Identifying unused antennas at tower site

[cpg178]

Recently set up a MTR3000 VHF Repeater in my area, I was able to save on antenna and coax costs due to the site having many abandoned antennas and coax feeds up the tower. The site owner informed me that I am free to use anything that is abandoned in place. I was able to positively identify a PCTEL MFB1500 series antenna with 1/2 heliax coming into the transmitter building that I am currently using with decent results, however there are multiple abandoned station masters and even a 2 bay dipole that are on the top of the tower that I would prefer to use as the current antenna is only about halfway up, and has zero gain. Some are just sitting there with no coax which makes it easy but others have coax which descends the tower in an unmarked spaghettied mess that's all tangled together.

My issue is that there are other live repeaters on site, I obviously can not disturb or interfere with any working equipment.

What is the best way to figure out what's what? Unfortunately, the site is nowhere near the model for neatness, there is only one coax feed that is labled out of the about 20 others, so its just a spaghetti mess so simply doing a visual trace is pretty much out of the question. I would love to just take my current coax up to the basically brand new VHF Stationmaster on the top of the tower, but its about 50ft too short and id like to use something already existing before buying new coax and either doing a brand new run to the repeater (~150ft) and having to terminate hardline. This is my first time doing this so any advice is appreciated.

 

[K4EET]

If you know which coax runs are not being used, put an antenna analyzer on it and sweep it. That will quickly identify the frequency band that the antenna is good for. Further testing could reveal the coax length, coax condition, and more. While you are at it, label the tested runs for others to know when the next person asks.

 

[freddaniel]

YES, sweep every unused coax in the building with an Anritsu 331 antenna analyzer or equal. It will show the operating frequency and provide a good indication of the coax condition using TDR function. Then attach a tag to each cable with the results. Otherwise you will need to trace each cable and mark them with something like three bands of colored electrical tape at each end. Like Y-Y-G for example. Twenty trips up or down the tower for 20 cables. Maybe the site owner will exchange site rental for time spent organizing his feed lines. I know it sucks, but someone needs to do it.

 

[prcguy]

Most antenna analyzers will not work at a repeater site due to massive amounts of RF at the site. An Antitsu 331 might, or maybe not. A cheap cable tracing toner might have some luck and I’ve used one on my home tower with ok results. With that you would connect the tone transmitter to an unknown coax then identify it with the tone receiver as it goes up the tower following it to the antenna. Since it operates at audio frequencies it’s less susceptible to high RF levels.

 

[prcguy]

Most antenna analyzers will not work at a repeater site due to massive amounts of RF at the site. An Antitsu 331 might, or maybe not. A cheap cable tracing toner might have some luck and I’ve used one on my home tower with ok results. With that you would connect the tone transmitter to an unknown coax then identify it with the tone receiver as it goes up the tower following it to the antenna. Since it operates at audio frequencies it’s less susceptible to high RF levels.

As an example of doing VSWR tests at a repeater site, the last repeater site I was at had about 10 watts of RF flowing down the coax from someone else’s transmitter picked up by our repeater antenna. That will smoke many antenna analyzers and at the least it will make it impossible to take any measurements. The backup plan is using a Bird wattmeter taking forward and reflected power readings, but when the meter shows 10 watts reflected with your transmitter off that method is out the window for any precise measurements.

I remember at least two other sites I’ve worked at with about 10 watts of power coming down the coax, so it’s not a unique problem.

 

[mmckenna]

I think the risk with sweeping the antenna is that it might tell you where the antenna is resonate, but that's it.

You'd want to know which antenna you were using. Was it a crapped out VHF antenna part way up the tower on crappy coax, or was it the good multi bay antenna at the top? Resonance alone won't give you what you desire.

Look for colored tape on the coax and see if it matches, look for notes left behind, or if you must, send a tower monkey up to figure it out.

 

[jeepsandradios]

Doing a full sweep of the antenna with a quality instrument will get you in the ball park. Return Loss, DTF and SWR. A drone will help with an experienced operator and a 107 under his belt. The only 100% accurate way is to get a guy to climb and map the site. Ive run into this for sites I've installed SAR stuff. Most of the time doing my sweeps, a little investigations with binoculars and or a drone and I've been pretty successful. Of course less lines means easier to find but if the tower is loaded it still gets tricky. If you do determine a antenna and location LABEL LABEL LABEL. If it was done correct early on you can be lucky.

 

[cpg178]

Thanks everyone for the advice, once spring comes we will get someone who can climb the tower and assist with this task, for now it works well enough.

 

[speedway_navigator]

My copper mountain VNA has a 23 dBm limit, I give an example in my ETA class showing overload could be a problem.
One of my students who is on this forum saw this excess power on a site he was working on just a few days after taking my class.

