accurate antenna current reading?

[secondchoice]

WFAN and WCBS (both 50 KW) are sharing a tower. With land costs so high in NYC I am surprised that more of this is not happening. I was just wondering, when two AM’s share a tower how you can get an accurate antenna non modulated current reading? Wouldn’t some of the other stations’ modulated signal (50 KW in this case) “bleed” into your reading?

 

[Tom Wells]

Yes. How many calibrated RF preselector circuits you can add into any measurement will only help.
( Think military radio radio shielding and any multiple passive/active/DSP you can imagine.)
At such proximity and high near fields, such selectivity is not possible with simple rolloff low-Q filtering,
and would probably require one or more tuned notch rejection and/or shunting filters for the sharing frequency.

Maybe even a carrier drop on the shared user ( or both) for reference on tuning the traps.

 

[Nostalgia]

Duplexing of two stations onto one tower is accomplished by a network of band pass and band reject filters. Both transmitters are not just connected directly to the tower!! WFAN would have a 880 band REJECT filter in their line to prevent WCBS from blasting back into their transmitter, and WCBS would have a 660 band REJECT filter in theirs to block WFAN. The antenna current meter would be after the stations ATU (band pass and matching network) and the reject filter so it would only be measuring the desired stations base current. Band reject filters blocks the specified frequency but is transparent to frequencies outside the filter range. Band pass filters are just the opposite.

Your example is pretty straight forward. But consider Hawaii where land is not only expensive, but scarce for an AM tower. They have a couple of towers with several stations on them. Consider that EACH station must have a reject filter for EACH OTHER station on the tower. I have seen this RF nightmare at the old KIKI tower (now gone) and the KKUA Kewalo Basin tower. Both were built by Alan Roycroft, now deceased, I believe.

 

[frankberry]

The stations don't actually log the antenna current. Each station logs their current into their input of the RF combiner. Measuring the impedance at the two inputs of the combiner is a good way to ascertain whether the combiner is operating properly. These impedance measurements can be done while the stations are on the air.
The combined antenna current wouldn't be of much use to either station.
I'm sure that the performance of the combiner is tested occasionally.

 

[Nostalgia]

OOOPs, typo correction:

The antenna current meter would be after the stations ATU (band pass and matching network) and BEFORE the reject filter....

 

[secondchoice]

The stations don't actually log the antenna current. Each station logs their current into their input of the RF combiner. Measuring the impedance at the two inputs of the combiner is a good way to ascertain whether the combiner is operating properly. These impedance measurements can be done while the stations are on the air.
The combined antenna current wouldn't be of much use to either station.
I'm sure that the performance of the combiner is tested occasionally.

So the AM signals are combined like FM's? I have been out of the biz too long! I thought one or both would be a shunt feed antenna and both would have their own transmission line to the tower.

 

[rfry]

So the AM signals are combined like FM's?

Yes... here is a link to a brochure from Kintronic, who is active in providing such hardware.

http://www.kintronic.com/resources/brochures/11.pdf