Is a VHF MSF a Waste of time with 15KHz Spacing

[krazybob]

I have three MSF 5000's. One is deployed and two are about to be. We have been struggling!!! But the one already on the air has suffered from interference for some time. It's input is 15KHz from the out of several repeaters. The band plan is 15KHz spacing. Nothing can be done about that. Making matters worse the MSF VHF has a 30KHz spacing and a 2 MHz receive window. It works real hard at hearing off frequency stuff, or so it appears. Does that matter? We use a Chip Angle dual band pass filter that cannot catch these -- too close. But we don't use a preamp!

What are some ways I might rein in a MSF? We tried the 15 KHz crystals and the audio tanked. Everyone sounded over-deviated. We may be able to get some 20KHz filters and that would be right down the middle. Will a UHF UniBoard work better? A 900MHz UniBoard with 25KHz spacing? Is there a UniBoard with a 20KHz spacing or a vendor that sells 20KHz IF filters? Will this be a waste of time since the MSF has a 2MHz receive window?

Bob, WA1MIK emailed me this and I don't think he will mind me sharing it.

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Here are the specs from the VHF Instruction Manual. You're not gonna like them!

Freq: 132-174 MHz
Channel Spacing: 30 kHz <<<<< This is the important one!
Receiver Bandwidth: 2 MHz (this is the front end bandwidth without tuning)
Input Impedance: 50 ohms
Frequency Stability: +/- 0.0002% from -30C to +60C
EIA Sensitivity 12 dB SINAD: -119 dBm / 0.25 uV
Other Acceptance Bandwidth: +/- 3 kHz
EIA Intermodulation: 85 dB
EIA Adjacent Channel Selectivity: 90 dB
Spurious Response Rejection: 100 dB
Squelch Sensitivity:
Carrier: 0.15 uV maximum at threshold
Tone Coded: 0.20 uV maximum
Digital Coded: 0.20 uV maximum

The modulation acceptance is +/- 7 kHz minimum, which I got from a UHF manual, but since the Uniboard IF response is the same, this would apply to a VHF station as well. However the UHF station is rated for 25 kHz channel spacing. The channel spacing might just be because that's the normal spacing for 5 kHz systems.

So as good as the MSF is, it isn't rated for the type of channel interleaving that's going on with your system and its neighbors on each side.

[KI4M]

Don't see this as a short coming of the MSF it is doing all it can. You are going to have to filter the crap out of the front end with either super tight preselectors and/or crystal window filters. Even with that you are going to see degradation of receive performance due to the amount of filtering required. Since you are in close proximity to the offending TX an preamp required to overcome the filter losses will just create more problems. This is going to be tough to make play and work decent.

[Bill_G]

Don't see this as a short coming of the MSF it is doing all it can. You are going to have to filter the crap out of the front end with either super tight preselectors and/or crystal window filters. Even with that you are going to see degradation of receive performance due to the amount of filtering required. Since you are in close proximity to the offending TX an preamp required to overcome the filter losses will just create more problems. This is going to be tough to make play and work decent.

There's a whole thread devoted to this problem explaining the back story - basically poor coordination at a specific site. He's installed the 15khz wide crystal filters on the front end, and if I understand correctly, they work. But, now his wideband users are getting clipped because their gear isn't running down the center. Cie la vie.

I'm not certain what Bob is asking for now. The physics of the problem are what they are.

[krazybob]

Thanks guys. I think that I am hosed unless I begin by changing repeaters. It has been suggested that I consider using a CDM 750 for RX only. Being newer it has a good front end capable of 20KHz operation. Its either that or accept that weaker signals may carry the output of other repeaters. I love the MSF and I'm looking for resources and suggestions. 15KHz crystals didn't work because they appeared to be 12.5KHz. We have a possible 20KHz vendor and they might help. But with a 2MHz receive window5t rated specifications anything within that window will be amplified. I think I need a good, single frequency repeater.

[Bill_G]

You're hosed by the problem period. Nobody makes a radio with the kind of rejection you require. You need to beat on the coordinators to get your split to match the incumbents. Or leave the site.

[d119]

You're hosed by the problem period. Nobody makes a radio with the kind of rejection you require. You need to beat on the coordinators to get your split to match the incumbents. Or leave the site.

I echo Bill's sentiments to a tee. Unless you are willing to narrowband the system AND THE USERS, time for a new frequency pair. I can tell you right now that 147.090 isn't going to move, they've been there since dirt was invented.

Just flip the pair and see if things get better/worse.

[Bill_G]

You're hosed by the problem period. Nobody makes a radio with the kind of rejection you require. You need to beat on the coordinators to get your split to match the incumbents. Or leave the site.

I echo Bill's sentiments to a tee. Unless you are willing to narrowband the system AND THE USERS, time for a new frequency pair. I can tell you right now that 147.090 isn't going to move, they've been there since dirt was invented.

Just flip the pair and see if things get better/worse.

Exactly. Just do it. Make your hi/lo split match theirs.

[Will]

You're hosed by the problem period. Nobody makes a radio with the kind of rejection you require. You need to beat on the coordinators to get your split to match the incumbents. Or leave the site.

I echo Bill's sentiments to a tee. Unless you are willing to narrowband the system AND THE USERS, time for a new frequency pair. I can tell you right now that 147.090 isn't going to move, they've been there since dirt was invented.

Just flip the pair and see if things get better/worse.

Exactly. Just do it. Make your hi/lo split match theirs.

Exactly, To the hell with the wanabe so called coordinators.

[tvsjr]

As I've already pointed out previously... the laws of physics aren't just a good idea, they are the law. What you're looking for doesn't exist, and the coordination scheme is farking retarded. Flip the pair. Coordination is a gentlemen's agreement - it's quite important in certain areas, but if you aren't causing interference to other coordinated systems, there's basically nothing anyone can do to you.

To another point... if your subscriber units are poorly aligned, you'll always have some sort of problem. I've had users whine about their $CHEAPRADIO not working on my 2m machine - sorry, it works fine, but it's not going to be happy with 7KHz deviation, 2KHz error, and totally jacked up PL dev. Fix your radio.

</soapbox>

[escomm]

To another point... if your subscriber units are poorly aligned, you'll always have some sort of problem. I've had users whine about their $CHEAPRADIO not working on my 2m machine - sorry, it works fine, but it's not going to be happy with 7KHz deviation, 2KHz error, and totally jacked up PL dev. Fix your radio.

Not only is this not an option the OP is willing to consider, he's actively refused it.

[d119]

Then I think we're done discussing this. Anyone disagree?