Re: [Repeater-Builder] Duplexers
Our local club recently installed a 2 meter repeater on a water tank adjacent to a cell site. Two cell towers are serviced by four buildings housing equipement, and we are having some desense due to noise pickup on the antenna. Running an iso-tee we found that our GE Mastr II receiver with GE preamp shows .6 uV for 12 dB SINAD using a dummy load in place of the antenna (.2 uV direct to the receiver bypassing the duplexer). With the antenna connected we see a 2 uV sensitivity for 12 dB SINAD. These readings are the same, with the repeater transmitter (40 watts) keyed or unkeyed. The noise floor is really decreasing the utility of the new repeater. The noise source seems to come and go as a quiet signal on the repeater input can become suddenly noisey, and vice versa - a noisey signal can become suddenly quiet. I hear the air conditioning units in the cell site buildings cycling and wonder if they might be the cause of the noise? Our antenna is only about 40 feet above the ground (at 7400 ft) while the cell antennas are at about 100 ft. So we are much closer to the cell site equipment buildings than the cell antennas. Two uV for the receiver sensitivity sure does limit the usefulness of the new repeater - Anyone have any thoughts? 73 - Jim W5ZIT [EMAIL PROTECTED] wrote: I am having some receive problems on my repeater and I am thinking that it might be desense. I am on 2M running a MASRII repeater with a Decibal Products band reject 6 can duplexer. While I can key the repeater from a pretty good distance the audio that makes it through the repeater drops off pretty quickly. I just had the duplexers tuned and they are tuned very well. So on to my question. If I were to take and seperate the recv cans from the xmit cans and run to 2 seperate antennas would that mess up the duplexer tuning? will 20' of vertical seperation plus the cans and the fact that I would be running through seperate cable, make a difference? Thanks, Vern KI4ONW - Yahoo! oneSearch: Finally, mobile search that gives answers, not web links.
Re: [Repeater-Builder] Duplexers
The duplexer is a DB-4048. I have had this repeater for a couple of months and it has always had problems. I don't have access to a service monitor or anything like that right now but I have a friend that has one however someone is borrowing it right now. From what I can tell the desense is pretty bad. I can key up the repeater a lot further than I can send audio through it. all of my cables are double shielded or better. Thanks, Vern On Mon, 24 Sep 2007 19:54:33 -0500 "Steve S. Bosshard (NU5D)" <[EMAIL PROTECTED]> wrote: > Decibel did make a 6 cavity notch duplexer - 4" full >sized cavities - > that would work nicely on a 110 Watt M2 station @ 600 >kHz. Isolated > TEE test into a dummy load - how bad is the receiver >desense ? If you > don't have some test equipment - signal gen, dummy load, >and a TEE > fitting with the side pin cut away (makes 60 dB lossy >coupling, there > abouts) you probably will need to find someone who does >locally that can > help. 73, Steve NU5D > > Eric Lemmon wrote: >> Vern, >> >> Did this problem exist before you had the duplexer >>tuned? Was anything at >> all done to your repeater system just before the problem >>was noticed? >> >> A band reject (notch) duplexer may be incapable of >>performing even >> satisfactorily at 2m. Please advise the model number of >>your duplexer, so >> we can understand your situation. Are all of your >>cables double-shielded? >> It might be a good idea to perform a noise-floor test >>using an "iso-tee" and >> a service monitor. >> >> Some helpful information about investigating desense >>problems can be found >> here: >> >> >> >> 73, Eric Lemmon WB6FLY >> >> >>
RE: [Repeater-Builder] Duplexers
