signal questions

[bobdavcav]

Here in Seattle as I'm sure happens in many big markets, we have four stations on one antenna. I have two questions,
1. How can four stations have different erps but transmit from the same antenna? Does this have to do with the transmitter power output as well as the other variables which I do not understand?
2. How can two stations operate from the same antenna with different haats? The two CC stations operate at 387 meters while the other two on that antenna are 385.
3. Last but not least, why does KUBE have such a different signal from KJR? Both run 98 kw at 387 meters.

 

[rfry]

Here in Seattle as I'm sure happens in many big markets, we have four stations on one antenna. I have two [three?] questions.

1. How can four stations have different erps but transmit from the same antenna? Does this have to do with the transmitter power output <snip>?

Yes, it does. Peak ERP is the product of the Z-matched power applied to the input connector of the transmit antenna and the peak power gain of that antenna at that frequency.

2. How can two stations operate from the same antenna with different haats? The two CC stations operate at 387 meters while the other two on that antenna are 385.

Technically they can't, although those HAAT differences are within the tolerances allowed by the FCC.

3. Last but not least, why does KUBE have such a different signal from KJR? Both run 98 kw at 387 meters.

If such is true at some specific receive location, that could be the result of factors such as their relative ERPs in that direction due to different "omni" pattern shapes from that common antenna, at their different frequencies.

 

[Lazy J]

Here in Seattle as I'm sure happens in many big markets, we have four stations on one antenna. I have two questions,
1. How can four stations have different erps but transmit from the same antenna? Does this have to do with the transmitter power output as well as the other variables which I do not understand?

For the non-engineers who may read this... You can have lots of different stations at different frequencies and different powers on the same antenna (assuming the antenna is built for that purpose). Each station has a separate transmitter and each transmitter goes into a combiner (basically a filter that keeps the other stations from feeding back into your transmitter) and the combiner "combines" all the stations and sends them up the antenna. You can put different power on each frequency and the antenna will send out each frequency according to the power you put into it. More power on my frequency doesn't mean more power on your frequency, even if we share an antenna.

2. How can two stations operate from the same antenna with different haats? The two CC stations operate at 387 meters while the other two on that antenna are 385.

This is basically just a paperwork issue. An FM antenna can typically be 5-20 feet tall. You are supposed to "center" the antenna at your licensed height. But as R. Fry pointed out, the FCC won't make a fuss over 2 feet. So maybe they split the difference and centered the antenna at 386!

 

[w9wi]

Here in Seattle as I'm sure happens in many big markets, we have four stations on one antenna. I have two questions,
1. How can four stations have different erps but transmit from the same antenna? Does this have to do with the transmitter power output as well as the other variables which I do not understand?

Yes, it has to do with the transmitter power output.

The antenna has a fixed "gain" -- the ability to concentrate the transmitter power along the horizon, where the audience is. (no PPMs on the Space Station )

Let's say "Antenna A" has a gain of 5.

- Station KAAA attaches a 20,000-watt transmitter to this antenna; its power is "amplified"* by a factor of 5 to 100,000 watts ERP.
- Station KBBB attaches a 10,000-watt transmitter to this antenna; its power is "amplified"* by a factor of 5 to 50,000 watts ERP.
- Station KCCC attaches a 1,200-watt transmitter to this antenna; its power is "amplified"* by a factor of 5 to 6,000 watts ERP.

And if, as "Lazy" suggests, you use a "combiner", you can connect all three stations' transmitters to Antenna A at the same time. Each station gets an ERP equal to its individual transmitter power, multiplied by the common antenna's gain.


* the power is not literally amplified -- the same amount of power is radiated in any case. "Gain" means that power is concentrated where it does the most good.

 

[littlejohn]

As Fry points out, the antenna may wel;l have diferrent gain at different frequencies, although this difference is not great in any systems I have seen. And as he points out, you'll have different patterns at different freqencies - and I have seen some of these which didn't do well.

The idea of ERP is fairly simple. A field intensity meter (signal strength) at a given location will read the same whether the radiator is a single bay driven with much RF, or if it is a (properly designed and installed) multibay radiator driven with lower transmitter power than the single bay. as W9WI points out. As long as the radio sees the same signal, that signal is the effective radiated power in that spot. How it got there matters not to the radio. It does, however matter to the pocket of the broadcaster.