Halfway between repeaters, mystery distortion?

DR04 · Nov 11, 2018

A mystery I need help with...

Details & background: I'm a volunteer firefighter/EMT and wear a pager (receiver) on my hip tuned to a VHF frequency (150MHz). In my basement I'm getting terrible reception on that pager, so I threw up a 4 foot tall Diamond VHF omnidirectional antenna (Diamond BC103) on the side of my house connected to a 2m ICOM IC-2300H listening on the paging frequency over an LMR-600 cable. It's pretty robust, I can pick up simplex 5w fireground traffic from 20 miles away consistently and have been known to pick up radio traffic (inadvertently) from 100 miles away or more all in that 150MHz frequency range. For Illinois I'm pretty high up, near 1100 feet. So the setup seems solid. But...

Kind of a strange thing happening to me sometimes. I've got one repeater 8 miles WNW from me pushing out emergency "paging" traffic at 155.820 MHz and a second repeater transmitting on that same frequency 9 miles SE from me. Despite being in a hilly area I have relatively good line of sight to both repeaters.

Sometimes when an emergency page is transmitted over those repeaters I get terrible distortion and noise, usually for the first 2 to 4 seconds of the transmission. Over the ICOM it sounds terrible (happens 1/4 of the time) and if my pager is connected to that antenna via an amplifier it sounds the same, sometimes preventing the pager from deciphering the tones and activating. The ICOM can't even sort it out sometimes and my squelch setting will turn off the reception. But if it's on my hip and I'm upstairs it usually doesn't happen on my pager.

I should point out, that with the exception of this strange distortion, and after it resolves, the reception is crystal clear again.

***Could the 2 repeaters be causing some type of distortion or interference since I'm darn near right inbetween the two? Like a slight latency on one is distorting the other's traffic?***

If so, I was thinking of throwing up a Yagi towards one of the repeaters and hoping it would help. But would appreciate any thoughts on this before I go crazy.

ko6jw_2 · Nov 11, 2018

You are experiencing phase distortion that results from the two signals being out of phase and creating distortion in the receiver. The two transmitters are not exactly on frequency and have slightly different arrival times. The first issue can be fixed (sometimes) by synchronizing the transmitters. The other is just physics and you can't do anything about it.

Our local county fire department has six different frequencies all of which have multiple mountain top repeaters. The phase distortion at my location can be anywhere between none to terrible. They try to synchronize every day, but it works for short periods of time. In other locations that are shielded from the other sites everything is fine.

A directional antenna is worth trying. You will just have to experiment. An attenuator may help too because the problem is too much signal not too little.

ofd8001 · Nov 11, 2018

And if nothing else, presuming both towers/transmitters are associated with the same system, bringing it to the attention of the radio shop might be good. They may not know there is an issue to check.

RFI-EMI-GUY · Nov 12, 2018

DR04 said:
A mystery I need help with...

Details & background: I'm a volunteer firefighter/EMT and wear a pager (receiver) on my hip tuned to a VHF frequency (150MHz). In my basement I'm getting terrible reception on that pager, so I threw up a 4 foot tall Diamond VHF omnidirectional antenna (Diamond BC103) on the side of my house connected to a 2m ICOM IC-2300H listening on the paging frequency over an LMR-600 cable. It's pretty robust, I can pick up simplex 5w fireground traffic from 20 miles away consistently and have been known to pick up radio traffic (inadvertently) from 100 miles away or more all in that 150MHz frequency range. For Illinois I'm pretty high up, near 1100 feet. So the setup seems solid. But...

Kind of a strange thing happening to me sometimes. I've got one repeater 8 miles WNW from me pushing out emergency "paging" traffic at 155.820 MHz and a second repeater transmitting on that same frequency 9 miles SE from me. Despite being in a hilly area I have relatively good line of sight to both repeaters.

Sometimes when an emergency page is transmitted over those repeaters I get terrible distortion and noise, usually for the first 2 to 4 seconds of the transmission. Over the ICOM it sounds terrible (happens 1/4 of the time) and if my pager is connected to that antenna via an amplifier it sounds the same, sometimes preventing the pager from deciphering the tones and activating. The ICOM can't even sort it out sometimes and my squelch setting will turn off the reception. But if it's on my hip and I'm upstairs it usually doesn't happen on my pager.

I should point out, that with the exception of this strange distortion, and after it resolves, the reception is crystal clear again.

***Could the 2 repeaters be causing some type of distortion or interference since I'm darn near right inbetween the two? Like a slight latency on one is distorting the other's traffic?***

If so, I was thinking of throwing up a Yagi towards one of the repeaters and hoping it would help. But would appreciate any thoughts on this before I go crazy.

Assuming this is a simulcast system, It indeed strange to have a phasing problem upon key up, that resolves itself. This could be a problem with one transmitter or both. I would report it to whoever maintains the system.

If this channel is used for paging and for voice, it could be that PL tone isn't being stripped immediately when one transmitter keys. Or, it could be a jitter problem with the link between the simulcast master equipment and the transmitter(s).

Or it could be a level problem at certain paging frequencies that has one transmitters deviation hotter than the other for certain tone groups. The transmitter modulation compensation balance could be out of wack.

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riccom · Nov 12, 2018

Ok i know this system, working in debeque, it is a phased issue, and with him haveing a nice setup like that, it maybe overpowering his pager and reciver, i know when i heard them in iowa, it had a cross talk warble in it also.

DR04 · Nov 13, 2018

Great insight and assistance guys. Thanks so much. With this phasing issue and/or timing issue maybe I'll give a directional antenna a try.

Follow up question then. Should I keep the yagi horizontal or vertical?

I can double check but would assume the transmitter is vertically polarized. But figured the horizontal orientation on my side pointed directly at the one repeater would help filter out other noise. What do you think?

riccom · Nov 13, 2018

Use a yagi reason being is you can pull the closer station in and not have to worry, now with a good dx day it will not be perfect bit will work much better then Omni

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RFI-EMI-GUY · Nov 13, 2018

The yagi should be verticle. Also, you can try pointing the yagi such that the back of it is pointed to the undesired site, the rejection of that site will be greater, even if the yagi isn't directly pointed at the desired site.

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Ubbe · Nov 13, 2018

Yagis often have bigger back loob than sideloobs.

/Ubbe

RFI-EMI-GUY · Nov 13, 2018

Ubbe said:
Yagis often have bigger back loob than sideloobs.

/Ubbe

the nulls between the back lobes are the direction to exploit for the undesired if the normal F/B ratio is not enough. every antenna and mounting arrangement is different, so you basically have to swing the azimuth to null the undesired.. Being that the TX are simulcast, that means looking for lowest distortion.

Halfway between repeaters, mystery distortion?

DR04

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ko6jw_2

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ofd8001

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RFI-EMI-GUY

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riccom

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DR04

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riccom

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RFI-EMI-GUY

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Ubbe

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