Batboard

I have a desktrac repeater with a small external cellwave duplexer sitting on top of the case. The cables from the duplexer to the radio look pretty darn small. Can I increase the reception quality by replacing the cables with larger cables? Does length matter? How do I select new cables? Did I really just ask if size matters, oh well you get what I mean.

Well if you want to talk vaig and not give any details, your going to get the same kind of answer.

It depends.

If your running this radio on the VHF frequencies, your probably wasting your time on the issue. If your using
this radio on 800 MHz, then maybe.

The concern I would have is you mention that the duplexer is small. This sort of hints that it may be a mobile
duplexer that someone has cobbed together to work in the lash up there. Normal repeaters require at least
70 to 100 db of isolation between the TX and RX ports. The higher the power the more isolation is needed.
Being a desktrac, your talking about being on the lower end of the requirements due to the lower power out.

Without knowing the separation between the TX and RX frequencies, you will not get much of an answer
on if you can improve anything. At UHF, the channels normally have 5 MHz between the RX and TX. At
800 MHz, this is 45 MHz. If your running this on the VHF band, there is no standard fore the separation
and your on your own.

Last item to talk about is the antenna. You can do much better by putting some good coax on the span
between the radio and the antenna. A good antenna will also make a big difference. Again no info on
what you have. One statement I will make is do not use any 9913 or LMR-400 type coax in a duplex or
repeater operation. These type of cables with the aluminum foil and copper braid shielding are well
documented to give noise problems when used for repeater service.

Not much of an answer to your questions, but then again, you didn't give much to go on.

Jim

WCHija wrote:I have a desktrac repeater with a small external cellwave duplexer sitting on top of the case. The cables from the duplexer to the radio look pretty darn small. Can I increase the reception quality by replacing the cables with larger cables? Does length matter? How do I select new cables? Did I really just ask if size matters, oh well you get what I mean.

Mornin WC - RG142 (the quarter inch double shielded brown stuff) with well made connectors is the best you can do with duplexer jumpers. RG214 is the next best choice. Anything else is either overkill with no practical real world gain, or a noise generator waiting to happen. Both RG142 and RG214 are flexible, dependable, noise-free, and easy to work with. Jumper lengths are not critical. 18 inches to 6 feet are normal. You will gain nothing cutting them super short. If you cannot make them yourself, premade cables are available from several vendors on the web.

I apologize for being so vague. I am using 2 GMRS frequncies with the normal 5mhz seperation. I have LMR 600 run to an Antennex FG4605, 20' above my roof line. I am trying to stretch my coverage a little more and noticed that a few areaa that I had coverage now barely works. Where I have coverage the system sounds great. Since the cable is new, the antenna is new, and the duplexer is old and the cables look pretty thin, I thought I would check that angle out. I do believe it is a small mobile duplexer as one post mentioned.

WCHija wrote:I apologize for being so vague. I am using 2 GMRS frequncies with the normal 5mhz seperation. I have LMR 600 run to an Antennex FG4605, 20' above my roof line. I am trying to stretch my coverage a little more and noticed that a few areaa that I had coverage now barely works. Where I have coverage the system sounds great. Since the cable is new, the antenna is new, and the duplexer is old and the cables look pretty thin, I thought I would check that angle out. I do believe it is a small mobile duplexer as one post mentioned.

Did you experience reduced range before or after the new ant and line kit were installed? Is it your talk in or your talk out range (or both) that reduced? Has the duplexer and jumpers been swept? Are you sure the Desktrac is working well? Their RFPA sections are known to fail.

Talk in range is definately reduced, not sure about talk out. The whole system except for the coax went in at the same time and it has worked well since the coax change. Not sure what duplexer swept means.

Ah. So this is a new problem. It used to work well, but now has grown a little deaf. Got it.

The kind of duplexer you have is a notch filter. The rx side notches the tx, and the tx side notches the rx. Sweeping the filter measures how well tuned it is, and lets you adjust it for the best rejection. The down side to notch filters is they allow all other in band freqs to pass with little attenuation. You could be experiencing desense from another xmitter or rptr in the same band. Do you have access to a service monitor to check your direct and effective sensitivity? That will tell you very quickly if your rcvr is working well, if your cables are troublesome, and/or if you have a local source of interference.