 

[rescue161]

Also, make sure that you do not use an antenna designated as a receive antenna for transmitting. What I mean by this is that a lot of commercial sites will have receive antennas that are tied in to multi-couplers at one height and transmit antennas tied into combiners at a lower height. If an abandoned antennas is at the same height as the receive antennas and you transmit on that antenna, you can desense all of the other in-use receivers. We have alarms set up for that at our sites and if we see that and can confirm that someone transmitted into an antenna that is near our receiver antennas, they will be promptly removed from our sites.

Best bet is to look at a site master diagram. The best case is that there is a vacant antenna at the top that you can use for receive and a lower antenna to be used to transmit. Then you could put multiple repeaters on those antennas using a combiner and receive multi-coupler.

 

[kayn1n32008]

A DTF instrument is your friend. Your other friend is deductive reasoning and elimination.

If you have multiple antennas at the same elevation and are resonant on the same band, you are going to need to climb and tone out the feed lines.

 

[KevinC]

Another old guy story...

I was testing and turning up a 700 MHz trunked site, the antenna system was already installed and not my responsibility at all. I did a quick sanity sweep on the TX and RX antennas and they both just barely passed at 700, but failed at 800 and were supposed to be broadband. I told the customer I couldn't turn it up like this and they needed a climb to do further testing. A few weeks later I returned to troubleshoot with the tower crew. First thing they did was confirm the model number, it turned out to be a 155'ish antenna. I guess harmonics let it barely pass in the 700 range.

So just sweeping to figure out the band of an antenna doesn't always work.

 

[cpg178]

Also, make sure that you do not use an antenna designated as a receive antenna for transmitting. What I mean by this is that a lot of commercial sites will have receive antennas that are tied in to multi-couplers at one height and transmit antennas tied into combiners at a lower height. If an abandoned antennas is at the same height as the receive antennas and you transmit on that antenna, you can desense all of the other in-use receivers. We have alarms set up for that at our sites and if we see that and can confirm that someone transmitted into an antenna that is near our receiver antennas, they will be promptly removed from our sites.

Best bet is to look at a site master diagram. The best case is that there is a vacant antenna at the top that you can use for receive and a lower antenna to be used to transmit. Then you could put multiple repeaters on those antennas using a combiner and receive multi-coupler.

Yeah, there is no site master diagram HAHA! This site is very poorly kept, and there is no RX antenna systems either. The active stuff on the tower is like 2 or 3 transportation agency repeaters (think school van services) that's about it. There is a HAM Radio presence at the site but they are on another tower adjacent, they actually have an impressive setup using a separate RX and TX antennas, but are running the RX through a massive bank of cavity filters and everything looks to be done properly, cant say the same for the other equipment in there!
I've taken time to look at the R56 manual and am making sure everything I am doing is done properly, it was a big investment into this equipment, the last thing I need is to have it get fried come the summer storm season.

 

[cpg178]

While a bit off topic, I did do some coverage testing and the portable talk in is quite poor. This was expected as when I was speccing this out it is for mobile coverage, the portables have no issue hearing the repeater but have a hard time getting in. Would a pre amplifier of sorts be of any help? I am using TRBO mainly, but have been testing with analog as I yet to have internet at the site so I cant hook up and get RSSI readings from RDAC.

 

[xmo]

"portable talk in is quite poor. ....Would a pre amplifier of sorts be of any help"
-----
Maybe, but doubtful on VHF.

What are your commissioning test results, effective sensitivity, etc?

If you have any questions about how to perform those tests, see the attached:

 

[cavmedic]

My copper mountain VNA has a 23 dBm limit, I give an example in my ETA class showing overload could be a problem.
One of my students who is on this forum saw this excess power on a site he was working on just a few days after taking my class.

I resemble that comment

 

[Ubbe]

he portables have no issue hearing the repeater but have a hard time getting in. Would a pre amplifier of sorts be of any help?

The MTR3000 doesn't have any duplexer of its own. What are you using and what power output are you using and what duplex spacing?

If you can follow xmo's pdf instructions and check receiver sensitivity directly at the MTR3000 and then from the antenna coax, to see that you do not have excessive losses, and then have someone transmit with a weak signal and you listen to the RX at the site and disable the TX and enable again while listening, you should have no noticeable change in reception. There can be intermod, SWR and other issues when using the live antenna so always do a live test that doesn't need any expensive instrumentation to do, just a hand from an extra helper with a radio transmission that are weak.

