Co-locating with broadcast transmitter

[Pump3]

Hey guys
Looking for some thoughts
I have a few repeaters on the rooftop of a hotel, about 70-80 feet on the other side of the roof is a 200 watt FM Broadcaster
I located my antennas as far away as physically possible from the broadcast antenna, but im fairly certain there is still some effect on my received signals, everything works okay, just the receive isnt quite what it should be....

i'm using a Vertex Mototrbo repeater, LDF4-50, and a Sinclair SRL-210-C2 and a Sinclair Q2220E Duplexer on 165/170 MHz

The broadcaster is on 104 MHz with a Crown FM300 Transmitter, set to 200 watts, using RG214 going to a Dominator antenna http://fmbroadcastantenna.com/dominator.html

When operating my repeater in trbo digital mode, quite often ill can be on/near the fringe of the repeater, but in areas where I would still expect the repeater to be usuable, the field units can hear the repeater, they will get the digital talk permit tone, and when the user talks, nothing comes across the talkgroup, but it appears to the user the radios working normally... I programmed the repeater back to analog mode and its obvious that there is some desense going on...

Over the weekend I installed a UHF repeater to do some testing on, I hooked the antenna right to the duplexer in the radio room while I had to leave during the installation to grab something, and it worked surprisingly well for being inside, after I installed the antenna on the roof, about 30 feet from the broadcast antenna, the repeater is completely unusable, tremendous desense, it worked much better inside the attic, I know its too close to the broadcaster but I was only putting it there to do a few days of testing, I do plan on moving it down the building to where my VHF antenna is but even still at the distance of 80 feet from the antenna, I feel like there is still some effect on my antennas, it might not be so bad on UHF once I move the antenna all the way down but I still want to know what some of my options are....

any filtering anyone recommend that I could put on my repeaters or the broadcasters transmitter?

Thanks in advance as always gents!

[Bill_G]

Have you talked to the broadcaster to see if he has harmonic filters on his transmitter? Some of those LPFM guys run barefoot and have no idea they are causing a problem. If he is the incumbent, you may have to pay for any filtering on his gear to get him out of yours. Your Sinclair duplexer is a good quality bandpass type. It should be knocking him down 70db or more. You'll have to do a desense test to measure how much he is clobbering you. You can try adding a pass/reject cavity in your rx line to notch him out. But, you may have to use a crystal filter with a preamp. Since his antenna is in the same plane as yours, you'll have some experimentation to do to eliminate him.

[DavidJ]

70 dB is probably not enough, you are probably just running into front end overload of these receivers, all your symptoms point to that. Modern receivers often have a fairly low third order intercept point and create a lot of hash internally when presented with more than a few milliwatts of energy at the input. Harmonic filters on the transmitter are not the answer, because your frequency is not a harmonic of 104 MHz. A simple quarter wave stub filter has worked for me in similar situations, you can create 30 dB or more of notch at the FM broadcast frequency with a piece of RG-8 and a coax tee, see for example http://www.kyes.com/antenna/coaxfilter.html. Put the tee in the line between the receiver input and the duplexer.

[kcbooboo]

I didn't research the duplexer you're using, but many of the pass/reject models don't have a real narrow bandpass; it's more of a tuned broad filter. You need something that will attenuate or eve notch the FM signal, such as the quarter wave TEE filter mentioned previously. Use a spectrum analyzer or service monitor to tune things up. You may need multiple filters to get sufficient attenuation. A BIG bandpass filter might help if you can find one, but a coax TEE is cheaper and easier.

If you can get your two-way antenna directly above or below the FM antenna, you'll get a lot more attenuation. If it's on the same plane horizontally you're just blasting FM signal into the front end of the receiver and you'll need a lot more filtering.

There's probably already a low-pass filter of some sort on the Crown transmitter, but that's mainly to eliminate the 2nd and 3rd harmonics. Still, there are FCC limits on emission out to +/- 600 kHz, which requires the signal to be at least 80dB down by that frequency. That still leaves dozens of milliwatts of energy legally floating around. 20dB reduces the 200w signal to 20w. 40dB gets it down to 2w. 60dB gets it down to 0.2w. 80dB gets it down to 0.02w (20 milliwatts).

Bob M.

[Bill_G]

70 dB is probably not enough, you are probably just running into front end overload of these receivers, all your symptoms point to that. Modern receivers often have a fairly low third order intercept point and create a lot of hash internally when presented with more than a few milliwatts of energy at the input. Harmonic filters on the transmitter are not the answer, because your frequency is not a harmonic of 104 MHz. A simple quarter wave stub filter has worked for me in similar situations, you can create 30 dB or more of notch at the FM broadcast frequency with a piece of RG-8 and a coax tee, see for example http://www.kyes.com/antenna/coaxfilter.html. Put the tee in the line between the receiver input and the duplexer.

That's good advise, and definitely worth trying. Super easy to make.

As for LPFM harmonics - true enough - no direct hits at 160 from 104 - but being so close to the transmitter, there is still hash in the output that needs to be suppressed. More than once I've found the operators (and site owners) didn't realize what was happening - or they knew but didn't have the cash - and ran without filters after the finals raising the noise floor for all the other users on the site. A friendly helpful hand goes a long ways towards cleaning it up for everybody.

[Pump3]

Thanks guys for the very helpful comments!
What would help more then anything is if the broadcaster only transmitted 50 watts ERP like their license says their limit is......but theres nothing I can do about that and I don't want to make any enemies..... but the station engineer is a acquaintance so theres no problem working with him to resolve this, but it will be on my dime

Unfortunately theres no way to get above or below the broadcast antenna with the layout of the roof, so ill see whats kicking around for bandpass filters but ill probably try the coax tee......thanks for the ideas guys!!