Hey Jim I have a few thoughts that come to mind. Based on my experience with VHF and UHF repeaters colocated in cell sites I would say that you may want to consider the following. First the AC unit motors etc will not be a problem to the excellent FM rec you have there with the GE M2 even with the preamp. Plain electrical noise is not going to be your problem. It is an RF issue. And Multi RCC sites are very often a big mess as far as RF goes. Its too bad you have to go through all this but it is a fact of life in crowded sites that are also the best ones to be on. The fact that your tests indicated the same actual values with the xmt on or off tells us that your duplexers are fine. And your rec tests direct connected are fine. Many cell sites are *real* bad with regard to rf on numerous freqs due to harmonics and mixing products/intermod. Some times you will only see your rf problem when 2 or more things occur at the same time aming it time consuming to pin it down. Sometimes the RF signal is slightly off your rcv freq but very strong and the skirts of the sweep are slightly coming into your channel. So you have to sweep the area with a good service monitor or Spectrum Analyzer to see what is going on. I use the IFR or R2200 plus the HP for this and it will show right up. It sounds like from the description you are in a very dirty site. You will have to consider 1. Moving to a new site even just a few hundred yards away on a new tower, or 2. Sweep with a spectrum analyzer to actually see if you can find the culprit(s) causing your problem and see if you can work with the equipment owner to clean it up. I doubt you will have much luck with this since if the techs at Verizon or Cell 1 ATT think their stuff is working fine they cant be bothered with your ham radio problems. They will tell you to take the site or leave it most of the time. 3. What I have had to do when the RF product was sitting just off our rcv freq and was fairly strong is install another cavity to clean it up. You may find it easier to use a pass type on your rcv channel unless you can definitely pin down the offending signal and reject it. Be aware that the preamp on your repeater can be a problem. GE and Moto reccomend those only for a base station and not a repeater unless you have a really clean site. They are notorious for intermod issues and most times are not worth the extra rvc gain in multi user sites. These suggestions are a start and may or may not do it for you but I would start with these basics. Keep an open mind to the fact that something at the site which is radiating some RF into your install may not be able to get fixed in practical terms and you may have to move either the frequency of your repeater or your site location, since I doubt you can get the cell guys/Paging RCCs to go away:) Let us know how this progresses 73 Glenn N1GBY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jim Brown Sent: Tuesday, September 25, 2007 8:20 AM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Duplexers Our local club recently installed a 2 meter repeater on a water tank adjacent to a cell site. Two cell towers are serviced by four buildings housing equipement, and we are having some desense due to noise pickup on the antenna. Running an iso-tee we found that our GE Mastr II receiver with GE preamp shows .6 uV for 12 dB SINAD using a dummy load in place of the antenna (.2 uV direct to the receiver bypassing the duplexer). With the antenna connected we see a 2 uV sensitivity for 12 dB SINAD. These readings are the same, with the repeater transmitter (40 watts) keyed or unkeyed. The noise floor is really decreasing the utility of the new repeater. The noise source seems to come and go as a quiet signal on the repeater input can become suddenly noisey, and vice versa - a noisey signal can become suddenly quiet. I hear the air conditioning units in the cell site buildings cycling and wonder if they might be the cause of the noise? Our antenna is only about 40 feet above the ground (at 7400 ft) while the cell antennas are at about 100 ft. So we are much closer to the cell site equipment buildings than the cell antennas. Two uV for the receiver sensitivity sure does limit the usefulness of the new repeater - Anyone have any thoughts? 73 - Jim W5ZIT [EMAIL PROTECTED] wrote: I am having some receive problems on my repeater and I am thinking that it might be desense. I am on 2M running a MASRII repeater with a Decibal Products band reject 6 can duplexer. While I can key the repeater from a pretty good distance the audio that makes it through the repeater drops off pretty quickly. I just had the duplexers tuned and they are tuned very well. So on to my question. If I were to take and seperate the recv cans from the x
Re: [Repeater-Builder] Duplexers
