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San Diego Metro Area Discussion Local area specific discussion for San Diego and Imperial Counties.

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  #1 (permalink)  
Old 06-27-2010, 12:16 AM
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Default TRS simulcast on 900MHz?

I was scanning the mid-900MHz band on my ham radio and was surprised to hear a familiar dispatcher clearing "station A." I left it on that frequency for a while and heard several other TRS tidbits, including medics talking with Mercy, SDPD talking with ABLE, etc.

Here are a few of the frequencies:

947.125
947.275
947.300

I turned on my barely-working trunk tracking scanner and determined that one of the channels (probably all of the channels, but I didn't have much time to play around) corresponded with a specific frequency in the 800MHz TRS pool. I listened to several transmissions, and it was definitely a static simulcast, no rotating frequencies. I found some voice/rotating control channels too. I'm hoping there's somebody out there can tell me what's going on here, I searched for info everywhere but couldn't find a thing about it.
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Old 06-27-2010, 3:47 AM
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Most of those ham HT's with wideband receivers have poor image rejection especially in the 900MHz range. I'm guessing you are hearing images of the actual 800MHz traffic. Study up on superheterodyne receiver performance issues especially IF (Intermediate Frequency) image rejection. Given the specs of your HT (receiver IF's, LO's, image rejection, etc.) you should be able to work it out mathematically.

I've seen some HT's using really low final IF values even in that band which makes them exceptionally prone to this issue. Common values are 10.7MHz, 10.8MHz, 10.85MHz, and 21.4MHz. I think some might go as high as something in the 40MHz range. Your HT's spec sheet should have the info.

Take that final IF and double it (i.e. if the IF is 21.4MHz then double it to 42.8MHz) then subtract that from the 900MHz signal frequency signal you are receiving. If the problem is due to images, then the result should be the real 800MHz frequency signal. Given your information, if they are images, then I'm thinking your final IF is in the 40MHz to 60MHz range as doubling those puts the numbers closer to the real frequencies in the 800MHz band.

There are other sources of "confusion" for superhet receivers besides IF images but images are highly likely and most common.

Another thing to test this with is another receiver with different internal design from the affected unit. If you can only hear the odd signals on the HT receiver but not on a different unit (like a scanner or, if possible, a good quality communications receiver or professional test receiver) then you know the signals aren't really there and could be due to any number of issues within the HT receiver including, but not limited to, poor image rejection.

-Mike

Last edited by Mike_G_D; 06-27-2010 at 4:04 AM..
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Old 06-27-2010, 1:17 PM
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Default Thanks!

That makes perfect sense, thanks so much!

-Zack
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Old 06-28-2010, 7:40 AM
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Those frequencies sound a bit high for the 900 mhz repeaters. There is lots of activity between 932 - 949 mhz. Radio station studio links to off-site transmitters are found around 950 mhz or so...
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Old 06-28-2010, 8:56 AM
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[QUOTE=Mike_G_D;1337414]Most of those ham HT's with wideband receivers have poor image rejection especially in the 900MHz range. I'm guessing you are hearing images of the actual 800MHz traffic. Study up on superheterodyne receiver performance issues especially IF (Intermediate Frequency) image rejection. Given the specs of your HT (receiver IF's, LO's, image rejection, etc.) you should be able to work it out mathematically.
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AFAIK there are NO "ham" HTs made and sold for the 902/927 mHz repeater band. We use mainly GTX Motorola HTs. New XTS2500s also can be had that do 900 but they're few and far between. There may also be a couple more Motorola models that do 900 but the fact remains that no one has ever taken on the task of making a 900 mHz ham HT.
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Old 06-28-2010, 1:17 PM
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Although they may not make "ham" HTs to transmit in the 900 MHz band, many can receive/scan in that range.
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Old 06-28-2010, 4:28 PM
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Quote:
Originally Posted by SDWolfman View Post
Although they may not make "ham" HTs to transmit in the 900 MHz band, many can receive/scan in that range.
Exactly my point! The OP was, I believe, referring to his 2m or 70cm or dualband or multiband HT which commonly have wideband receivers capable of receiving up through 900MHz quite easily though not, as I was trying to explain, without major compromises in terms of rejection of out-of-band signals.

It is possible the signals are legitimate (maybe some form of link transmitters) but, given the type of receiver used (ham HT wideband receiver), I suspect out-of-band interference, most likely images.

-Mike
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Old 06-30-2010, 11:13 PM
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FWIW, no trunked system will use an analog form of linking. It will be digital, be it microwave or 900 (ISM).
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Last edited by WayneH; 06-30-2010 at 11:19 PM..
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Old 07-01-2010, 2:45 AM
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Thanks Wayne! I kinda thought so.

-Mike
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