Resonance Filters

Hello. I am doing some work for a small non-comm FM station. At this time they have a LOT of refelcted power (I cannot get the transmitter to push more than 140 watts, without getting about 25 back, and the transmitter dumping). I have had the line going to the antenna replaced, the antenna checked and retuned, all to no avail. We are replacing the transmitter very soon, but the existing transmitter / exciter has been serviced and are said to be performing within normal limits. The station has two resonance fitlers (I do not have the name/model at the current second, but will soon) Besides the Bird Wattmeter, there is nothing else installed on the line. I am thinking it has to be the filters. My question(s): Can a resonance filter cause this reflected power? If so, how do we re-tune these?
Thanks for your help in advance!

I'm sure that you meant to say BandPass Cavity Filter, rather than resonance filter.
Not uncommon on modern solid state rigs. They very easily could be causing
your problem, if mis-tuned or damaged. You should get someone familiar with a tracking generator, such as
a good 2-way radio shop, or a Ham Radio buddy that may have one (lots of the repeater gurus do). Hopefully they have N style connectors at that power level, as that will make it easier to hook up to a trackeing generator. They need to be checked for problems & excessive loss, as well as the tuned center frequency. Standard 2-way radio filters are too narrow to pass the full bandwidth of an FM broadcast station, so hopefully what you have is designed for your purpose. The broadcast FM filters will have a wide enough passband to allow for full bandwidth deviation, whereas a standard 2-way radio filter will not. I've seen guys get bit by that.

Hope that helps,

Dave in Sacramento...


> Hello. I am doing some work for a small non-comm FM station.
> At this time they have a LOT of refelcted power (I cannot
> get the transmitter to push more than 140 watts, without
> getting about 25 back, and the transmitter dumping). I have
> had the line going to the antenna replaced, the antenna
> checked and retuned, all to no avail. We are replacing the
> transmitter very soon, but the existing transmitter /
> exciter has been serviced and are said to be performing
> within normal limits. The station has two resonance fitlers
> (I do not have the name/model at the current second, but
> will soon) Besides the Bird Wattmeter, there is nothing else
> installed on the line. I am thinking it has to be the
> filters. My question(s): Can a resonance filter cause this
> reflected power? If so, how do we re-tune these?
> Thanks for your help in advance!
>

I am kind of curious why you would have the cavities (filters) on the output of the transmitter in the first place.

Do you share a site with other FM stations?

What kind of transmitter is it?

Most modern transmitters at that low of a power have a low pass filter built into the box, (not external) That's more common with higher power (5,000 and above) transmitters.

> Hello. I am doing some work for a small non-comm FM station.
> At this time they have a LOT of refelcted power (I cannot
> get the transmitter to push more than 140 watts, without
> getting about 25 back, and the transmitter dumping). I have
> had the line going to the antenna replaced, the antenna
> checked and retuned, all to no avail. We are replacing the
> transmitter very soon, but the existing transmitter /
> exciter has been serviced and are said to be performing
> within normal limits. The station has two resonance fitlers
> (I do not have the name/model at the current second, but
> will soon) Besides the Bird Wattmeter, there is nothing else
> installed on the line. I am thinking it has to be the
> filters. My question(s): Can a resonance filter cause this
> reflected power? If so, how do we re-tune these?
> Thanks for your help in advance!
>


Where in the circuit is the wattmeter? Does the Bird show the reflected power or does the transmitter?

You should have the box setup this way;



Transmitter ---> filter ---> Bird wattmeter ---> feedline ---> antenna



Try taking the filters out of circuit and see what it does. Try putting the exciter into the antenna directly and look at the reflected on the exciter. Most exciters have some type of wattmeter built in. Just becareful because the values you posted is a VSWR of 2.46. This will dump any transmitter. Also, is the line foam or air filled?


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