Batboard

Help!

I'm getting killed by VHF adjacent channel interference! I'm working in a system that uses 155.205, bordering a system that uses 155.220. To add insult to injury, both systems are fairly large, and we are in a very small overlap area, so nobody is going to be changing their entire system just to keep me happy. Last, but not least, both frequencies are using the same TPL tone.

I can shrug and live witht the occassional splatter on the mobiles and portables, but it's a real PITA on the bases and repeater, where it comes in full quieting.

Obviously, I'm looking at some sort of pass or notch cavity, but with the spacing that close, I'm not sure just what it will take. Any input or suggestions on solutions would be greatly appreciated.

Thanks!

Change your PL. Then start the process of narrowbanding your system.
You asked for a solution and these two are the best ones unless you wish to consider a third.

Change your frequency.

Changing your PL will not make your "adjacent channel" interference go away. It will simply keep you from hearing it. Narrowbanding will give you more relief, because you will be making the "window of interference" smaller. If the other guy can be persuaded to narrowband that will help matters even more.

You can play with filters and other fixes until "the cows come home" but your solutions are above.

Sadly, as mentioned above, both frequencies are used on a county-wide basis by multiple agencies.

The odds that either county is going to completely rework its system because we are being inconvenienced are exactly nil.

So filters, or continue to live with it, it is...

I hate to put it in these words, but it seems like a classic case of some dumb :o head in charge of the communications that doesn't know their head from their ass.

No one in their right mind would ever build a system on an adjacent channel to another sytem. One of them was there before the other. The last one in should have know better. Kind of strange that the frequencies would have even been approved by the frequency coordinator.

It would seem that there are few choices in this case. I would start by removing the SOB in charge. After that there are only 2 choices. Move the system to another frequency pair or live with the problem. Start with who was there first.

Crystal filters on the receiver won't work as the frequncies are too close to each other.


Jim

HumHead wrote:Sadly, as mentioned above, both frequencies are used on a county-wide basis by multiple agencies.

The odds that either county is going to completely rework its system because we are being inconvenienced are exactly nil.
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So filters, or continue to live with it, it is...

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You asked for solutions. If filters were a solution, they would have been offered to you. Filters are NOT a solution. Don't let someone take your money and tell you they are. The frequencies are adjacent.

You do have solutions, and of course you have stubborn people, now it's a matter of "is the problem going to annoy enough people to make them want to do what it takes to fix it."

If they are unwilling to do anything....then the problem persists and that's the way it is.

I think I read that the other guy is simplex 155.2200 and you're on a repeater pair at 155.205. What's the other side of the pair and is the 155.2050 your repeater input or output.

If 155.2050 is indeed your repeater output, then I'd start with detuning the base front ends until they can hear your repeater and not your neighbor(given that your repeater carrier is good and strong).
Also, do you have omni antennas on the bases? I bet you do, and I also bet that if you put a good(NOT some bullcrap 3 element) high gain high front to back ratio yagi on the base(s) you could swivel it around here and there and further minimize your neighbor.

As far as notch or band pass cavities, I disagree with charlie and jim. That is, if you got some dough to throw around. The engineers up at TX RX can come up with some pretty wild stuff if you have the bucks. If it were my system I'd try a decent notch or lo pass cavity on the receive side after the duplexer just to see. It just might attenuate neighbor enough to make you happy(ier?)

BTW---what kind of duplexer do you have?

All is not lost, Hum, I'll kinda guarantee that my suggestions will at least help your issue.

KINDA.

Thanks for the input gentlemen.

I just looked hard at the numbers and realized that we are talking about two 25KHz channels 15KHz apart. That does start to make filters look a little tough.

Unfortunately, as technology and politics collide, I don't think that either party is going to be willing to completely change their system. I'm not sure who was there first (I'll look it up later), but both systems have been on the air for many years. The problem has simply become more apparent recently as our agency only recently added the base station and repeater (currently under construction).

The problem that I really need to solve is with the repeater, since once everyone is on the new repeater frequency, they simply won't be on 155.205 that much anymore. If I can keep the splatter out of the repeater, the problem should again become pretty much invisible.

The repeater is being put on a completely new frequency pair, but includes a low-power, selectable, directional link to the old simplex frequency. I currently have a five element Yagi on the link radio. The Yagi is currently aimed to cover three of our county's towers, with the offending tower on the adjacent channel just off past the -3dB point.

The base antennas are, of necessity, omnis, but the base is staffed so infrequently that I can probably live with just trying to minimize the problem, especially once our operations move primarily to the new repeater.

