Antenna tech question

[Rudy3145]

Here's the scenario. Our district covers about 30sq miles with 1 UHF Repeater site. 100 watt Radio with a super station master 10db gain antenna. We recently built a new tower. Same repeater is being used but we have new cable and new antenna (same model). But now were about 40ft higher. You'd think we get better with receiving and transmitting but it had in fact been not as good as our old tower which was 40ft lower.

Were stumped. Any suggestions?

 

[rescue161]

Was a new duplexer used? Was the same type of feed line used? What type of connectors were used?

If it is a new duplexer, then I'd suspect it as not being tuned properly. If you were using hardline before and now are using coax (LMR400), then there will be degraded performance, especially with the increase in length of feed line.

 

[UberGeek]

Here's the scenario. Our district covers about 30sq miles with 1 UHF Repeater site. 100 watt Radio with a super station master 10db gain antenna. We recently built a new tower. Same repeater is being used but we have new cable and new antenna (same model). But now were about 40ft higher. You'd think we get better with receiving and transmitting but it had in fact been not as good as our old tower which was 40ft lower.

Were stumped. Any suggestions?

Is the cabling and connectors of a lower quality, or have higher losses?

Unless the cable used has less or the same loss per foot at the frequency, you'll see that.

So, the question is: What was the old cable spec, and what is the new cable spec? And, what was the feed line length in the old install, vs the new install? Longer feed line = more loss, especially at UHF.

 

[Rudy3145]

Our tech thinks were overshooting our coverage area (30sq miles, really??) with the 10am gain antenna. But here's the thing, with testing with radios the coverage is worse far out all the way into coverage area. So in my opinion the overshooting is out the window. Coverage in our area is not horrible (were dealing with it for now) but there is still some static in the immediate area. 1st tower was a telephone pole, 10db super station master was on top around 100ft. New tower, antenna is on top at approx 140ft. Same 100 watt MTR with decent brand new cable and new same model super station master 10db gain.

 

[zz0468]

There's far too many variables involved to offer a solution here. But we can suggest some measurements to make that would lead you in the right direction.

First off, I am aware of some cases (as in, more than one) of brand new 10 db antennas being bad, right out of the box. So, just because the antenna is new, doesn't mean it's working right.

Next, do a swept return loss measurement and TDR of the cable. This will show any anomalies in the cable and connectors.

Next would be to measure the actual loss of the entire cable run.

Next, find some places outside the far field of the antenna, a few miles away would be good, that's relatively free from obstructions. Calculate the predicted field strength that the system SHOULD be putting out there, and compare to what's actually measured.

Finally, getting back to the antenna, be aware that just because it looks good when it's swept, doesn't mean it's radiating in the direction you expect. It could have significant uptilt or downtilt that would produce the symptom you describe, but could look just fine when you measure return loss.

Your tech is going to be very busy.

 

[Rudy3145]

thanks for the input. funny enough, the 1st brand new antenna that went up there was bad. So we got a new one under warranty. The 2nd one was tested and found to be ok out of the box.

 

[rfradioconsult]

Any difference in your physical tower location, did you run a coverage plot for the old and new location?

 

[Rudy3145]

New tower is about 10 feet away from old one

 

[prcguy]

A 10dB antenna with no down tilt is fine up to about 1000ft above average terrain before you need to consider overshoot. There are countless 10dB UHF sticks with no downtilt on 6,000ft to 9,000ft mountains in So Cal and they work great with no dead spots out to 75mi and more.

Is the new antenna at the very top of the tower and unobstructed or mounted down alongside a metal part of the tower?
prcguy

 

[Rudy3145]

At the very top position

 

[prcguy]

Then everything should be suspect and carefully checked and don't assume anything is perfect.

Last time I checked out someone else s repeater that was having problems I found one of the female Heliax connector center contacts splayed apart and probably shorting, a duplexer that was somewhat mis-tuned and some other problems.

I would start with things that have changed like the new feedline and connectors looking for obvious screw ups then measure the loss from the radio end to the antenna end. You can do that by measuring the transmit power at the transmitter, then attach the feedline and measure power at the antenna connection with a load. Check with all jumpers attached, one messed up connector could be at fault. Then calculate the loss of the entire feedline run and compare to published loss specs for your length of cable.

Its good to bench check the repeater with duplexer connected for receiver sensitivity with the transmitter on and off using a service monitor. That will tell you the overall sensitivity of the system and if the duplexer is mis-tuned the sensitivity will be affected when the transmitter comes on.
prcguy

At the very top position