Thanks for the great info as soon as we get the service monitor back we are going to try these things.
I have already seperated the 2 sides and have seen much improvement so I think that this is really my problem. I do have a question about duplexers in general. I am sure that this is a dumb question but.... What is the purpose of notching out the receive frequency on the transmit side? Since I have 6 cans couldn't I move one of the cans from the transmit side to the receive side to give me 4 on the receive and 2 on the transmit? Thanks, Vern On Tue, 25 Sep 2007 23:46:12 -0600 Nate Duehr <[EMAIL PROTECTED]> wrote: > [EMAIL PROTECTED] wrote: >> I had them tuned because I had just bought them and >>didn't >> really trust that they were right. They were very far >>out >> so it's good that I got them tuned. I was having the >>same >> problem as now though very poor receive. Right now I >>have >> a radio on there for receive that was getting about 30 >> miles of coverage as an Echolink link node with home >>made >> antenna and now hooked up to the repeater using a big >>Tram >> Dualband antenna through the duplexer I am lucky if I am >> getting 3 miles. >> >> So I don't think the repeater's built in receiver is the >> problem which leads me to either desense or a bad >>antenna >> cable. Transmit is getting out very well and the swr is >> almost 1 to 1 so I think the cable is OK. I am running >> LMR 400 up the tower 95% of the way. I just have a >>short >> coax jumper that goes into the antenna. >> >> I am going to try to split them and see what I get. >> >> Thanks, >> Vern >> KI4ONW > > Before you do that. Have someone transmit a weak signal >(or use an > iso-T and transmit it in yourself, as someone else >mentioned) into the > repeater while you're at the site, listening to the >receiver while the > transmitter is on. > > Turn the transmitter off. If their signal gets better, >you're fighting > desense. It's that simple to find out. > > To find out exactly how bad it is, feeding a weak signal >into the > receiver with an iso-T and measuring the audio coming >from the repeater > receiver with a SINAD meter is the "next level" of >knowing what's going > on. (I've seen people do this by ear with practice and >get close, but > you need to see it on a meter first or have someone >demonstrate to even > try it. Hey... sometimes when you're starting out you >don't have the > gear, we understand...) > >Feed a weak signal (usually 12 dB SINAD for these tests, >as a standard > starting point) and then turn the transmitter on. The >weak signal will > disappear or be noisier if you have a desense problem, >as mentione above. > > Increase the signal generator to the point where the >weak signal is the > same as before (usually 12 dB SINAD is used when you >have a meter). > > The difference between where the signal generator was >level-wise when > you started, and where you end up, is how MUCH desense >you're fighting, > and how much more isolation you need in the overall >system to make it > work. Plus if gather numbers like this, folks here can >tell you > "ballpark" numbers to expect from your particular radio >and setup. > > Also be forewarned, some antennas simply don't "duplex" >well... it's > difficult to explain, but you'll find antennas that >throw all sorts of > crap around when used in duplex operation, that are fine >for simplex. I > know nothing about the Tram antennas, but "dual-band" >antennas for > repeater operation, sets off warning bells for me. > > Use the best cables for interconnect you can possibly >buy! Having nice > double-shielded stuff built onto the duplexer by the >manufacturer, only > to run lossy/leaky crud from the repeater to the >duplexer, is just > asking for trouble. If you used your LMR 400 for that, >good... it'll > work in most cases, just fine. Many people do have >problems with LMR > 400 in duplexed service, other's don't. There's a long >thread about it > around here somewhere in the archives... > > If you can afford/get hardline - always do it. 1/2" will >work fine at > VHF unless you have an enormous run, and you might want >7/8" for UHF, > depending on the length of your run. Keep an ear to the >ground and > scrounge hardline any which way you can. Hardline >connectors too. > They're not cheap. > > You can test your "inside" setup by replacing the >antenna with a GOOD > dummy load rated for the power you're pushing, and that >is a solid 50 > ohm load. (Don't use a cheap one for this. Find >something big and > stable. I found a 500W Bird load at a hamfest once for >$12, best > purchase that year.) See if the system desenses itself >when not hooked > to the outside antenna. If it does, you have something >wrong right > there in the repeater itself. Stop and figure that out. > > I could go on and on, but will stop and give the >admonishment my elmers > gave me... "MEASURE IT"... don't guess. Beg, borrow or >steal test gear > and get someone to show you how to use it. You can >"stumble" into > correct setups without it, but you can KNOW how well >your repeater > performs if you measure. > > - Receiver sensitivity -- put a very weak calibrated >signal directly > into the receiver and measure the 12 dB SINAD point. > > - Useable receiver sensitivity -- do the same test, but >with an iso-T or > directional coupler of known loss (measure that too!) >and see how much > more signal you need to have the same receiver >performance through the > duplexer, and final cabling. > > [If you have a pre-amp this becomes more important to >see if the gain > has driven the receiver into the noise floor at the >site, and/or if > you're overdriving the receiver with too much RF.] > > Those are good starting points, both with the >transmitter on and off. > > Nate WY0X > Nate WY0X
