Batboard

Hello, we have a 450MHz simulcast system utilizing mainly older Spectr Tac receivers. Over the last couple of years we have been modernizing the receiver sites with the newer Astro Tac receivers. We have experienced one new problem with these receivers though. Where the Spectr Tacs experience little to no interfearance, the new Astro Tac receivers seem to allow clear reception of various traffic from other emitters. The audio quality of these errant receptions is excellent, almost as if they belong on the system. Looking at the specs of the two receivers the Astro Tac looks better on paper. We have tried in vain to search the software for some way to create selectivity. We have played with the PLL/carrier squelch settings and don't see anything else in the software that could allow reception of errant traffic. We are now to the point where we feel we need to put tanks on each of the channels to isolate them further. The Astro Tac receiver only seems to be manually adjustable at the front end, and detuning it just to reduce spurrious receptions is not the way we want to go. Has anyone else experienced this problem with the Astro Tac (Quantar) receivers?

Thank you

Eric
Houston

Hello Eric,

This sounds like an interesting 450 MHz Astro-TAC Receiver problem. Although you've provided some history on your Simulcast system, I don't see information regarding the receiver sites "if" they are colocated in High RF Transmitter Density Sites with other close frequency (12.5 kHz) either side your Carrier Frequency.

The other thing I'm interested in, "Are your Astro-TAC Receivers being connected to Active Receiver Antenna Multi-couplers?" If so, "Are these Multi-couplers set for specific Amplification Gain Levels or Attenuators for the previous Spectra-Tac Receivers?"

I would like you to clarify whether the T5589A Astro-TAC Receivers are all set to the Carrier Squelch operation mode rather than using Tone or Digital PL Decode on receive?

Does your system require the older 12Kb SecureNet operation?

Do your previous UHF Spectra-TAC Receivers have one of the following:

Model # C04RTB1108A and Serial # 273 or 373ABC1234

Model # C04RTB3108A and Serial # 273 or 373ABC1234

Model # C04RTB6108A and Serial # 273 or 373ABC1234

Dan

Dan,

Thanks for the reply.

Our simulcast system receiver sites in question are located in fire stations, and are essentially isolated from high power transmitters. We have researched the local channels in this metropolitan area and find no users who are within 450kHz, and they are using DPL. The channels are wideband, and we utilize tone decode. Our channels are divided into 8 fire channels and 8 ems channels. We boost the RF input with a small amp with a max gain of 10dbm and then split the signal either with a single 8 or 16 way splitter, or a series of four way splitters depending on the site. (They are not all uniform, and the ems channels are not located at every fire receiver site.)

As for programming, we originally had the Astro Tac receivers programmed for Carrier and PL. We have since reprogrammed them to PL only but this has not cured the problem.

A quick comparison on the bench seemed to indicate both the Spectra Tac receiver and the Astro Tac receiver have similar roll offs on either side of the carrier, and both seem to drop out at approximately +/- 3.5kHz. We did notice that tone falsing is slightly higher on the Astro Tac than on the Spectra Tac though. The Spectra Tac closes squelch at +/- 1Hz while the Astro Tac can often get as far as +/- 2Hz off and still open. Not the most scientific test however.

Our issue is that the problem has been absolutely isolated to these four new Astro Tac receiver sites because we can isolate at the comparitors. The signal quality is ideal, and the users can key up and carry on both sides of the conversations. It is not isolated to one entity either, as some of the traffic appears to be police car to car communication, while another common entity is a spanish speaking company of some sort. There is the posibility that it is environmental in nature, as the problem seems to happen at dawn or dusk, and one of our technicians claims the police traffic indicated it was from a town three hours from here. We have toyed with the idea of placing tuned cavities on the affected channels (isolate to a couple of our fire channels in particular) but the Astro Tac rolloff doesn't seem to be limited by much when using one.

Any help you Moto experts can offer is greatly appreciated.