[kcbooboo]

Depending on the antenna gain and feed line, they might actually be running 50 watts ERP, however losing 150 watts seems like an awful lot, assuming the TX is putting out 200 watts. They might have bought a 200w TX and are running it at a much lower power. I've heard that some LPFM stations exceed their licensed value. Friend or no friend, it might be worth pushing the issue.

Bob M.

[Pump3]

No its definately 200 watts, theres a multi function LED screen on the front that can display SWR, Power, PA Voltage, Current Draw, Temperature ect...put it on power and its at ~200-201 watts, where they are located in the building its only about 30-40 feet of RG214 and they are at their antenna....i'd say their ERP right now is atleast 350 watts

As a side note its interesting to feel RG214 that has 200 watts continuously being pumped out of it, depending on what wavelength you feel the coax at there are warm and cool sections of the coax....

[DavidJ]

200 watts transmitter power out, about 1 dB of loss in the coax, and a 5/8 wave j-pole antenna that claims 3 dB power gain at the horizon yields about 320 watts ERP. If their license says 50 watts they have a problem. This is in addition to the problem of excessive VSWR that is causing hot and cold spots in the transmission line -- lots of reflected power to the transmitter can cause any PA to generate spurs. Look at the spectrum and see how much noise is being generated away from the carrier -- there may be bigger problems than just overload.

[Pump3]

According to the Crown Transmitter the VSWR is 1.26, I will definitely take the time to haul the SpectA up there my next visit...

[Bill_G]

If the guy ignores his licensed ERP, then who wants to bet he has no post xmit filtering either? Comb generator. Since you know him, talk to him rather than calling the FCC. Keep the FCC card in your hand though. I don't know how big your market is. But, if he's clobbering you, he's clobbering others too especially when they get at elevation with his antenna.

[tvsjr]

Doing some quick math, by Friis' equation you should be seeing his signal at about +21.39dBm at the base of your antenna. That's definitely going to cause you some challenges.

Have you looked at the RX port on the duplexer to see what the level is like there? Looking at the spec sheet for your duplexer, it doesn't look like there's much out of band rejection, so it's probably not helping you a great deal.

My suggestion would be two-fold. First, add a circulator to the output of the duplexer, with the third port terminated to a 50-ohm load, or use an isolator that has the load built in. This will keep any RF coming back down the coax out of your gear. Then, add a crystal filter on the RX side of the duplexer. I've used the 4-pole connectorized models from MtronPTI (previously Piezo) in the past with great success. Their model 4133 is 20dB down at +/- 30KHz, 40dB down at 50KHz, with an ultimate attenuation of 60dB across most of the RF spectrum. That should be sufficient to knock any harmonics down into the noise, and drop his fundamental to -40. This may give you the improvement you're looking for... if not, you might need an additional notch filter.

Beware, crystal filters are not without their issues... the 4-pole model will cost you about 7dB of insertion loss. You may want to add a preamp as well.

For UHF, similar situation, but you'll need to use a window filter instead of a crystal filter.

This is the right way to do it, but it won't be cheap. Those crystal filters run north of $500, if memory serves.

[Pump3]

Thanks tvsjr, that's good advice, and definitely the "professional" way of doing it, unfortunately there's no affording that right now, what I think i'll start out with is building two coax stubs, one for my repeater -> duplexer rx port to notch his frequency, and possibly one on his coax to notch my input frequency, or possibly a reject can on his coax tuned to my input frequency and a good pass can on my rx leg.....I have to see what I have kicking around....

[DavidJ]

Thanks tvsjr, that's good advice, and definitely the "professional" way of doing it, unfortunately there's no affording that right now, what I think i'll start out with is building two coax stubs, one for my repeater -> duplexer rx port to notch his frequency, and possibly one on his coax to notch my input frequency, or possibly a reject can on his coax tuned to my input frequency and a good pass can on my rx leg.....I have to see what I have kicking around....

There are two things that can be happening, and doing the above will help you understand which it is (maybe both). The situation you describe is not unusual; I have run rs within 80 feet of a 60 kW ERP FM station with no ill effects, so it is doable.

The two possibilities are (a) there's too much broadband energy swamping the front end of your receiver, and (b) there's too much energy on your receive frequency. More bandpass filtering can help (a) if the signal is really getting in via the antenna terminal. Lots of receivers aren't really well filtered and shielded for use in high RF environments, so it could be entering the receiver on power or audio lines, or through the housing. If it's (b) there isn't anything you can do at the receiver; you will need to install a notch on the transmitter.

[Pump3]

Poked around with the SpecA, the noise floor is really bad all over VHF and doesnt start to look normal until about 350 MHz, thats with the antenna right on the analyzer, doing the same test from the duplexer RX port only makes a marginal improvement, so I scrounged up a Sinclair Bandpass cavity and added that between the duplexer rx port and the repeater rx port and it made a huge improvement, completely knocked down the high noise floor I was seeing everywheres

Have the UHF 460MHz repeater running with just a Sinclair MR356 and it works A-1, mounted a SRL-310C4 at the other end of the building near the VHF antenna as far away from the broadcaster as possible, long run of coax but it seems to be working great....will have to do some more testing on the VHF repeater before I can say with certainty if theres an improvement in the field or not....

Thanks for the help guys!

[Wowbagger]

One thing to look out for is front-end overload on your spec-an. Your high noise floor *may* have been due to overloading the front end of the spec an rather than being a real measurement. Not that I am saying a bandpass filter is a bad thing to have anyway, just that your high noise floor measurement may not have been real, but caused by large signals overloading the front end of the spec an, and bandpassing them out gave you a true reading.