I dealt a lot with MTR2000 VHF and I wouldn't expect MTR3000 do be any worse. It was extremely reliable, never a component failure and the receiver was very sensitive and could take a lot of RF power without degrading. I know that external preamplifiers have been used with other VHF repeaters and worked well. When the noise level increase almost in a linear fashion when the signal degrade it instead stays more noise free until at the squelch level when the signal more or less vanishes at the same time as without a preamplifier. It can reduce noise but does nothing for the level where the squelch operates. It's dependent of the RF noise level at the site and of course the quality of the amplifier and how much RF the duplexer are passing out its RX connector.

/Ubbe

 

[cpg178]

The MTR3000 doesn't have any duplexer of its own. What are you using and what power output are you using and what duplex spacing?

If you can follow xmo's pdf instructions and check receiver sensitivity directly at the MTR3000 and then from the antenna coax, to see that you do not have excessive losses, and then have someone transmit with a weak signal and you listen to the RX at the site and disable the TX and enable again while listening, you should have no noticeable change in reception. There can be intermod, SWR and other issues when using the live antenna so always do a live test that doesn't need any expensive instrumentation to do, just a hand from an extra helper with a radio transmission that are weak.

I dealt a lot with MTR2000 VHF and I wouldn't expect MTR3000 do be any worse. It was extremely reliable, never a component failure and the receiver was very sensitive and could take a lot of RF power without degrading. I know that external preamplifiers have been used with other VHF repeaters and worked well. When the noise level increase almost in a linear fashion when the signal degrade it instead stays more noise free until at the squelch level when the signal more or less vanishes at the same time as without a preamplifier. It can reduce noise but does nothing for the level where the squelch operates. It's dependent of the RF noise level at the site and of course the quality of the amplifier and how much RF the duplexer are passing out its RX connector.

/Ubbe

I am using a TXRX 4 Can duplexer and my spacing is 5mhz on VHF, I am currently pushing 100w from the repeater, but I plan to dial it back a bit as it is not needed to push that much output power.

 

[Ubbe]

I am using a TXRX 4 Can duplexer and my spacing is 5mhz on VHF, I am currently pushing 100w from the repeater, but I plan to dial it back a bit as it is not needed to push that much output power.

Looks to be an extremely good can filter with Vari-Notch that gives more than 85dB isolation. Those notches can be very narrow and then be temperature sensitive that requires fine tuning at the site if the temp are different than at the place they where originally tuned.

You probably need a signal generator to measure sensitivity directly to the coax connector to the antenna and then using a 50 ohm load and T-sniffer to insert a signal thru a 30-40dB attenuation and set the signal generator to the same 12dB Sinad level, just when you begin to hear gunshots in the audio. Then connect the antenna coax and it should not degrade in sensitivity or there are an external interference. Then start the TX power to check that the antenna doesn't act up, the more power the more are the risk to interact with something at the site. Compare how it works with a dummy load to check the can filter tuning. There are many cases where water intrusion in the antenna creates havoc. Professional radio technicians always installs a new antenna and don't reuse old ones, those can be donated to HAMs.

If you got 5W portables then run 25W from the repeater and if mobiles have 25W then run at least 50W. Mobiles and portables move in environments that could have a lot of interference from electronics and they usually doesn't have as good receiver and filters as the repeater have, so repeater output should always be higher to overcome that higher noise level.

/Ubbe

 

[cpg178]

Looks to be an extremely good can filter with Vari-Notch that gives more than 85dB isolation. Those notches can be very narrow and then be temperature sensitive that requires fine tuning at the site if the temp are different than at the place they where originally tuned.

You probably need a signal generator to measure sensitivity directly to the coax connector to the antenna and then using a 50 ohm load and T-sniffer to insert a signal thru a 30-40dB attenuation and set the signal generator to the same 12dB Sinad level, just when you begin to hear gunshots in the audio. Then connect the antenna coax and it should not degrade in sensitivity or there are an external interference. Then start the TX power to check that the antenna doesn't act up, the more power the more are the risk to interact with something at the site. Compare how it works with a dummy load to check the can filter tuning. There are many cases where water intrusion in the antenna creates havoc. Professional radio technicians always installs a new antenna and don't reuse old ones, those can be donated to HAMs.

If you got 5W portables then run 25W from the repeater and if mobiles have 25W then run at least 50W. Mobiles and portables move in environments that could have a lot of interference from electronics and they usually doesn't have as good receiver and filters as the repeater have, so repeater output should always be higher to overcome that higher noise level.

/Ubbe

Thanks for the advice, I am going to turn the power down for now and see if that has any improvements until I can get the proper equipment needed to the tests recommended. I am finally getting a network connection tomorrow, so I can finally use RDAC and get some measurements as well too.