Is this other police department and whoever los mexicanos are far away but on the same frequency but different PL? Or are they on a totally different frequency and different PL?

If it's the same frequency, the having it come and go with darkness isn't unusual. Why it's breaking your squelch without a valid PL however is not normal.

Different freqeuncy & different PL, I suppose it could be a harmonico f some sort but that stuff is way over my head so I won't even try to give you advice on it.

That's the problem, we are unable to narrow it down. We really need to stick a spectrum analyzer at the site(s) and monitor that traffic. We are unable to determine who exactly the extra traffic is, so we don't know what the carrier or PL is. And we have been unable to get to a site quick enough to catch it. Once on air the traffic can linger for thirty minutes or more, but getting to one of the remote sites quick enough at that time of day is difficult. We really need to do a better job of detecting the emission and analyzing it. Short of sticking a guy at the site with a bedroll and pillow though...

Hello Eric,

Where are these four (4) Astro-TAC ATAC Antenna receiving sites located in reference to your Simulcast transmitting sites; co-located, within 10 air miles or within 20 air miles and in which direction North, South, East or West from the center of Houston?

Are these four (4) Astro-TAC Antenna receiving sites located on high ground elevation and/or mounted on tall towers?

What is the Above Sea Level A.S.L. and Height Above Average Terrain H.A.A.T. for each of the individual antenna sites?

Are these four (4) ATAC brand new Antenna receiving sites that were incorporated into the Simulcast Radio System or Did they already exist?

What type of UHF Omni-Directional Gain Antenna is mounted out at each receiving site: +3 dB, +6 dB, +9 dB or +10 dB?

What Tone PL Code or Frequency in Hz are you using?

Since your System Service Technician is aware of the Police Department Voice Traffic being from a town within a 3 hours drive time from Houston. Then, What is the distance in Air miles from that town to each of the individual four (4) ATAC Satellite receiving sites?

Does your System Service Technician know the RF Channel Frequencies for Transmit & Receive of the other town’s Public Safety Base Station and Subscriber units?

By the way, What are Houston’s Public Safety RF Channel Frequencies for the Fire and EMS service being interferred on?

Did your Public Safety Department hire a Creditable Radio System Engineering Consultant Firm prior to this Simulcast System Up Grade?

Is Houston’s Public Safety Fire and EMS Simulcast Radio System, Self- Maintained (by your own radio maintenance technicians) or Maintained by /\/\ Field Service Technicians & Shop?

As nmfire10 has pointed out, some of these interfering signals could be coming from Mexico. Okay “if” we assume these signals are from Mexico then not only would you need a Service Monitor with a Spectrum Analyzer display but also a Yagi antenna to start Direction Finding DF the interfering signal for a better signal heading.

Dan

Excellent questions Dan. The answers are varied and will take time to compile in detail.

Does this inteference happen on the same RF channel/frequency? Same site or more than one site?

Most receivers are set to AND, requiring BOTH a vaild PL and carrier squelch opening to un-mute the audio. The other half, is setting the muting to OR, loss or either PL OR carrier.

You could try a 6 db pad in the line to the paticular receiver to evaluate the possible receiver generated mix or intermod. A 3 or 6 db pad will drop the mix or intermod by a factor more than two times greater than the attenuation.

There are a lot of 'dirty' transmitters out there too. Just a little spur, your good receivers and you hear it.

Perhaps get a Spanish-speaking individual to listen to the interference (or a recording of it) to narrow things down. It's possible it could even be a dirty amateur radio or even a bootleg business FREEquency used nearby. Many of our local department stores around here use frequencies that a MSS blew out of their behind. It woiuldn't be the first time! If you get somebody very local in the neighborhood running scabbed frequecies (such as security, plant maintenance, trucks carrying illegals in, etc.), the source of interference may be right under your nose in plain view. Hopefuly, the source is in the USA. If en Mexico, you'll be S.O.L. and have to deal with it on a technical level rather than a legal one.