I do not. I am going to let a local MSS come out and take a look at it. Is it worth it to put a better duplexer on it? Will I have better reception going into the repeater? GIGO.

The notch duplexer is used by thousands of people for a long time. Proven performer. They just don't function well on a busy hilltop. You shouldn't be hasty in replacing it until you know why you're having troubles.

got it thanks.

Bill_G wrote:The down side to notch filters is they allow all other in band freqs to pass with little attenuation. You could be experiencing desense from another xmitter or rptr in the same band.

If this is indeed the problem the addition of a narrow band pass filter on the rx side tuned to the input freq would probably eliminate or at least reduce
the problem. Of course the usual jumper cabling measurement and type should be observed.

fineshot1 wrote:

Bill_G wrote:The down side to notch filters is they allow all other in band freqs to pass with little attenuation. You could be experiencing desense from another xmitter or rptr in the same band.

If this is indeed the problem the addition of a narrow band pass filter on the rx side tuned to the input freq would probably eliminate or at least reduce
the problem. Of course the usual jumper cabling measurement and type should be observed.

My preference is to change to a pass duplexer - the inverse of a notch. Adding cavity filters to cavity filters is where you start finding out about reactance in real world applications. If you have the test equipment to sweep the entire chain, then you can get away with it. Otherwise, you'll get some interesting results. Critical cable lengths come into play, and the law of unintended consequences starts shining it boot looking for a place to land it.

WCHija wrote: I have LMR 600 run to an Antennex FG4605, 20' above my roof line.

First the Antennex FG antennas just have a 20 gauge steal wire in them. They are very poor antennas on duplex/repeater systems.
The LMR 600 can lead to noise generation due to the different shield materials.

Antenna suggestions? I use this for GMRS.

A folded dipole. Quieter rcvr operation and better xmit pattern. Select gain based on haat.

Any reccommendations on brand or model number? What criteria should I use to select the gain? My HAAT is about 30'.

The Andrews DB404 is a good choice. But, just like the duplexer, don't throw money at the problem until you've identified the source of poor performance. Has a tech looked over your setup yet?

You can test your existing Antennex for physical problems with a simple tap. Key the rptr and gently tap the ant mount with a wooden stick. If you get a lot of noise, check for loose connectors, failed connectors, and a failed antenna.

Those are great suggestions. I had scheduled a tech to come out but my work schedule changed and they had, legitimately, bigger fish to fry so I could not find a time to have them come out. I will try the stick test. In theory, if the system worked fine woudl changing the antenna to the DB404 and the LMR 600 to heliax improve coverage much?

WCHija wrote: In theory, if the system worked fine would changing the antenna to the DB404 and the LMR 600 to heliax improve coverage much?

First try testing the system into a known good dummy load at the output of the duplexer. Then move the dummy load to the end of the LMR600 cable run, in place of the antenna. These tests will show any desence that is due to the cable.

One question, is your desired coverage in one general direction or all directions?
"antenna is 20' above my roof line" What is the overall antenna height above ground?

WCHija wrote:Those are great suggestions. I had scheduled a tech to come out but my work schedule changed and they had, legitimately, bigger fish to fry so I could not find a time to have them come out. I will try the stick test. In theory, if the system worked fine woudl changing the antenna to the DB404 and the LMR 600 to heliax improve coverage much?

In theory, yes. In reality, probably no. What's the difference in performance between a Ferrari and a Corolla in 5 o'clock traffic? Again, without knowing why you are suffering degradation, throwing hardware at it may not address the problem though you might get lucky and replace the right piece blindly. It could be an externality you haven't accounted for like a spur off your laptop display, or your neighbor's new plasma TV, or lucky hot spot from a UHF HD broadcaster, or a sewage lift station PLC, or someone's sloppy ethernet cabling radiating from his new low cost gigabit router comb generator. A better antenna could actually make your situation worse because it would more efficiently transfer the interference to your rcvr. It is amazing how many consumer devices do not actually comply with Part 15.

http://en.wikipedia.org/wiki/Title_47_CFR_Part_15.

Will's suggested test is perfectly valid. I would prefer to do the same test with a service monitor and an iso-tee so you can actually measure the desense. But, his test will tell you if you have a gross problem with your cable and/or antenna. Hopefully, when you do get an opportunity to have a tech check your equipment, he performs this and other tests.

http://www.repeater-builder.com/tech-in ... esens.html