Possibly a skip problem?

[djcris]

So, I am fairly new to the radio service field. I have tinkered around with radios and electronics since I was a kid, but no formal training (as of yet). I am a rookie service tech in South Arkansas and we are having issues with a local police dept. They keep having problems with Rx and Tx on their mobiles and portables. There are a few key areas in town (one about 3 miles away from repeater and 1 a few blocks away from the repeater) that have bad reception. They have also stated that they do pickup other depts occassionally.

They are using the same setup as the FD and are both in the same building and on the same tower, same repeater/antenna setup. FD is having no issues, and you can even swap over to the FD freq on one of the PD radios, and it is perfectly clear. I checked both repeaters with a wattmeter at all connections, and are identical. 34w forward power at the antenna coax connection (leaving the repeater room) and 0 reflective power. I swapped over connections so that the FD is using the PD antenna and PD is using the FD antenna (just for kicks) and still having same issues. PD has static and FD clear. so, that made it clear it is not the antenna. I swapped repeater radios around, and still same issue. It doesn't matter if it is a new mobile or an old mobile, and even the same issues using the radio in my van (I know it is installed correctly). PD static and FD clear. And yes, both depts have narrowbanded, and all radios are NB as well.

Is there anything I am overlooking? Any input would be greatly appeciated. We don't learn unless we ask, right?

 

[Bruce42]

Skip Problem?

I have a series of simple questions to aid in finding the problem.

1. What are the actual frequencies? Skip usually just bombs low-band but not normally at the same location each time.
2. Have you checked the noise levels at the bad locations on both the PD and FD frequencies to see if there is a difference? Listened in carrier squelch?
3. Are the troublesome locations near any bank or gas stations? I have heard interference on what I assume were multiples of the microprocessor clock rate from gasoline pumps and ATMs. And it is often only one or two channels wide so it will not affect multiple channels. This is "narrow band" noise.
4. On low band I would look for flourescent lights or pumps or other electrical devices. However, these are usually "broad band" noise and would effect both agencies.
5. Set up a scanner to read out the tones of other agencies being heard on the PD frequencies. This may lead towards more information.
6. Look up the frequency on both the FCC and the RR databases to see what may have been recently licensed on that frequency.
7. A real stretch would be to find the harmonic of the Local Oscillator of an FM receiver on the UHF channel. I have heard this several times and if the LO is being modulated by the music (poor power supply regulation) it can false tone decoders now and then. Example: 104.3 + 10.7 = 115.00, 115.0 x 4 = 460.000 MHz. Because of the normal wide deviation of FM stations, and the poor power supply regulation, the signal can be very wide. Look for background music radios in restuarants, gas stations, stores, etc. I found more than 3 examples of that in my experience.
8. Report back your findings to this forum so we can finish the job and help educate the next chap.

Bruce..... WA6YLV

 

[djcris]

The actual frequencies of the repeaters are as follows: Police Tx 155.490 Rx 159.090 and Fire Tx 154.325 and Rx 154.010. I was over there yesterday with a Sr. tech and we did take the service monitor out all the way around the city limits and the bad spots. (Farthest distance was 2 miles) We were getting a very poor to no signal at most of the locations on the service monitor and no other noise or close signals were present. The FD was very clear and strong at the same locations. Just to entertain a thought we had, we went back to the repeater room and swapped the antenna connections. So now, the PD is using the FD antenna on the top of the tower, and the FD is using the PD antenna just under it. We went back out to the same locations, and both signals are now fairly strong and clear.

I found out that the repeaters were both replaced in January due to lightning. Appearantly, the antennas and coax have not been changed. The only thing me and the other two techs can think of, is that somehow when the lightning struck, it may have altered the tuning of the PD antenna just enough so that it won't work well with their frequency. The PD said things haven't been right since the lightning. All three of us (the techs) recommend replacing the antenna and possibly coax.

Also, we did have a radio with us all day that was set for carrier squelch. Just to see if we ever heard anything that wasn't supposed to be there, and never did.

Does this mis-tuning sound like a possibility?

 

[Bruce42]

Skip or Not?

