY DANGEROUS. The cavity filter in this case if it is tuned to 105 MHz, your frequency, prevents the other users frequency of 92 MHz from passing through to the output stage of your transmitter and therefore there’s no mixing and no new frequencies are generated. The transmitter output stays clean.

This phenomenon is called inter-modding or intermodulation products.

So again, if you are operating on a shared site and you don’t want a crashed airplane on your conscience, YOU NEED a cavity filter.

A Cavity Filter Is A Good Idea With An Old FM Transmitter

If you have an old transmitter, pre mid 90s that does not have a CE Mark or FCC Certification / Verification. Also an old transmitter may have misaligned filters, dried out capacitors etc. any number of problems in the output tuning or modulation section, or simply built to a lower spec due to the regulations being less stringent all these years ago. A cavity filter will clean up the output and in most cases (not all, but most) will bring it back to EU and FCC specifications.

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This is a double coaxial cavity filter very with high Q quarter wave lines made of copper and silver plated with PTFE insulators. Normally this is installed between the transmitter and the antenna. The variable tuning and coupling of these quarter wave lines makes it possible to wrap this filter tightly around the passband of the 200 KHz wide FM signal. This in turn has the two following benefits:

It eliminates or at least attenuates all spurious outside the passband.
It prevents strong signals from other users transmitters, usually on shared sites with close by antennas, from entering your transmitter, mixing inside and creating new spurious signals

The filter is built solid from thick aluminium and is 3U 19 inch rack mounted. Temperature compensated versions are available on request.

Technical Features

Ideal for use with Aareff Transmitters

Insertion Loss
0.4 – 0.9 dB

200 Watt Max.

Frequency Range
87.5 – 108 MHz

(-3db) 200 – 300 KHz

50 Ohm

10 – 40°

less than 1.15 : 1