Repeater Antenna - Poor Performance

[mm]

Simple solution, it sounds like the bad N connector was on the receive side of the duplexer which would not effect the transmitter output power measured nor the antennas vswr.

A excessive loss on the receiver side would cause a major receiver loss and thereby a reduction of talkin range.

The proper way to test a repeater is with a dual directional coupler in the duplexer output coax going to the antenna, one port of the coupler connects to an RF generator set for the calculated receive sensitivity taking into account the directional couplers port attenuation.

The other port of the dual directional coupler measures the transmitter power, also taking into account the couplers loss on the port measuring the transmitter power.

This effective repeater sensitivity test will show any issues on the antenna, receiver and transmitter components/tuning.

By replacing the antenna with a 50 ohm load also shows the effect of improper duplexer tuning and/or site noise on receive sensitivity.

The difference in receive sensitivity between the 50 ohm load and the antenna should be less than 3 db.

 

[alcahuete]

Does the bad cable have nickel plated connectors? These can create some low level IMD which will raise the noise floor in the receiver and limit range, but its usually not severe like you are describing. It would still be interesting to know if it has nickle plating and the replacement does not as in silver plated or something else.

I honestly don't know. They are not new cables. That's really interesting...wish I knew now.

 

[lmrtek]

I suspect you now have desense due to a poor heliax connection.
You need to do a effective sensitivity test using a iso T.

 

[alcahuete]

Well, I think I'm ready to call it a win. Looks like it definitely was that connector. Tested all day today and things are working great. 30+ miles without breaking a sweat.

Thanks guys for the help and suggestions!

 

[freddaniel]

Problems of this nature are almost always [98%] bad cables or connectors between the radio and duplexer. They do not look bad or test bad using almost any metric, but they allow receiver de-sense to occur. The problem is usually RF leaking to the surface of a cable and entering the receiver side. The plating on the connectors is usually the problem.
I first observed the problem around 1978 when a friend who worked at Disneyland put up a new UHF repeater on the Matterhorn and it barely covered the park. He finally discovered a couple PL-259 connectors from Radio Shack on the jumpers were the problem. Poor plating was the issue.
The jumper coax can also cause the problem if the cable is LMR400 type, or RG8 that is not the best quality or is old. If you ALWAYS use double-shielded sliver-plated coax such as RG214, 223, 400, 55 and use silver-plated connectors, you will always have success.
Many repeaters using less are suffering from undiagnosed receiver de-sense because they work almost OK.

 

[KC4YIN]

Just curious, what antenna did you use for this repeater setup?

 

[alcahuete]

Just curious, what antenna did you use for this repeater setup?

Was a Laird 4507.

 

[Ubbe]

Many repeaters using less are suffering from undiagnosed receiver de-sense because they work almost OK.

Is that common i US that people set up repeaters and they do not measure it's performance?
I had the impression that everybody that works with repeaters, at least myself and everybody I know of, connects a T-led to the antenna coax with a 30-40dB attenuation insert where you check for overall performance in both standby and transmit mode and check for any de-sense problems from your own transmitter or from other sources at that location. You cannot do it any other way or you are just guessing how the repeater are performing. I can't imagine anybody spending resources of setting up a repeater and then just power it up and are done with it.

/Ubbe

 

[jim202]

You didn't say if these were crimp style connectors or of the solder the center pin variety. I never crimp the center pin on any connector I use. I always solder the center pin on the coax cable. That way I know it won't give me any problems down the line. I do like to use the crimp coax connectors. But I also use the correct crimp tool and solder the center pin.

The use of LMR type coax cable is a big no no in repeater use. You may not see any problem when first installed, but down the road you will. The non similar shielding on that type of cable becomes a big diode detector once moisture makes it way into the shield. People will tell you that if you weather seal it up tight, the problem will not develop. But given enough time, moisture will migrate into the shield and cause you problems.
So the best way to avoid the problems with the LMR type coax is to not use it in the first place.

 

[prcguy]

In my experience, most repeaters in the US do not get a final desense check with signal generator into a T connector or directional coupler when installed. I do it but have never seen anyone else do it. I also take everything needed to tune a duplexer when I visit mountain tops to install or repair a repeater. In many cases the repeater can work fine on a test bench and the bumpy ride up the mountain detunes the duplexer.

Is that common i US that people set up repeaters and they do not measure it's performance?
I had the impression that everybody that works with repeaters, at least myself and everybody I know of, connects a T-led to the antenna coax with a 30-40dB attenuation insert where you check for overall performance in both standby and transmit mode and check for any de-sense problems from your own transmitter or from other sources at that location. You cannot do it any other way or you are just guessing how the repeater are performing. I can't imagine anybody spending resources of setting up a repeater and then just power it up and are done with it.

/Ubbe

 

[Project25_MASTR]

In my experience, most repeaters in the US do not get a final desense check with signal generator into a T connector or directional coupler when installed. I do it but have never seen anyone else do it. I also take everything needed to tune a duplexer when I visit mountain tops to install or repair a repeater. In many cases the repeater can work fine on a test bench and the bumpy ride up the mountain detunes the duplexer.

Some do, some do not. I got a talking to when I transferred from one branch to another because I took the time to make those reference marks (for future service) and check the duplexer for desense because I was wasting time...same company, different branches. Another thing I've noticed, techs don't want to carry 50 lb service monitors up multiple flights of stairs...

Most of the jobs I'm around, are warehouse/campus repeaters. For 97% of those installs, they simply don't have uplink signal levels low enough to care if the repeater has 4-6 dB of desense (and I agree, it's not the right way to do things). Also, LMR gets used often because...return business when it fails in 5 or 6 years.

 

[alcahuete]

You didn't say if these were crimp style connectors or of the solder the center pin variety.

They were the good solder connectors. I never use crimp.

The use of LMR type coax cable is a big no no in repeater use.

As mentioned in the very first post, it was 7/8" heliax.

Either way, this was resolved on November 13, as I mentioned above. It has been working great ever since. Had it out to 50 miles the other day. I'll take it.

 

[12dbsinad]

In my experience, most repeaters in the US do not get a final desense check with signal generator into a T connector or directional coupler when installed. I do it but have never seen anyone else do it. I also take everything needed to tune a duplexer when I visit mountain tops to install or repair a repeater. In many cases the repeater can work fine on a test bench and the bumpy ride up the mountain detunes the duplexer.

If any one of our techs did not do a proper set-up of a repeater, they would be fired.