It sounds to me as though you may have an intermittent in the antenna or feedline system. Although you had commented on swapping them earlier w/o any difference, now you get a difference. This suggests it could even be in the connectors.

I agree with your suggestion to replace the antenna, the feedline, the connectors and the lightning arrestor any time you get a known lightning strike. Of course, here in CA we don't get anywhere near as many lightning strikes on the local stuff but we get blasted on the mountain tops.

On many of our mountains tops we have 120 foot towers but the top of the tower is steel pipe welded to the tower. The highest antenna is side-mounted. This way, the tower takes the hit, and hopefully the antennae and radios are spared.

After you replace the antenna, try it on the ground level for SWR, and then cut it open to see what may have happened inside. I have seen Stationmasters that blew open like a peeled banana before. There may be a minor fault inside that changes with the wind if the top antenna is a 22 foot Super Stationmaster or the like. They can really flex with ice and wind forces and get noisy, if not intermittent. That is why we usually use snubbers on the side mounted high-band gain antennas to prevent the flexing.

Good luck and let me know how it turns out.

Bruce... WA6YLV

 

[djcris]

That was my mistake on the first post. The first day I swapped the antenna feeds around, I did not have a service monitor available to check the signals. Just the test radio I have in my vehicle. So, I couldn't get a true figure as to what the signals were doing. That day, and the few days before it, several people were having issues with skip. The rural fire freq was having bad problems getting a dept that was about 150+ miles away. Happens every fall and spring. We are used to that. A few local radio stations, I was having trouble picking up 30 miles away, and was getting stations I've never heard before.

So, we are convinced it is a mis-tuning issue related to lightening strike (since that is when the troubles started) combined with a little bit of skip that just makes that problem stick out even more. We are working to get a new antenna and feed line work order approved, and go from there.

FYI, these antennas are dipole antennas. PD has 4 lobes and the FD has 8 lobes. I think it is a 50' tower. I will try to post a pic of the set up, if I can. Whoever installed them, did not direct the lobes in a 360 pattern. They are all pointing the same direction. Which, doesn't make sense because the tower is in the center of the town. And, you would want full 360 coverage, not just from the west. Although, the town is only 2 miles East-West by 4 miles North-South.

 

[Bruce42]

think its the antenna

Very interesting. The surprise to me is that the FD antenna on top seems to be OK and the PD below it is the one that suffered. If the lightning took out both repeaters it is not surprising that the antennae may have suffered.

As far as the antenna patterns, I agree that the antennae should be set for an omni pattern. But for a 2x4 mile town and 43 watts to those gain antennae it really shouldn't matter much. I would think that they have portable coverage everywhere in town, particularly since it is repeater operation. You could probably use a 1/4 wave on a plate (inexpensive ground plane) up on that tower for a temporary antenna until the new one comes in and still continue to cover the town.

Glad it turned out to be something solvable rather than the esoteric problems I described. You just threw me off with the mistake on the description of swapping antennae and both worked. Let me know when everything is back to full service. And congratulations on having the courage to ask for help when stumped. Not everyone will admit they need some help. But we all do once in a while.

73's Bruce WA6YLV

 

[BushDoctor]

repeater noisy

check the exciter and the cabinet for antenna relays in use and not needed in repeater operation i have seen lightening strikes do lots of weird stuff in my 35 years of being a mororola field service tech look carefully at the exciter for traces of carbon or tracks and defective components u might have to swap units with ur backup to to determine if it is an exciter or antenna/hardline problem.

 

[motorola_otaku]

The actual frequencies of the repeaters are as follows: Police Tx 155.490 Rx 159.090 and Fire Tx 154.325 and Rx 154.010.

Did your FD used to use 154.37 for the repeater output? I know your town used to be a regular down here in the Houston, TX area but I can't remember if it was on 154.37 or 154.325. FYI, the 154.01 input freq is used down here by Pasadena VFD (167.9) and Angelina County (192.8) as a base station/repeater output, so that may be who you are hearing since the path is fairly frequent.

 

[mastr]

The antennas look like DB228 and DB224 respectively. Mounting with elements pointed toward the tower is said to be omnidirectional. WISCO International (Decibel Products / Dipoles DB228)