At 9/25/2007 05:20, you wrote: >Our local club recently installed a 2 meter repeater on a water tank >adjacent to a cell site. Two cell towers are serviced by four buildings >housing equipement, and we are having some desense due to noise pickup on >the antenna. Running an iso-tee we found that our GE Mastr II receiver >with GE preamp shows .6 uV for 12 dB SINAD using a dummy load in place of >the antenna (.2 uV direct to the receiver bypassing the duplexer). With >the antenna connected we see a 2 uV sensitivity for 12 dB SINAD. > >These readings are the same, with the repeater transmitter (40 watts) >keyed or unkeyed. > >The noise floor is really decreasing the utility of the new repeater. The >noise source seems to come and go as a quiet signal on the repeater input >can become suddenly noisey, and vice versa - a noisey signal can become >suddenly quiet. > >I hear the air conditioning units in the cell site buildings cycling and >wonder if they might be the cause of the noise? Our antenna is only about >40 feet above the ground (at 7400 ft) while the cell antennas are at about >100 ft. So we are much closer to the cell site equipment buildings than >the cell antennas. > >Two uV for the receiver sensitivity sure does limit the usefulness of the >new repeater - > >Anyone have any thoughts? Anything with a CPU in it could be the culprit @ 2 meters. Anytime I drive within ~200 ft. of a local "Foothill Transit" bus the squelch on my mobile radio blows open; within 50 ft., normally full quieting signals are completely captured by the noise. I suspect it's the electronic scroll sign on the back of the bus. Fortunately I haven't seen this on 440 MHz, but with faster CPUs finding their way into everything it may only be a matter of time. Bob NO6B
Re: [Repeater-Builder] Duplexers
>The noise floor is really decreasing the utility of the new >repeater. The noise source seems to come and go as a quiet signal >on the repeater input can become suddenly noisey, and vice versa - a >noisey signal can become suddenly quiet. It looks like you have a 10dB degradation. Many times this is from phase noise from other transmitters, but intermod can give you the same problems. I would look for a VHF paging transmitter if phase noise is suspected. This type of troubleshooting requires a good spectrum analyzer in most cases. I typically use the RX part of a duplexer on the front end of an ANRITSU 2721B to keep the first mixer in the spectrum analyzer happy. I hook up a 10dBd Yagi to the duplexer and start looking for the noise floor to rise. Once I have found the site that is causing the increase in noise floor, then the hard part comes, getting the other site to cooperate in further testing. What we ultimately want is for the other site to completely shut down for the time it takes to test the sensitivity of the receiver that is experiencing the degradation. Since you mention that the degradation is intermittent, you may be able to monitor other signals and see a correlation between when the suspect transmitter keys, and an increase in the noise floor in your bandpass. If the problem is because of intermod, then it becomes a little more difficult as you have multiple culprits. The spectrum analyzer you use should have a noise floor of around -120 dBm @10 KHz bandwidth. 73, Mark N5RFX
Re: [Repeater-Builder] Duplexers
The 10dBd Yagi I mentioned is for 900MHz, something smaller would have to be used for VHF :) 73, Mark N5RFX
[Repeater-Builder] Re: Lightning Damage, new UHF antenna needed
Hi John, It's on the campus of Southern Polytechnic State University in Marietta, Georgia, about fifteen minutes north of Atlanta on I-75. Jeff --- In Repeater-Builder@yahoogroups.com, "Maire-Radios" <[EMAIL PROTECTED]> wrote: > > We have used a DB-420 450 to 470 on our ham repeater for years with no problem. > > Where are you guy's at? > > John > > > - Original Message - > From: Jeff > To: Repeater-Builder@yahoogroups.com > Sent: Thursday, September 20, 2007 11:30 PM > Subject: [Repeater-Builder] Lightning Damage, new UHF antenna needed > > > Hi all, > > Well, it finally happened. We got hit by lightning. And it blew the > antenna in half on the 444.325 machine here at the college. > > It wasn't a great antenna to begin with, obviously. We'd like to go > back with something much better if possible. We're thinking maybe a > multibay folded dipole antenna of some sort with an omni pattern like > the DB 404, but if there's a better choice we'd love to hear it. > > There don't seem to be a lot of choices in antennas tuned for Amateur > Repeaters. Most of the ones we're seeing are Part 90 antennas > (450-470 MHz). I don't know how much trouble it would be to retune > one or in the case of phased folded dipoles if it's even possible. > > What is the consensus Best UHF Repeater Antenna, considering all the > above (omnidirectional, probably no more than 100 watts)? > > Thanks in advance, > > Jeff/KD4RBG >
