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TRX-2 with preamp intermod?

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scotttish

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Nov 14, 2011
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Little Rock, AR
#1
I apologize in advance, if this subject is answered here already. Couldn't find one specifically. I have this issue with my TRX 1, and now with the 2. I am using the Moonraker pre-amp to bring in weak signals that are 30-45 miles away and I am doing pretty well with MOST conventional frequencies coming in strong, while others do not so well. With the preamp on, the scanner completely skips digital P25 completely, and brings in DMR with a greater signal bar of 4-5, but tends to skip DMR more easily. Can anyone explain what is going on, and can I correct this with a specific DCI filter for digital, etc? Thanks!
 

EricCottrell

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2,274
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Boston, Ma
#2
Hello,

It is likely nearby signals are overloading the preamp, which is not difficult to do.

GRE/Whistler models before the TRX-1/TRX-2 have signal overload problems on VHF High when there are strong FM stations nearby. The TRX-1/TRX-2 does not suffer from this problem due to improved filtering. Putting a broadband amplifier ahead of the scanner reduces the effectiveness of the front end filtering. It is likely several strong FM stations nearby are causing some of the problems, so a FM Band filter may reduce the problem. There could be strong paging and land mobile signals nearby as well. There are filters for the paging frequencies, but it can take some work to figure out what to buy,

In general it is better to improve your antenna rather than trying to make it up with a broadband preamp. It is more effective to have the antenna outside, as high as possible, and in the clear.

73 Eric
 

scotttish

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Little Rock, AR
#3
Thanks Eric! My current outdoor antenna that I'm using, is approximately 25 feet off the ground using fiberglass poles. I have one of the best intent as it can be used for the multiband frequencies. I have a few local VHF County sheriffs office repeaters that I can tell interfere with my scanner, and give off a dingdong sound in their transmission. I use the scanner attenuater and this helps a little bit. Thank you for your suggestions and I continue to welcome additional input on this matter. Many thanks!
 
Joined
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Stockholm, Sweden
#4
Amplifiers always has internal noise that will add to the signal. Good amplifiers have 2dB noise or less.
Moonraker have several amplifiers but they do not specify their internal noise figures. Usually that means they are pretty bad.

An amplifier will boost the signal but also the atmospheric and earth noise. If an interfering signal, within the frequency range of the amplifier, are too strong it will create a "barrier" on the transistors or ic circuits input, and the wanted signal will have to overcome this barrier which means that weak signals are actually attenuated instead of strengten.

Amplifiers will often make the signal strenght meter increase and show a high value without any signal and will jump to full scale even with a noisy signal. This is normal when using amplifiers.

The best use of an amplifier are when it's connected between antenna and coax which will stabilize the impedance variations from a broadband antenna and also compensate for the coax loss.

/Ubbe
 
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