At 30 watts on a 5 MHz split in GMRS, you're looking at a minimum of 70 dB of required isolation between the .30 uV receiver and transmitter. This can be accomplished rather easily with a 6 pack mobile duplexer (assuming you can get the full 75 dB of isolation out of a properly tuned high end flat pack). Otherwise you're looking at a minimum of 34' vertically or 794' horizontally to accomplish the bare minimum 70 dB of isolation. That figure, however, assumes there is perfect shielding, no crosstalk or tower interaction and proper grounding/bonding across the board. In reality with your setup of LMR400 and Heliax, I'm guessing you might get 40-45 dB of isolation at the 30' separation mark, knock off another 3-10 dB for the interaction between the feedlines.
Hopefully you've got sufficient forced air cooling running on the transmitter Maxtrac - these tend to get VERY hot during operation, even worse after you exceed the 10-15% duty cycle. Good luck with the project! On Sun, Jun 28, 2009 at 9:47 AM, Richard Fletcher <[email protected]>wrote: > > > I know you all will laugh at this one, but they say necessity is the > mother of invention. A while back a friend had the chance to try a tower > site and wanted a simple and quick way we could check coverage. > > So with 2, ¼ wave NMO whole mount antennas and 2 18” satellite dish pans. > I mounted both antennas. Since these mounts were not shielded inside I used > a couple strips of foil tape to insure the shielding. > > Then panned them face to face with one then being on the top and other > one inverted on the bottom. Then we dropped 2, 50’ feeds of 9913 to a Custom > MVP Repeater on a car battery at the base of the tower. The Dish was set in > the tower structure and laid down inside pretty nicely. We then had a couple > different folks access it from various areas and we drove around checking > coverage on a GMRS freq. The surprising thing was that with the RX as the > top and the TX was inverted. There was enough isolation that there was no > desense on the 15 watt TX and the receiver was running a nice .2 uv, 12db > sinad. The location worked so well that he made arrangements on an adjoining > tower at the same site. He had his GMRS there for the full term of the > license and removed it when everyone started using cellular’s instead. (This > was back in the MID 90’s). Oh and when he went permanent he did use a real > antenna and Duplexer. But it was the same Custom MVP radio. > > Just thought I would share that since we were discussing dual antennas > and no Duplexer options. > > Regards > > -Richard > > > ------------------------------ > *From:* Barry <[email protected]> > *To:* [email protected] > *Sent:* Saturday, June 27, 2009 8:46:58 PM > *Subject:* RE: [Repeater-Builder] Where to install my repeater antennas > > > You are going to need a lot more isolation than you are contemplating , > either fit cans on a common stick or separate by a couple of hundred feet > > ____________ _________ _________ __ > > To: Repeater-Builder@ yahoogroups. com<Repeater-Builder%40yahoogroups.com> > > From: agrimm0...@yahoo. com <agrimm0034%40yahoo.com> > > Date: Sat, 27 Jun 2009 21:01:58 +0000 > > Subject: [Repeater-Builder] Where to install my repeater antennas > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > This isn't the most difficult question to answer, and I figure it can be > answered relatively easily. I currently over the past few weeks constructed > a GMRS repeater for my neighbor hood to use. It is constructed of 2 maxtrac > radios one set at just 30 watts tx. I plan to use 2 antenna's so no duplexer > is needed with some good Heliax cable running at least to the TX antenna. I > have 40 ft of tower that I plan to mount the antennas on. One antenna is a > Celwave (RFS) 6db gain antenna. And the other is just a plain 1/4 wave NMO > style mount antenna. Which is the best way to install these antennas on my > repeater? Celwave on the transmit end and at the top, or 1/4 wave on the > Transmit and celwave on the receive? Also which antenna should be mounted on > top of the tower? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > ____________ _________ _________ _________ _________ _________ _ > Looking for a new car this winter? Let us help with car news, reviews and > more > http://a.ninemsn. com.au/b. aspx?URL= http%3A%2F% 2Fsecure% > 2Dau%2Eimrworldw ide%2Ecom% 2Fcgi%2Dbin% 2Fa%2Fci% 5F450304% 2Fet%5F2% > 2Fcg%5F801459% 2Fpi%5F1004813% 2Fai%5F859641& _t=762955845& _r=tig_OCT07& > _m=EXT<http://a.ninemsn.com.au/b.aspx?URL=http%3A%2F%2Fsecure%2Dau%2Eimrworldwide%2Ecom%2Fcgi%2Dbin%2Fa%2Fci%5F450304%2Fet%5F2%2Fcg%5F801459%2Fpi%5F1004813%2Fai%5F859641&_t=762955845&_r=tig_OCT07&_m=EXT> > > >