[Repeater-Builder] Quantar and PL
We recently replaced an aging UHF machine with a Quantar for a local ham repeater. Now it seems that some users are not able to key up the system. Turns out their radios (all VX6's) have fairly low tone deviation. Tests on the Quantar show that it needs at least 300 htz to key it. This seems reasonable to me but the users all say "Well, my radio used to work with the old repeater. So fix the new one." Is there a way to increase the sensitivity to PL tones in a UHF Quantar? Is this desirable? Al, K9SI
RE: [Repeater-Builder] Quantar and PL
Al, Something doesn't sound right here... most Yaesu portables- including my own VX7- have far too much tone deviation as delivered. This is common with many Amateur-grade radios, and Alinco is the worst. The CTCSS deviation is usually not adjustable in the small portables, so the manufacturers apparently think that more is always better. I don't have experience with the VX6, but I would be surprised if the CTCSS deviation wasn't close to 900 Hz. Perhaps these users modified all of the radios to pad down the tone deviation, but I think that 500 Hz is ideal. I will check my Quantar service manuals at work for confirmation, but I suspect that the tone sensitivity is fixed. I wonder if there is another factor at work here, such as the purity of tone coming from the VX6 radios, and the tone accuracy. Does the Quantar work with other radio brands/models? Maybe it doesn't like raspy tones. 73, Eric Lemmon WB6FLY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Al Wolfe Sent: Tuesday, September 25, 2007 9:24 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Quantar and PL We recently replaced an aging UHF machine with a Quantar for a local ham repeater. Now it seems that some users are not able to key up the system. Turns out their radios (all VX6's) have fairly low tone deviation. Tests on the Quantar show that it needs at least 300 htz to key it. This seems reasonable to me but the users all say "Well, my radio used to work with the old repeater. So fix the new one." Is there a way to increase the sensitivity to PL tones in a UHF Quantar? Is this desirable? Al, K9SI
Re: [Repeater-Builder] Duplexers
At 9/25/2007 07:59, you wrote: > >The noise floor is really decreasing the utility of the new > >repeater. The noise source seems to come and go as a quiet signal > >on the repeater input can become suddenly noisey, and vice versa - a > >noisey signal can become suddenly quiet. > >It looks like you have a 10dB degradation. Many times this is from >phase noise from other transmitters, but intermod can give you the >same problems. I would look for a VHF paging transmitter if phase >noise is suspected. This type of troubleshooting requires a good >spectrum analyzer in most cases. I typically use the RX part of a >duplexer on the front end of an ANRITSU 2721B to keep the first mixer >in the spectrum analyzer happy. I hook up a 10dBd Yagi to the >duplexer and start looking for the noise floor to rise. Once I have >found the site that is causing the increase in noise floor, then the >hard part comes, getting the other site to cooperate in further >testing. What we ultimately want is for the other site to completely >shut down for the time it takes to test the sensitivity of the >receiver that is experiencing the degradation. > >Since you mention that the degradation is intermittent, you may be >able to monitor other signals and see a correlation between when the >suspect transmitter keys, and an increase in the noise floor in your >bandpass. If the problem is because of intermod, then it becomes a >little more difficult as you have multiple culprits. > >The spectrum analyzer you use should have a noise floor of around >-120 dBm @10 KHz bandwidth. The spectrum analyzer method is of course an excellent way to look for interference sources. However, a noise floor of -120 dBm @ 10 kHz RBW is a bit on the deaf side when you consider that your repeater RX will be almost 10 dB better than that. You might consider adding a preamp & an additional pass cavity in front of the analyzer to maximize sensitivity if at first you don't find anything. Bob NO6B
Re: [Repeater-Builder] Duplexers
[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
