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Filters in ATU

C

CaptBob92

I've got a question that maybe someone can help me with. I recently began working with an AM station that is located in the vacinity ( within 2 miles ) of two other AM stations. My station is a nondirectional. I recently found that one of the other station's signals is getting into my transmitter tank circut and coming out on a different freq.

In checking the ATU I found the normal tee network with transmission line attached on one end and on the other what appears to be two filters one a coil and cap in series and the other a coil and variable cap in series the filters are parralelled with each other with the antenna on the output. I'm thinking one of these passes the frequency my station operates on and the other blocks one of the two other stations frequencies.

But here's the ringer,,,,also in the ATU, but no longer connected, is what appears to be another filter. This is made up of a coil, which at first glace might be taken as the static discharge coil but it seems to have a few less turns, connected in series with a glass variable cap. The cap is shattered and doesn't seem to have any indication of value on it. nor does the coil. It appears the coil was connected on the output of the tee network through the variable cap to ground. The ATU is divided into two chambers with the tee net and this disconnected filter in one and the filters plus a static coil and ammeter in the other.

My question is what do you think the disconnected filter was used for,,maybe to filter the freq that is now getting into the tank? The antenna is a folded dipole 1/4 wave.

Thanks for any help.
 
Did the station used to play the Rolling Stones? ("Shattered"... sorry).

Sound like you are describing a classic pass-reject filter: one leg is series-resonant (pass) at your carrier frequency, but becomes inductive or capacitive at the interfering station's frequency. The other leg is then set to to be the opposite sign at the interfering freq (via the resulting sum of the series cap and coil), such that the whole circuit is parallel resonant (i.e. reject) at that freq.

The coil-cap to ground may have been in place to transform the impedance into which the T was looking, or could have been a filter at the other frequency.

All tricky stuff, of course, depending on what the original designer and subsequent tinkerers had in mind. Remember that beyond just matching the tower and rejecting interfering signals there are also circulating currents and circuit voltages -- i.e. component values -- to consider.
 
I would start by digging through the station's engineering files and see if someone was kind enough to leave a schematic of the ATU and the filter components. Unfortunately, chances are probably slim that they bothered to take a few minutes and sketch out a diagram for those who may come after.
 
If not, you might just do the math on the mixes and check it with a spectrum analyzer a mile away, etc. If it's really not spitting out bad stuff, all is good.
 
"My question is what do you think the disconnected filter was used for,,maybe to filter the freq that is now getting into the tank? The antenna is a folded dipole 1/4 wave."

That LC network was obviously put in for a reason. The blown capacitor probably explains why you are seeing one of the other stations in your transmitter tank circuit.
One of the other stations may have some information about the blown reject filter.
If worse comes to worse, find a variable vacuum cap and install it to replace the blown cap. You may need to add a fixed cap in series or parallel to give you the ability to notch the offending frequency from your transmitter's tank circuit.

Trial and error unless you can get more information.
 
From your description it sounds like a common pass-reject configuration employed in diplexers. The circuit is series resonant (short) at your frequency and parallel resonant (open) at the other station's frequency. In such circuits junction voltages and currents can be quite high. Much more specific information is needed. The pass/reject circuit, ATU phase and transmissionline phase rotation all play an important role in providing the correct sideband impedance relationship at the RF generation point. Remember, the transmitter's RF output network will also affect these.

The pass/reject filter is usually physically shielded or in its own enclosure in the ATU cabinet.
 
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