I created a presentation for our local ham radio club to explain the environment our repeater engineer was dealing with. The problem many great sites have is that their prominence attracts multiple transmitters with each addition making it harder for the repeater to pick up the signals we want out of the electromagnetic flood. Thus the situation I describe with this Downing Mountain repeater is common to many.
I called this “Tracking A Transmission”. We follow a signal from the operator’s mouth through his radio and up to the repeater overlooking Hamilton Montana. Once the repeater gets the information, it transmits it out from a large antenna with 50 watts of power pushing it as much as 50 miles away. To get a clean signal to and through it is radio engineering magic that few people can do well. You will see why in a moment.
We start with a radio operator wanting to communicate with his buddy one valley over.
As with most handheld ham radios, we are using 5 Watts of power output that the FCC allows. But the radio’s antenna sends that out in every direction. Therefore only a tiny bit of it goes in the direction we need to reach our friend the repeater.
Let’s look at the one direction we want our signal to go, and the distance we want it to travel, losing strength in that desired direction along the entire journey. One of our Electrical Engineers did the math for a 50-watt mobile unit as we have in our vehicles with an antenna on the roof a mile from the repeater. That signal is more than ten times as powerful as our little handheld transceiver.
This tiny little 0.0063 MicroWatt (0.000000063 Watt) signal finally arrives at our repeater site where our antenna is less than 100 feet from a huge commercial FM broadcast antenna putting out 25,000 Watts! The FCC has rules about how sloppy any transmitter can be. They call the transmissions outside of their desired frequency “spurious emissions”. It is a very small percentage of the desired signal, but scale is our enemy here. In the case of our commercial radio station, the stray signal is about 250 MicroWatts. Compare that to the signal our repeater is trying to hear for us. This is kind-of like trying to find a grain of sand in the state of Texas.
Oh, but that is not the end of the challenge. Over the last twenty years four more commercial broadcast stations have installed transmitters on this site. No big deal for any of them. They don’t try to receive anything… yelling without listening is their job.
Now we have a 1,250 MicroWatt jungle our repeater engineer has to hack through to pick up the signal we want repeated from our station on Downing Mountain. I have attempted to draw a picture of our engineer focusing this ocean of stray signals to help the repeater find ours.
One of the tricks he used was to switch our repeater from nearly obsolete wide band signals to a newer narrower bandwidth. All radios sold in the last twenty years have the capability to use the older or the newer bandwidth, depending on how they are programmed. This means the focusing portion of our repeater system delivers less spurious signals for the repeater to wade through.
The number and complexity of components and functions in the repeater assembly is far past what most of the advanced-licensed Amateur Extras know. Our engineer can attempt explanations, but most hams hear more noise than signal from those attempts.
That I can have my signals from several valleys and 17 miles away repeat cleanly out of that Downing Mountain repeater is nothing short of amazing. The engineer who is pulling this off has my respect, and my support.
Thank you, Jeff (aka: US Communications and Disguised Antennas).