From the input that I've received, and some head scratching, here's how I see my best (most realistic) options at this point:
1) See if I can get the offending tower to back their power down a bit, without compromising their coverage.
2) See if I can tighten the receiver front end to 20KHz spacing to cut the amount of overlap without turning the audio into garbage.
3) Rotate the Yagi further away from the offending tower. While this will move our primary tower out of the center of the main lobe, given that our main tower is line-of-sight less than a mile away, with a 200watt ERP, I'm guessing that we will still be just fine, especially given what an adjacent channel base, located further away, is doing in that position currently.

Thanks again, more news to follow...

Hello.

Notch filters for RX are very common in broadcasting.
The IFB, the little radio sets the remote crew has stuck in the ear, works with a 15KHz spacing from a real big signal, thousands of watts.
The IFB is very ultra heavy on selectivity, fair in sens, around .7 microvolt.
Point being filters do work, and work well, but you are going to pay for good ones.

Jim202:

BTW- If you want a handle on how bad of a mess the frequency coordination situation is, the frequency originally coordinated for our repeater output was 155.235.

Yup, as in 155.205, 155.220, and 155.235 all right on top of each other.

You can't make this up. Some days I really question the whole point of being robbed by the frequency coordination system.

That sounds like an interesting potpourri of freqs. I'll run that thru the TX RX intermod software Tues. And report back. Simple math already tells one that it is a complete $&*&%^*UP.

APCO did this to me---I have 2 repeaters, PW class. APCO licensed a city govt. output on the input of 1 of my repeaters, and the input on my output of another repeater. City of Smyrna uses this repeater to tote old farts around. They're approx. 30 miles away.

There are a lot of great people at APCO.

They have a huge job to do with limited resources and people wanting perfection where none exists.

There is no such thing as a PERFECT two-way radio dealer.
There is no such thing as a PERFECT frequency coordinator.
There is no such thing as a PERFECT radio system. (one that does everything a customer could ever desire now and in the future without costing any money).

It appears that you are pissed off at a lot of people KH. I too have an elephants memory. When someone does me wrong I don't utilize their services again. This works great when I have a choice in vendors or contractors. However.......

If you piss off your frequency coordinator and or the FCC, now who is going to help you when you have a problem?

Large organizations, especially non-profits like APCO have good people
and bad people. My experience with APCO is this: Frequency coordination is a team effort. It takes effort from the radio shop (dealer), the coordinator, and sometimes the customer, to make things happen. If you go to APCO and say: " Find me a Frequency and I need it quick." Do you think you'll get more help then you will if you say. "I need your help, can you send me a list of some possible frequencies you think may work and I'll check them out and see if there are any problems on my end".

I normally like to sit quietly on this board and give advice when I feel it will help someone. That appears to be the function of this board....helping people.

No one is helped when you bash an organization like APCO or apparently one of it's coordinators that you may have had a problem with.

You have some great advice KH and I enjoy reading it, but please consider toning down the bashing of organizations or individuals on the board.

I have no love for some of the stupid frequency coordinators in the past. Not sure they have gotton any better. Haven't had to deal with them in the past 6 years or so.

The last real classic, was a Public Safety system was operating a repeater for several years and one day they received a complaint. Seems another system was placed on the repeater output about 30 miles away. You could take a clear day and see the runway of the airport where new system was at. They put the transit buses on the same channel. Even used the same CTCSS. Then they have the nerve to say they were being interfered with.

Seems their solution was to change the tone squelch of the buss system and live with the interference from the Public Safety repeater system. The fire and EMS people hardly ever heard the busses.

As for your filters solving the problem, I doubt you will get them to work. As I mentioned even crystal filters have their limits. Most of the limitation is called insertion loss. The closer the frequencies are, the more loss any filter will insert into the receive path. Someplace along this game, you run out of signal and end up hearing nothing but white noise.

The same thing happens when you try to tighten up the IF system. Yup you can get IF filters that have a narrow pass band. Problem is again someplace along the line, you will drop the audio pass band to the point the audio will sound like crap. Noise and the received audio going outside the passband of the IF are the most likely results. This causes the audio to chop and sound distorted.

Have fun solving this problem.

Jim

You might try checking with the FCC, 155.205 and 235 require special coodination due to their close spacing. Make sure that the you are in compliance and then check with the interfering party, to verify, that they are also in compliance. There may also be a non-interference clause in their license.Who was operating first? You may have some pull with the FCC if you were operating first, and the interfering party came last. We had similar problems, with a school district, who would not give us approval for coodination, untill we put in a letter, to the FCC that we would correct any interfenece, that was traced to our operations.