Powering of pre-amps

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JMM-BDA

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I apologize for asking such a mundane question as this, but can someone please explain how power is supplied to an item such as a pre-amp? I've opted for the type of pre-amp that needs to be powered at the pre-amp itself, rather than by a power-injector.

And before anyone thinks this post is stupid, I live out in the Atlantic, and we have absolutely no suppliers of connectors & adpaters here whatsoever. Over the last couple of weeks I've purchased online an antenna & various accessories such as filters & preamps, only to discover once they arrived that no cabling was included or the connectors on the items were not the same as the ones depicted in the photos, causing me to have to order additional adapters online for such prices as $2.99, then spend close to $50 to ship them here, and then have to wait 2-3 days for them to arrive. :)

So, in order that the same thing doesn't happen when my pre-amp arrives (tomorrow morning), I want to place one final order for any items that I will need to power the critter.

In looking at the pre-amp itself, it looks like there's a single wire protruding from the pre-amp casing, to which I need to connect a 12-v supply. To where do I attach the other side of the power feed? In other words, if I connect the (+) end of my power feed to that lead protruding from the pre-amp, where & how do I connect the (-) end of the feed?

Thanks in advance for the above & have a great day.
 

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JMM-BDA

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Hey there Hoser, thanks for your fast reply.

No, they make 2 models: one model needs to be powered externally, such as with that power injector you refer to ( http://www.scannermaster.com/LNA_AIR_Pre_Amplifier_p/24-531043.htm ). The other model (which is the one I ordered) is powered right at the pre-amp itself, through that protruding lead ( http://www.scannermaster.com/LNA_AIR_B_Pre_Amplifier_p/24-531044.htm ).

That power injector you refer to, I already have - I got it to power this filter/pre-amp ( http://www.scannermaster.com/product_p/24-531224.htm ), but I am not impressed with that filter/pre-amp unit at all ..... it amplified one frequency beautifully (124.5), but blocked all the other frequencies completely (ranging from 118.1 up to 128.5) which I used to be able to hear faintly. And since removing that filter/pre-amp & going back to straight feed from my antenna, I can now no longer hear those other faint signals anymore, and un-amplified communications that I used to hear strongly, I can now almost not hear at all. It's almost as if something has affected the sensitivity of my scanner, so I don't wish to use any power injectors anymore, for fear that's what screwed up my scanner. But I digress. :)

Anyway, the pre-amp that I ordered gets its power from a separate power cable - somehow.

Thanks again for your reply, though.
 

rbm

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Here's what I do for that type of pre-amp.

http://www.radioreference.com/forums/showpost.php?p=860180&postcount=5

RG6 with a messenger wire is often used for satellite TV reception. You may be lucky enough to have a source close to home.

The + 12 volt lead goes to the feedthru on the amplifier, the - 12 volt lead can be carried by way of the shield on the coax (to the amplifier case). An easy way is to connect the negative lead to a coax grounding block and then connect the coax to the input & output of the grounding block. Similar to the image below. You can also connect the negative lead to the back (case) of the radio that the coax connects to.

Rich
 
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hoser147

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Depending on the scanner it may not be able to handle the RF it is getting with the preamp. That is the same effect I had with my GRE amp and it was ran off a battery. Good Scannin Hoser
 

JMM-BDA

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RICH: I had seen your post with the pic during the past couple of weeks, but I couldn't remember where I had seen it - thanks for jogging my memory. That answers my question perfectly. Thanks.

As for the grounding block you mentioned & attached, a couple of months ago I came across something similar while searching the net for something else, but for N instead of F connectors. When I inquired in another post a week or so ago, I was told that as long as I had a good ground elsewhere (i.e. at my lightning arrestor), that one of those that you nentioned are not needed. I guess there's no such things as too many grounds, so I'll try searching again for the type I saw previously. Thanks.

HOSER: Mine's the BCT15, and I could've sworn that I had better reception before I inserted the pre-amp in the circuit. Are the BCT15s known for this problem? And if it has indeed been affected, I assume there's no way to undo what I've done, is that correct?

Thanks guys for your answers - they were all helpful in answering my questions & concerns.
 

rbm

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HOSER: Mine's the BCT15, and I could've sworn that I had better reception before I inserted the pre-amp in the circuit. Are the BCT15s known for this problem? And if it has indeed been affected, I assume there's no way to undo what I've done, is that correct?
That particular preamp is tuned for operation only within the air band. I would guess that anything outside of the range of 118-137 MHz would be suppressed if not almost eliminated. So I assumed that you only were concerned with air band coverage.

Also, if there is a strong signal nearby, it could be amplified to the point where it completely overloads the receiver making reception worse.

You may want do some comparisons with and without the amp.
After installing the amp at the antenna, the feedline loss will reduce the possibility of overloading the receiver. 20 dB of gain directly into (and at) the receiver is quite a lot.

Rich
 

rbm

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Without going off on a tangent too far:

Something I used to do prior to moving to my current location in 1993 was to use fixed gain amplifiers at the antennas and then variable attenuators at the receivers. That allowed me to back off the signal seen by the receivers to an optimum level.

At that time I lived within sight (less than 2 miles) of 20-30 transmitter sites. I was most of the way up a large hill (small mountain) and had a big RF problem if I tried to run too much gain.

This setup allowed me to copy the 245-270 MHz milsat signals mostly full quieting. During that time there were a lot of clear voice communications up there. I was able to listen to the invasions of Panama and Grenada, the Dick Rutan and Jeana Yeager Voyager flight around the world, and quite a few other things I won't mention. I doubt I would have been able to hear most of that without a good preamp.

Using a low noise amplifier at the antennas and variable attenuators at the receivers allowed me to adjust the gain for stable, usually interference free reception.

I forget if I got the attenuators from Radio Shack or from Grove at the time. But they worked great for my needs.

I see something similar at Scannermaster though I can't vouch for how good it is.

http://www.scannermaster.com/ATT_20_Variable_Attenuator_p/24-531021.htm

Sometime in the late 1980's, I got an amplifier from Grove that had an amplifier module for mounting at the antenna and a control box at the receiver that allowed for gain adjustments. It also worked very well. I had to rebuild it a few times because of nearby lightning hits and eventually it just became unrepairable from all of the weathering and repairing.

Rich
 
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