Any recommendations on a Pre-amp?

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Scanner-geek

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I just moved to a new NYC apartment and the scanner reception is terrible. I use a BC 996t/785D/710xlts for UHF and VHF high bands. Outdoor antennas are not a option. I have 2 questions:

1) Do pre-amps really work (i.e., will I be able to pull in more UHF signals)?

2) If so, can anyone recommend one? I'm willing to spend the $ if it will really help.

Thanks in advance,

Scannergeek
 

Josh

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Pre-amps do work, but if you're in NYC, you're probably not going to benefit from one. A typical scanner pre-amplifier will work from often 30-1000Mhz perhaps... this will amplify the signals you want to hear, but also everything else raising the noise floor and lowering your scanner's selectivity, thus opening it up to garbage noise and intermodulation which could actually make your reception worse. Big cities are inherently overly RF-filled environments, and often it doesn't even take a signal pre-amp to pull in the intermodulation and other noise.

-Josh
 

rbm

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Pre-amps are like politics and religion.
When you try to discuss them you will almost always offend some people. So I usually just keep quiet when these discussions arise. (However, you did ask.)

Unfortunately, it's impossible to know how well (or poorly) a pre-amp will work in any given location.

The best way to find out if a pre-amp would work for you is to borrow one before you buy. Unfortunately most of us don't have a friend with good, spare pre-amps sitting around.

Other than that, you're gambling the price of the pre-amp.
Then there's the problem of risking a little money on a poor pre-amp, or more money on a good pre-amp.

The following only applies if you don't have overload or intermod problems already.
If you do have any problems without a pre-amp, then a pre-amp is not your friend.

Here are a few things to consider.

Try one.
Not just any pre-amp but one with reasonably good specs. Unfortunately many pre-amps intended for scanner use only talk about gain and not much else. The Jim M-75 pre-amp is good one to try because the gain is variable from -10 to + 20db. It has a relatively low noise figure but most other specs aren't given.

You'll want an amplifier with gain that is reasonable for your situation.
If you're only using a short feed line, or using the pre-amp at the scanner end, you DO NOT want a huge amount of gain. That would almost surely overload your radio.

So you'll want something with a low noise figure, gain that is reasonable for your location, and one with a high IP3. (The higher the IP3 the better.)
"IP3 is an industry standard figure of merit used to rate the linearity of receivers and other signal processing systems. High IP3 equals good linearity. Good linearity equals immunity to intermodulation distortion caused by strong signals on adjacent frequencies."

High IP3 is not a guarantee that a pre-amp will work for you though.
One with a 35 dBm IP3 will be less prone to problems than one with only a 10 dBm IP3.

This isn't a recommendation but here's a link to a reasonable low noise ampifier with decent specs. for an example.
LNA-1400
• Frequency Range: 100-1400MHz
• Gain: 20dB
• P1dB: +17dBm
• IP3: +35dBm
• Noise Figure: 0.8dB
http://www.rfbayinc.com/LNA/LNA-1400.pdf
 

prcguy

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I was recently in NYC and was surprised at the level of RF hell that exists there. My scanner was really pissed off and even portable FM broadcast radios had problems unless you collapsed the antenna or moved the radio to null out interfering stations. I would investigate band pass and notch filters before considering a preamp and the best pre amp I own would get blitzed there without some pre-selection. Here is a pic of the typical NYC skyline in the area I was staying.
prcguy
 
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rbm

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One thing that I neglected to mention but important to remember:

A higher S-Meter reading is not an indication of pre-amp goodness.

The S-Meter (or bars) may indicate much higher with a pre-amp than without, but the signal could be much more noisy.

On weak, marginal signals a pre-amp 'should' make them quieter and easier to listen to.

For example on my Icom IC-R7000's I can make the S-meters read anything I want.
If I'm listening to one of the milsats at 255.55 MHz for example I could have a full quieting signal that only gives me an S-5 on the meter. If I crank the gain up I can make the meter read S-9+10 and the signal would be very noisy and close to unintelligible.

