Batboard

Hello all again;

Have a question for the RF guru's that I just can't seem to answer myself.

I have a four site voting system with one of the sites (main tx site) with a co-located RX'r that is located at a high spot within the district. I would like to get the best performance out of my lower sites by adding receive bandpass filter/pre-amp assemblies (angle linear) at each receive site. My other thought is that I will not install one on the main high RX site, as they operate in the T-Band and are susecptible to alot of interference, so I don't want to beat the RX of the main site up and cause voting issues.

Can anyone see an issue by doing this? I don't feel that it will negatively affect the "voting characterisitics" of the audio that is processed by the spectra tac. If the signal is good and clean getting into the south RX site, than it votes there, regardless if the amps are in play? at least thats my thoughts.

I have always used Angle linears three filter preamp assembly on almost every analog repeater that I have installed (I know what you are thinking). The assembly and amplfier works great, and it really makes a huge, huge difference in RX performance. In some many words it makes system pop and come alive. I obviously do some site RF recon prior to install to make certain that I am not going to amplify any offending stray signals on the input, etc.
P.S. I use these on my wide area Digital system and it makes a huge difference...


Thanks, Rob

Mornin Rob - The purpose of a voting rcvr system is to improve the reciprocity of the inbound and outbound paths by filling the under served rx areas. There comes a point of diminishing return where the rcvr areas overlap. As more rcvrs serve an area, the less likely additional gain will improve performance. Low sites are more subject to fade than high sites, especially low sites below a well developed tree canopy, or within a dense multiple story urban area. High gain will cause inappropriate reception of "lucky shot" multipath making line equalization to the voter critical. So, you need to have a very good understanding of the system footprint before adding rcvr gain in an attempt to improve things.

Bandpass and pass-reject filters are always a good idea at a high site since they are subject to local desense from co-located xmitters (both present and future), and their antennas might be in the beam path of a nearby site. However, they may be an unnecessary expense at a low site. The rcvr may already have sufficient rejection built in. Adding gain to make up for insertion losses is prudent, but again may be unnecessary if the rcvr areas have a large amount of overlap. The losses may improve your noise floor while increasing your rejection. These are all considerations of system design. Sometimes it is best to build all sites the same to keep your assumption about each site the same.

You should download Radio Mobile, and model the system coverage to see if adding preamps will help, or just spend money.

http://www.cplus.org/rmw/english1.html

OR - If you send me system details, I can produce some coverage maps for you.

Don't overlook the possible use of tower-top preamps. They newer versions are very reliable, can be specified with very tight filtering, and also make up for any losses in the cable between the receiver antenna and the receiver equipment.

If you have concerns about co-channel interference, I would look to pair this with the use of antennas with a degree or 2 of downtilt so the maximum gain is below the horizion.

Hello all,

We do have "some" overlap in the system, but it is not tragic. Our interference doesn't exactly come from other T-Band LMR stations, but rather broadcast TV from PA and NJ, it just is a massive signal and beats up the front ends. The main TX and RX site is on 300' of earth and 120' of tower in a district that goes from almost sea level to over 310' of elevation like five times from north to south of the district.

Bill,

I understand what you are saying about the multi-path issue. What I am maybe not explaining fully is that lets say for example, you are operating in the south end of the district, you would mainly vote out of our south rx'r and would sporadically handoff to the central or high RX/TX site, and back and forth if you were moving around on portable.

Now, (temporarily deleting the voting system) I know that if I install a filter/preamp assembly on the south site that I will increase the coverage in the south side of the district, and where I was marginal but usuable into that site (but not to the main RX high site) that I will be almost full quieting with the pre-amp. So how does that change things in the grand sceme of the entire voting system other than it will vote more on the south site if I increase the " full quieting = clean" audio into the south site?? hope that makes sense.

P.S. This was the voting system we were discussing about two months ago.. Since installed it sounds awesome, and I am super happy with the performance of the system. Switches sites flawlessly, has great audio (even for a 12.5 system, argh) and it turned out to be a bad console priority and SQM module. And to boot, I am mixing RTNA and microwave circuits, and have a co-located RX'r with the voter and I had no issues with line levels or the equalization of them. People talk about needing the roofing filter or caps to attenuate high end audio, but I found with the built-in line equalization settings of the astro tac rx's it quelled that need, so proof is in the working product, LOL.

Thanks, Rob

A few thoughts come to might that you might consider. One would be to put an ISOTEE into the system and/or read the site noise floor at the coax or after the receive cans. This has the benefit of establishing a point for determining improvement. If you have a receiver that opens at -130 dBm, and you have a site noise floor of -100dBm, there's 30 dB of dynamic range that your receiver is unable to take advantage of. It can't use it because the noise level is above the signal level. Another thought would be to re-position the antenna on the tower to place the -2dB null or so in signal toward the source of interference -- the T-band television transmitter. By favoring the pattern away from that transmitter, you'll be improving your present receive dynamics.

Another thought is to disregard the main receiver altogether. I know of at least one ham system that uses a single paging transmitter with an aperture-coupled cavity centrally located between four receiver sites. He doesn't have a voter or dedicated links ("user-steered voting" if you will), but the system has exceptional coverage given our terrain. This allows you to retain the minimum amount of filtering cost at the transmitter site, and deploy that into your receiver sites.

Excellent advise Bob, and I completely agree. You need to know each site's effective sensitivity before adding preamps. If the noise floor is much higher than the rcvr threashold, you'll gain nothing. An amp may even make things worse. In some severe cases I've improved performance with attenuators.

Note to Rob - sorry I missed your question above last week. With so little overlap in your system, each rcvr becomes critical. Rcvr coverage overlap is good. None is bad, and suggests the inbound and outbound paths are unequal for a reason. Whether it's a matter of desense at one or all rcvrs, or that the satellite rcvrs are inappropriately low is hard to say. Perhaps there are no other choices for better rcvr locations. ie: The main site is on a hill, but all the rcvrs are in fire stations that happen to be at the lowest elevations thus yielding very small footprints. I understand your description of the TV "wind" (as I call it), and have battled it myself many times. Cavities with tight skirts may be your best friend here. Adding elevation may be more helpful if that is possible. Adding gain may work against you. I would need more detail to understand the system better.

I think I understand what you are trying to do: Improve the performance of the satellite receivers without screwing up your voting.

All considerations of noise floor and type of pre-amp aside, if adding pre-amps does improve site performance you may have mixed results. If your voter likes site 3 (for example) it may vote a signal on it even though site 4 (or 1, or 2) is a better choice. Before the pre-amp, site 3 may have never heard the signal in the first place, or it was so poor that the voter wouldn't have considered it. That's what you are going to run into, and you will have to be super critical equalizing your voter channels or you are going to have miss votes.

As far as not having overlapping sites, Bill_G is exactly right. There is no redundancy. Lets say you have a lightening strike, or a backhoe digs up your fiber, or a trash truck backs into your guy wires and pulls down the tower while emptying a dumpster, or a idiot decides that your microwave waveguide would make a great rifle target, or a million other possibilities that takes down a site - then you have a big hole - that would be made smaller if your other sites had improved performance and were able to fill in where before they wouldn't have even heard a signal in the first place.

Sounds to me like the potential positives outweigh the potential negative (which is solvable).