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February 13, 1998
 
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ARTICLE FROM AUG 1996 

A MITREK REPEATER

Part 2 of 3 

In part 1 of our series, we tuned up the receiver of our Motorola Mitrek. The next part is not so east if you don't have the right equipment. 

I started from the basic transmitter coil and slug settings as given in the Mitrek manual. Using my HT and a digital VOM, I started on the first strep of the tune up. This entailed a measurement after the first three stages of the transmitter. On the plus side, I heard the carrier on frequency right away, and was able to peak the three slugs just fine. 

The trouble started after this triumph. The next metering stage was after FIVE MOVE STAGES!! No matter what I tried, I could not find the magic spot on all five slugs to give me any reading on the VOM at all?? Now what? 

Well, as you all know from last month's JOURNAL the answer was provided by Dave Metz, WA0AUQ and a rf probe. By the way, if you receive your JOURNAL via e-mail, the schematic was not included. It is available in ISIS format from WA0AUQ at davemetz@muscanet.com . If you need it in another format, contact us and we'll see what we can do. 

Since Dave covered the RF probe so well, we'll just leave it at this: If you are tuning a Mitrek (or comparable radio) GET ONE!! By testing at every can with the probe, tune up is a snap. 

Once tuned up on both receive and transmit, it is time to modify the radio for full duplex service and get it hooked up. If we are now thinking full duplex, we better think out the antenna system a little. 

Duplexers are great, duplexers are wonderful, duplexers are EXPENSIVE!! At least new ones are. There are bargains to be had if you look very carefully under all tables say about 6 hours BEFORE the hamfest starts. Otherwise they are gone! But if you look hard enough, long enough, you can usually find something suitable. It just SEEMS like it takes forever. 

Do you NEED duplexers? Good question. The Lancaster Missouri, repeater worked the two antenna system for years and worked well. Of course there was 100 feet of vertical separation between the antennas. That is one of two ways to separate the antennas. But it takes a LOT of separation. On the order of several wavelengths to get enough isolation. The ARRL Handbook states that you can get 64 db of isolation for 2 meters with a separation of only 25 feet. Unfortunately, 72 db is generally considered the minimum. And since the feedline of the upper antenna passes right past the lower antenna, that 72 db may never be achievable without some VERY expensive coax. 

Oh!! How about horizontal separation? Got lots of coax? Because it takes one half MILE of separation that way. If you do it that way, you don't need this conversion because you will have a transmitter at one site and a receiver at another. Of course, you'll need a transmitter and receiver on another frequency band to get the audio from the repeater receiver to the repeater transmitter. Make that another band, or you'll be right back where you started with the desense. 

Yep, you might as well start looking for those duplexers NOW if you want your repeater to fly. As luck would have it, Clyde, N0ZAK was able to trade for a good set of duplexers. Maybe we can get Dave, WA0AUQ to write an article about duplexers tuning. Just let me say that they don't work right if you get the "goes-inta" and the "goes-outta" reversed! 

NEXT MONTH:FULL DUPLEX MODIFICATIONS 

Michael Muldoon, KE0BX

Comments to muldoon@se-iowa.net 
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