You're about to take the big dive, aren't you? You want to put a GMRS repeater on the air. Where do you start? This month we are going to talk about commercial sites as possible locations for your repeater. First, some basics.

Obviously, first you have to determine what area you want your repeater to cover. Find a location as near the center of your desired coverage area as possible. Since GMRS is in the UHF band, GMRS signals are subject to the ‘line of sight’ rule. This means, the higher you can get the repeater antenna above average terrain, the bigger your coverage area will be since the communicating antennas need to ‘see’ each other for clear signals.

Here is a predicted range calculator you can use to get a basic idea of how far you will be able to clearly use your repeater based on elevation, power output, frequency, and antenna gain. (Courtesy Decibel Products)

Begin on the left under the appropriate power output level. Look down the line from your power level until you get to the height of your antenna above terrain. Now, move to the right to the ‘db’ column. Count up one notch in the ‘db’ column for each db of gain of your antenna (minus cable and other losses). This is called your ‘net antenna gain’. Now, move to the 450 MHz column to the right and the number you land on or near will be your predicted range in miles.

Example: A repeater puts out 10 watts and is 1000 feet above terrain. So in the first column, I choose the notch at ‘1000’. Now, knowing that the antenna gain is 5 db, and that there are 2 db in losses through the duplexer and cables, this gives me a net antenna gain of 3db. Moving to the ‘db’ column, I count up 3 notches. Now, I move to the right under the ‘450 MHz’ column and read the number to predict my range, about 23 miles. Using this basic table, you can determine if the site you chose would give you the range you expect.

Since most commercial site owners want your business just as much as you want to have your repeater at their site, the process of approaching the site owner or operator is usually very low key and relaxed. There are, however, some things you will need to get that you probably wouldn't need for a home installed repeater.

First, you will need liability insurance. The sites in central California usually require at least $300,000 in liability coverage per individual lease. Your home owners or renter’s umbrella coverage may cover this, it's worth the phone call to find out. This helps protect the property owner in case your gear burns the place down, or someone trips over your power cord and sues, etc.

Next, you will need to have your antenna and cable mounted on the tower by a professional climber. Ask the landlord to suggest one. They usually have a favorite climber that is familiar with that site already. Very few site owners will allow individuals to mount their own antennas. You usually don't get to pick where your antenna will go on the tower yourself. You can always suggest a particular leg of the tower, depending on which direction you don't want your signal attenuated by the tower itself. The site landlord will be able to tell you what type antenna brackets to use, how much cable you will need, how many hangers, etc. to buy. Some sites use combiners, and do not allow tenants to install their own antenna systems, but more on that later.

Equipment wise, you will need to have good coaxial cable for the run to your antenna and for interconnection cables between your radio and duplexer, etc. For an antenna run of up to 50 feet, ½ or 7/8 inch Heliax (TM) is the bare minimum. For longer runs, use larger cable. Connectors between your repeater and duplexer should be double shielded, such as LMR-400.

You will need to use a grounding kit and lightning arrestor outside the building right where your cable enters, and attach it to the site ground system. There is usually a ground bus bar at the cable entry for this purpose. Inside, at the repeater, you will be required at most sites to use a pass or pass/reject duplexer. This helps keep your transmitter emissions from interfering with other receivers at the site. Also, you will need to install an isolator. An isolator works like a one way street for RF. It allows the output from your transmitter to pass to the duplexer with little attenuation, but any RF that comes out of the duplexer back toward your transmitter goes into a dummy load and is dissipated before it reaches your transmitter. On the output of the isolator, you will need to use a harmonic filter. Some isolators have a harmonic filter attached already. This is one of the best ways to prevent intermodulation at a busy site, and is a standard.

In any repeater installation, you should install an A/C line surge protector. Most high altitude sites get lots of spikes and garbage on the power lines from storms and other stuff. Protect your gear. Even the Radio Shack $10.00 surge protector is better than nothing. Some sites have backup power from a generator or battery bank fed inverter. Make sure you find out if you're A/C plug is on the backup system or not. Some sites have both backed up plugs and non-backed up plugs, and allow only certain systems to be on the backup power system. Since some of the more popular sites don't have the room on the tower to put as many antennas as they would need to meet the demand for the number of repeater at the site, they use combiners. A combiner can mix the output of many transmitters and feed them into the same antenna at the same time. For the receiver, a multi-coupler is used which takes the received signals from one antenna, filters the transmitters’ spectrum out and amplifies the signal, then distributes it to several receivers.

When you sign the lease to be at a site on a combiner, you will be asked what kind of connectors you want at the ends of the transmitter and receiver cables. When you go to the site to install your repeater, the landlord should have the cables marked, and run to where your repeater cabinet has been assigned to be. You do not need a duplexer, isolator, or harmonic filter if you are connecting to the combiner since these are built in to the combiner and are an essential part of how it works.

Usually, the receiver antenna is mounted high on the tower and will provide fantastic ‘ears’. Transmitter combiners, on the other hand, are lossy in nature. Depending on how many transmitter ports the combiner has, you can lose half of your transmitter power or more in the combiner. This means, if your transmitter output is 50 watts, you will get only about 25 watts to the antenna. Not really a big deal, but something to remember. Since most repeaters need better ‘ears’ than ‘mouth’, this has a tendency to equalize your repeater to mobile and portable coverage anyway.

Signing a lease for commercial site space is a big step. As long as you are prepared financially to fulfil the terms of the lease, you will be more than pleased with the performance boost a high altitude, professionally maintained site can give your system. Your repeater will become a reliable communications tool and not a toy. Next month: stories from the trenches.

Gummers^(TM) is a trade mark of Paul Shinn