AM Transmitter distance to tower

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AM Transmitter distance to tower

Post by AMRADIO WV » Mon Jun 20, 2016 9:28 am

I bought an AM radio station and I am new to the industry. I have a question our transmitter is about an 1/8 of a mile to a quarter mile from the transmitter, do we loose power because of the distance. Should I move the transmitter closer to the tower? I know with FM you haven to increase power to make up for distance what about AM.

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Deep Thought
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Re: AM Transmitter distance to tower

Post by Deep Thought » Mon Jun 20, 2016 10:27 am

It's not all that uncommon for the transmitter to be some distance from the tower itself, and that can provide some measure of protection from lightning. A quarter mile might be somewhat excessive, but I've seen some DAs where the far tower is >2000' from the phasor.

At AM frequencies the transmission line loss is negligible in most installations. For example, 7/8" foam Heliax has an attenuation at 1 MHz of 0.0345dB/100'. That's an efficiency of ~92.5% at 1000 feet. 1-5/8" cuts that to 95.5%, which isn't really worth the extra cost. You'd be better served to make sure the line is matched to 50j0 at the ATU because a mismatched line will dissipate far more than the attenuation losses.
Mark Mueller • Mueller Broadcast Design • La Grange, IL •

Tom Osenkowsky (RIP)
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Re: AM Transmitter distance to tower

Post by Tom Osenkowsky (RIP) » Sun Jun 26, 2016 4:06 pm

If your station is non-directional (a single tower) you measure your power at the tower base. This is accomplished by reading the base current without modulation and multiplying that reading by itself and multiplying that number by the licensed base resistance, in other words I squared R.
For example if your base ammeter reads 4.5 Amps and your licensed base resistance is 49.4 Ohms your power would be 1000 Watts or 4.5 times 4.5 times 49.4. Any power lost in the transmission line or ATU is compensated for by increasing the transmitter power until your base current reads 4.5 Amps.

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