More gain, more S-meter, but much worse.

Rich
 

Scanner-geek

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Thanks guys for the info. In general, NYC is a nightmare in UHF for intermod and noise. This is particularly true in open areas, like in a car (but I wouldn't know about that). Where I now live (on a low floor in a courtyard of a highrise) the reception is awful. There is absoulutely no noise or intermod now, even with a mag mount antenna outside my window. I am worried about the noise a preamp may pull in, but I'm running out of options. I can't receive half of the manhattan precincts, but got them all no problem from Long Island. There's just too much concrete and metal in the way. What a drag. I would love to try a jim 75 before buying it, but they're not easy to come by.

Should the fact that I have no noise/intermod now lead me to believe that it might not be a major problem with the preamp? Guess I won't know until I try.
 

w5cyc

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I'm with the majority opinion here. I'd try all kinds of other things first -- bandpass filters, notch filters, or even better, getting your antenna higher up in the air if you're wanting "more" UHF signals before I'd go the pre-amp route. I've owned a couple. One from Grove for a base station, and one that was Japanese for a handheld.

Both gave me an extra toggle switch to play with and another red LED to look at when the lights were off, but other than that, didn't really get me "more" signals than I was already getting.
 
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prcguy

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With the building and antenna conditions you describe, a pre amp should give a noticeable improvement if there were no overload problems. Doing a quick calculation estimating an 8dB noise figure for a typical scanner and using 3dB feed line loss, a 0dB gain antenna and a high performance UHF pre amp with 20dB gain and 1dB noise figure I see a 7.8dB system improvement with the pre amp at the radio and a 12.2dB improvement with the pre amp right at the antenna. This might work on Long Island. In reality the pre amp without a preselector in NYC will get blitzed, cause lots of intermod and your reception will probably get much worse. You don't know till you try it so if you can borrow a good pre amp for testing it may save you from an expensive mistake.
prcguy
Thanks guys for the info. In general, NYC is a nightmare in UHF for intermod and noise. This is particularly true in open areas, like in a car (but I wouldn't know about that). Where I now live (on a low floor in a courtyard of a highrise) the reception is awful. There is absoulutely no noise or intermod now, even with a mag mount antenna outside my window. I am worried about the noise a preamp may pull in, but I'm running out of options. I can't receive half of the manhattan precincts, but got them all no problem from Long Island. There's just too much concrete and metal in the way. What a drag. I would love to try a jim 75 before buying it, but they're not easy to come by.

Should the fact that I have no noise/intermod now lead me to believe that it might not be a major problem with the preamp? Guess I won't know until I try.
 
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Thanks guys for the info. In general, NYC is a nightmare in UHF for intermod and noise. This is particularly true in open areas, like in a car (but I wouldn't know about that). Where I now live (on a low floor in a courtyard of a highrise) the reception is awful. There is absoulutely no noise or intermod now, even with a mag mount antenna outside my window. I am worried about the noise a preamp may pull in, but I'm running out of options. I can't receive half of the manhattan precincts, but got them all no problem from Long Island. There's just too much concrete and metal in the way. What a drag. I would love to try a jim 75 before buying it, but they're not easy to come by.

Should the fact that I have no noise/intermod now lead me to believe that it might not be a major problem with the preamp? Guess I won't know until I try.
I think what he's trying to say is the building is attenuating all the signals so the overload potential may not be there. I was living in an apartment on the same side of town as some FM stations and got overload but in the complex more so in the building the strong signals was attenuated so overload and desense wasn't so bad. So on my base scanner i used a RS TV mast mount preamp to compensate for the attenuated signals so i could hear the Railroad better. had to get the gain in the right spot as to keep from electronic interference but still get good signals. If the antenna was outside that preamp would've killed the scanner for sure.
 
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