mobile repeater distance problem

lunzac · Post by **lunzac** » Wed Jul 05, 2006 10:57 pm

hey all.... first off let me discribe my setup. it is a mobile repeater setup in my 2000 f150.
2 UHF M120 mobiles on GMRS 650 freqs
6 cavity celwave duplexer
Antennex 1/4 wave OR Maxrad 5/8 wave on NMO roof mount.

problem- i can press tx from a handheld mobile up to 4-6 blocks away and hear the audio on a 2nd handheld. from 6 blocks to about 1.5 or 2 miles i can still key up the repeater and hear the rf carrier, but do not get any audio.

things already checked
-got duplexer professionally tuned by a radio shop.
-SWR is below 1.5:1

just wondering where to start looking for a problem. different antenna? try using 2 antennas and not using duplexer?

thanks

AEC · Post by **AEC** » Thu Jul 06, 2006 12:53 am

I can see your problem right away...

You are using garbage antennas for the repeater....NOT GOOD!

If you must use mobile antennas, at least go with a high gain antenna, say at least 5.0 dB, that will flatten your signal plus allow the antenna 'capture' area to cover that same terrain.

1/4 wave anything is lossy and poor for any sort of repeater, especially a mobile version.

For increased performance on receive, place a good GaAs FET preamp on the RX and tune the transmitter to no more than 10 watts if possible, any more than this, and you will drain the battery, plus you will not see any improvement from that system.

If your truck has the room, locate a commercial mid gain collinear, maybe even a DB408 and mount it in the frame rails of the truck for maximum gain and signal penetration on both TX and RX, plus the height will not be that 'offensive' as it should be stationary while in use.

Most of all....tone both the input and the output of the repeater, you don't need garbage getting in and interfering with your HT as well as any other users that may be operating through your mahine.

Years ago, I built five VHF and UHF mobile repeaters and they worked wonderfully, but I used a G6-440 collinear as the antenna, and ran 1/2" Heliax to the repeater, with a Celwave mobile duplexer.

I also used a dual ferrite circulator on the transmitter side with a 50 watt termination load, and grounded the hell out of it, to the body, bonded the doors and even the cabinet was shielded from every component in the repeater, IDer, Receiver, Transmitter and low power 440 amplifier, then to a shielded space for the duplexer.

All connectors for RF were bulkhead, and cables were attached to every port and grounded, same goes for the audio, control and power into and out of the cabinet.

Feedthroughs were used very liberally as they need NO Rf on them at all, and they also had ferrite beads and toroid chokes on the B+ leads entering the cabinet...NO RF leaked from these 'boxes', so nobody could sniff them out.

Just get a good high gain antenna, that's the first step, and toss out those loaded 1/4 wave maxsplatter radiating dummy loads.

A 1/4 wave is useless as a receiving antenna under any circumstance!

Mobile repeaters are a passion of mine, I'm going to build a few this year and use solar panels with them so they can be instantly set up as emergency repeaters for SAR work, as a loaner for hams on a rescue mission.

AEC · Post by **AEC** » Thu Jul 06, 2006 1:08 am

Aside from the post I made earlier, the reason you can't hear audio from the machine from a distance is that the transmitter is desensing the receiver and the repeater can't 'hear' the handheld any longer.

How much power is this repeater running?

If full bore...drop it!

And as I stated earlier, install a good GaAs FET preamp in the receive line, it makes a world of difference!

Just make absolutely certain not a single milliwatt of RF can get into the receiver, it will blow the GaAs FET before you can unkey.

The same goes for the bare receiver...NO RF can get into the radio, it even a single milliwatt of RF is on the RX input, your receive just died.

The best duplexer is a BpBr(Band Pass, Band Reject) style.

An excellent duplexer I use is the Telewave TPRD-4544, also of the BpBr style, six cavities....-120dBm port isolation, not the skimpy -89dBm you find on cheap cans and mobile duplexers from Sinclair(I had 4, all rotten junk)...sorry to the Sinclair lovers out there.

Real isolation means REAL reception....pin drop or better!

RKG · Post by **RKG** » Thu Jul 06, 2006 5:54 am

I lean more toward AEC's second suggestion; get out the specs for your duplexer and see what they say for max power and, more importantly, separation. If the separation is on the order of 40-50 dB, the max power that you should be Tx'ing is on the order of 25 watts. Also, be sure that the duplexer is tuned for the freqs you're using by someone who really knows how to tune it (duplexer tuning is as much art as science, in my experience).

While 1/4-wave antennas are seldom used for repeaters (since their primary advantage is bandwidth and repeaters are usually anchored to a single freq), we've used them in similar applications (either for appearances sake or, frankly, just because they were handy) with no adverse consequences. However, we use M-branded quarterwaves.

nmfire10 · Post by **nmfire10** » Thu Jul 06, 2006 7:43 am

I find it hard to believe one brand or another is going to make a significant difference when it comes to a 1/4 wave. It's a 6" stick. I could make one with a coat hanger and it would work just as well as the motorola version.

440roadrunner · Post by **440roadrunner** » Thu Jul 06, 2006 8:10 am

your problem may not be with the repeater but with desense at the mobile end. Ask yourself this question---

WHY would you "hear the carrier" and "not the audio"? That relationship SHOULD NOT CHANGE with distance IF YOU CAN REALLY HEAR THE CARRIER

whatnext · Post by **whatnext** » Thu Jul 06, 2006 8:26 am

The problem is the desense on his " 2nd handheld".. He is holding one in one hand and using it to tx while holdiing a second handheld and trying to hear himself thru the repeater.. Lets see 2 miles away from a GMRS portable repeater using 1/4 wave antennas and trying to hear on a GMRS handheld Probably a Maxon.. Good luck..

HLA · Post by **HLA** » Thu Jul 06, 2006 8:44 am

height is your friend too, get that antennae (no matter what it is) as high as possible.

440roadrunner · Post by **440roadrunner** » Thu Jul 06, 2006 4:24 pm

The problem is the desense on his " 2nd handheld".. He is holding one in one hand and using it to tx while holdiing a second handheld and trying to hear himself thru the repeater..

That is what I was trying to say in my roundabout way

RKG · Post by **RKG** » Mon Jul 10, 2006 11:11 am

There is a difference among various manufacturers of 6" end-fed 1/4-waves: the connector button or pin under the cap. Some are far more resistant to aging or losing contact because the stick gets bent or whacked.

That said, my only point was to avoid implying that I had any special knowledge about the particular brand of antenna that the fellow had identified.

lunzac · Post by **lunzac** » Sat Jul 29, 2006 5:24 pm

AEC... where would i go about purchasing a GaAs FET preamp?

440roadrunner · Post by **440roadrunner** » Sat Jul 29, 2006 7:13 pm

Lunzac, before you go around worrying about installing a preamp please RE READ the posts on this thread CAREFULLY