If there are two or more AM's sharing the same tower, how does the antenna current measurement from one station not pick up the other station(s) current?
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Antenna current on shared towers
- Thread starter secondchoice
- Start date
Each station sends its signal into a diplexer unit which does two things: it tunes the transmitter to the characteristics of the tower and it rejects the signal from the other station.
The combiner works so that the tower accepts each station's signal but rejects that of the other so it does not feed back into the transmitter of its tower partner. At the same time, it "tunes" the output of the transmitter to match the impedance and reactance of the tower at the frequency of each station.
Antenna current is measured for each station entering the combiner/antenna tuning unit where the signal of the other station is not present.
I have only built one combiner (for 570 AM and 805 AM) many years back, but that system is relatively simple in theory.
You could measure current going out of the tuner/rejection network with narrow passband networks to send each station only to the meter. I've never worked on a diplexed directional, but I'd guess that this is how the individual tower currents are read. Perhaps someone with more diplexed or triplexed system experience can explain how this works and correct me if I did not explain this well (trying to keep it in lay language)
Here is a picture of my diplexer with me at the unit, around May of 1966 https://davidgleason.com/1964-1970-Adding-More-Stations.htm
Thanks Dave. The reason I asked this question: I found my old soldering gun while going thru a couple of boxes: I remember after a visit from lightning having to wait till signoff going out to the dog house and replacing a melted AC outlet while holding a regular 2 C battery flashlight. That which solved the tower lights not working. Thankfully there was a schematic and a good 110 volt mechanics light at the station. All I had to do was replace a burned-out diode and a blackened resister which were easy to see. The station manager gave me an extra $20 bucks. Five years and two stations later, while talking to a contract engineer, I found out I most likely saved the station at least $200.
I have a two-station diplexed antenna, there's one base current meter that reads the RMS value of both signals. Individual station current can be checked during maintenance with one TX off. Otherwise individual TX power is monitored pre-diplexer, and the filter losses are known.
I never saw a need for combined RMS base current measurement. I know the characteristics of the combiner, and wanted an actual antenna current reading for each station at all times. Where I installed there was no government requirement for metering and as my installation was the first diplexed AM in South America, there were no rules. It was written up in "Electronics Magazine" later that year, including the math, by my local consulting engineer who also manufactured the coils and put together the box for the combined ATU.
You're correct, a single base current reading for combined stations isn't a requirement. Some stations I've seen use one to confirm base current against the calculated/licensed current at the common point/base for the individual stations. Problem is; you have to turn the other station off to check the reading. In one instance, the single base current ammeter was just left over from the days where there was only a single station feeding that tower.
Ultimately the station licensee must maintain antenna operating parameters in accordance with licensed values. How this is done depends on how the engineers designed it, operational requirements and budget of the station, and interpretation of FCC rules and policy.
There are two commonly used meter types, thermocouple ammeter, and toroidal sampling transformer. There are details the engineers work out for each situation.
I think antenna sharing is an example of engineering helping owners.
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From a pure physics standpoint, sharing antennas is always a compromise for one or more stations. The towers/antennas were cut to a length at the original station frequency and, depending on vintage, the entire system may have been designed for enhanced bandwidth at the original frequency. Diplexed compromises include certain inefficiencies and potentially non-linear performance for both stations involved. That includes if you diplex with one or more stations, you can pretty well forget a future of broadcasting in full MA3 mode.
I agree Kelly A. Antenna sharing is a solution or compromise, and not necessarily first choice of engineering. Engineering rises to the challenge and gets the job done. Consider 660 and 880 in NYC. 660 has advantage of lower frequency, 880 gets a half-wave radiator. Both get a fine location with a noteworthy salt-water propagation path to coastal south New England.
I have always wondered what the big deal was about antenna amp. reading in a non directional AM. You have the transmitter final voltage and amp reading. If total power out of the transmitter is correct for the engineered efficiency of the antenna, how can a station be operating with more power than licensed? I can see if there is a severe problem with antenna but wouldn't the transmitter will trip if the plate amps gets too high. I totally get the directional antenna currents dividing up the power between the towers. .
Reading the base current on a non-directional AM station from time to time is a way to see that the impedance of the tower has not shifted.
A change in licensed base current could indicate a problem with the ground system or with the antenna tuning unit or transmission line.
A change in licensed base current could indicate a problem with the ground system or with the antenna tuning unit or transmission line.
secondchoice- Good questions, keep thinking about this, but please don't obsess over it.
Most AM stations do not have a transmitter that is capable of producing significantly greater power than authorized for daytime operation. Transmitters are expensive. Electricity cost is real.
Context for non-directional AM stations:
Antenna Input Power is the product of the square of the antenna current and the antenna resistance at the point where the current is measured, when the station is unmodulated.
Antenna input power is determined by the Direct Method, which is an observation (or remote indication) of antenna current as indicated by RF voltage or RF current.
In certain circumstances stations may temporarily use the Indirect Method which is the indicated transmitter parameters in conjunction with transmitter efficiency value.
Recent AM transmitters have a FM style percentage transmitter power output (TPO) meter, which is calibrated to read 100 percent when the authorized power is produced. Usually, the meter can be switched to read power reflected back to the transmitter from the transmission line, antenna tuning unit and antenna.
Stations are required to calibrate metering accuracy as needed and maintain authorized operating parameters.
Most AM stations do not have a transmitter that is capable of producing significantly greater power than authorized for daytime operation. Transmitters are expensive. Electricity cost is real.
Context for non-directional AM stations:
Antenna Input Power is the product of the square of the antenna current and the antenna resistance at the point where the current is measured, when the station is unmodulated.
Antenna input power is determined by the Direct Method, which is an observation (or remote indication) of antenna current as indicated by RF voltage or RF current.
In certain circumstances stations may temporarily use the Indirect Method which is the indicated transmitter parameters in conjunction with transmitter efficiency value.
Recent AM transmitters have a FM style percentage transmitter power output (TPO) meter, which is calibrated to read 100 percent when the authorized power is produced. Usually, the meter can be switched to read power reflected back to the transmitter from the transmission line, antenna tuning unit and antenna.
Stations are required to calibrate metering accuracy as needed and maintain authorized operating parameters.
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I get the long term implications of antenna current, but back when I started (1968) we when suppose read antenna current with the meter readings. You really couldn't get an "accurate" reading unless you killed the audio. To be totally truthful, I would get readings with no modulation at sign on and copied them unless there was a change in plate voltage or current for the rest of the shift. Funny thing the readings were the same all day, Those old Collins and Gates AM tubed transmitters really stable! Was meter readings every hour or half an hour? I am having trouble remembering details from over a half a century ago. I feel old now.
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