Agency Having Major Skip Problem

[DC615]

The agency in which I work is currently having a few difficulties receiving skips that I thought I’d bring to the board for discussion/guidance.

Location:
Nashville, TN area

System Information:
*4 VHF MTR2000 repeaters on 4 different towers; 2 have UHF voting panels
*2 voters on 1 of the towers which also house a repeater (VHF RX radio into a UHF TX radio)
*Fleet radios: CDM 1250 & 1550s, HT 1250 & 1550s

Recently one of the voters was moved from a tower in which the antenna was located about 40 feet. It has now been moved to a new tower site which is 180 feet. We are now receiving very clear skips from another agency in KY on that channel. The tower which houses the 2 voters recently had a repeater added to it. As soon as is was put online skips started with it as well. I found this skip to be coming about 300 miles away in GA, mostly unintelligible, but I started getting decoded MDC IDs and some intelligible traffic which I finally found where it was coming from after listening for about 8 hours.

We are currently using TPLs on all repeaters, and I thought I’d ask the board about what they thought of using DPL. I messaged someone the other day about this, and they stated that 3db of gain is lost for quieting of the channel. Of all the research I’ve done on looking at other agencies across the United States, I hardly see DPLs in use, and when I do they are on the low end of the scale.

We are getting ready to reprogram the entire fleet, which takes about 8-10 hours, and before we do we don’t want skips coming in.

Questions:
1) What PL format would you use?
2) What is the difference between TPL and DPL?
3) Inverted DPL?
4) What is the best configuration to use with what we have?
5) What are the pros and cons?

As always I thank everyone in advance for your time and help!

-Daniel

[Tom in D.C.]

I don't believe that rearranging your PL/DPL/whatever setup will materially
help your problem with the overly sensitive receivers. My first inclination,
assuming you must stay with the "improved" location of the voting receivers,
would be to install antennas that do not favor the direction from which you're
receiving the unwanted signals. Obviously if the unwanted signals are using
the same tone coding as your system a change in coding is in order, but if
they're not the same code then your system's signals should capture the channel
when you use it.

Without going into a lot of technical stuff about PL vs. DPL, one of the basic
things to remember about using DPL is that the inverted forms can result from
running the signals through a repeater, although many PS agencies do so and
manage to make their systems work very well with DPL.

[Will]

I am making the assumption that the "skip" IS coming into the repeater receivers.

First thing is to verify that your receivers are actually correctly set to PL.
Then make the AND/OR squelch modification or programming on the receiver(s).
"Skip" can ride in on the tail with the normal Motorola squelch configuration.

Also try to verify that the distant user is using a PL different than yours. Call them and check...

[DC615]

Oh this is a definate PL match issue. I've spoke with a HAM from Atlanta that confirmed the GA issue, and my admin has checked with KY.

The voters were moved to a better location for coverage. The repeater is on the east side of the county, and the voter was at a station on the west end of the county. The voter was moved further west to a hill that had shadowed some of our signal.

The skip from Atlanta started as soon as the repeater was brought online.

Yes the repeaters indeed are set to the proper PL. I haven't heard any tail "ride in" in years.

Still unsure about what to do about this PL issue.

[xmo]

If the co-channel interference is audible because of a common PL code, then changing to a different PL code, or to a DPL code, can reduce the annoyance.

There should be no degradation of your system performance using DPL vs. PL. Some people prefer PL on the supposed basis that in can be decoded farther down into the noise, however in this case that could actually be detrimental.

In any case, however you choose to mask the co-channel signals, you must be aware of the effect on your system coverage. Once the interference is inaudible, your users have no way to know when the system is operating optimally [no co-channel signals are active] or is being degraded. The result is apparent intermittent communications in weak signal portions of the service area.

Such are the challenges of frequency re-use - particularly at VHF.

Determination of your coverage in the presence of noise and interference, including co-channel interference, is explained in TIA-TSB-88 which is increasingly being used as a reference in the design of new systems.

[fineshot1]

This "skip" or ducting has been happening all week long all up & down the east coast.

xmo nailed it above as i could not improve on his post.

Last year i had to deal with a similar problem but i am up in ocean county nj and
was getting a lot of band opening and ducting problems on one of our vhf channels
so i just changed it from PL to DPL and that solved it for us. But i had to do a lot of
research on that frequency before i decided on a DPL as I did not want to go from
one problem to another so choose your new PL or DPL wisely(look before you leap).

EDIT: I used google search's and some other search methods to do the research.

[DC615]

Thanks for everyones help! I'll do some homework and let everyone know how it goes.

[RKG]

The reason (or, I should say, one of the reasons) why I prefer PLs to DPLs is that not all models of subscriber equipment (even from the same manufacturer) define "invert" vs. "normal" the same way. This means you may have to program some equipment to Rx "normal" and some to Rx "invert," and if so, you lose the "direct" feature without dedicating a separate channel to direct.

[commstar]

"The tower which houses the 2 voters recently had a repeater added to it. As soon as is was put online skips started with it as well."

I am a bit confused here based on the above sentence. Yes , true 'tis the season for lots o'ducting but the above statement indicates to me that the problem may be a permanent condition as opposed to ducting. There is no mention of quiet times or times without the co-channel interference.

One of my UHF machines (on a mountaintop), I could regularly access from a friends mountain property about 175 miles away.

Obviously, I am not contending this was useable in the area 175 miles away but I could access and use the machine from that one point on his property.

I cannot explain it except looking at the topo contours there was not much between the two points and the use of a Super StationMaster. Goes without saying, The gain from that SSM was likely the final determining factor in this, but, it still 'on paper' should not have worked as it did. Beyond that, I cannot explain it except for a very hot machine located on a high point which is something that we all pray to be blessed with.

My point being, perhaps a little interagency contact and goodwill coordination might be a fruitful thing, perhaps the 'offending' agency is using a yagi on one of their sites (i've seen stranger things) and they would not mind turning it a degree or two to get you out of their funky hyper-functioning lobe.

Just changing your PL etc. will likely work and prevent you from hearing the other agency but your machines are still getting douched by their system 24/7 which will still have a deleterious effect your receivers in the form of a higher noise floor to weed through to get to your signal.

Who cares as long as your users dont HEAR it- right? I say no.

I would work through this part of the problem regardless as public safety systems are critical use and criticism comes not from the day-to-day but the critical moments in time when fringe xmit and receive come into play. The interference inherit in this situation comes into play in that scenario. Who knows what the other agencies mad scientist of of a tech is up to- give them a call organization to organization there should be some goodwill there - usually is.

Send them a recording of what is going on on your end. They will not want the blame in the media of one of your people getting hurt because of their radio system. I say call them, they are degrading the efficiency of your system and there is likely an easy solution at hand with some mutual cooperation.

Sorry about the verbosity. Old habit. Glad words are free. Good luck, let the board know how your problem resolves.

[Dan562]

Recently one of the voters was moved from a tower in which the antenna was located about 40 feet. It has now been moved to a new tower site which is 180 feet. We are now receiving very clear skips from another agency in KY on that channel.

The receive antenna height improved signal level by +12 dB so have your Field Technical Staff insert a 10 to 12 dB 50-Ohm RF attenuator in front of that receiver only bringing the increased signal level back to the original level. Since you have a wide area voting system, your skip / interference should be immensely degraded.