Question on pre-amplifiers...

[kmacka]

http://www.scannermaster.com/Pre_Amplifiers_s/45.htm

I was browsing around in the above link and was looking at picking up one of these pre-amps. I just have a question on these units. There are two different types of these pre-amps for air, which is the type I am looking for.

One of these gives this description:
Tuned low noise preamp. Range 118-137 MHz, gain typ. 20dB, BNC-connectors, supply 12V via coax.

The other ones gives this description:
Tuned low noise preamp, range 118-137 MHz, gain typ. 20dB, BNC-connectors, supply 12V via separate wire.

What is the difference between these two. Do these two pre-amps needs to be powered? One says supplied via separate wire the other says supplied via coax, what does this mean? Someone help, thanks! lol

 

[kmacka]

One other thing. Does anyone have any other websites that offer other types of pre-amps, I appreciate any links, thanks.

 

[tonsoffun]

http://www.scannermaster.com/Pre_Amplifiers_s/45.htm

I was browsing around in the above link and was looking at picking up one of these pre-amps. I just have a question on these units. There are two different types of these pre-amps for air, which is the type I am looking for.

One of these gives this description:
Tuned low noise preamp. Range 118-137 MHz, gain typ. 20dB, BNC-connectors, supply 12V via coax.

The other ones gives this description:
Tuned low noise preamp, range 118-137 MHz, gain typ. 20dB, BNC-connectors, supply 12V via separate wire.

What is the difference between these two. Do these two pre-amps needs to be powered? One says supplied via separate wire the other says supplied via coax, what does this mean? Someone help, thanks! lol

Hey Kmacka
Thats exactly what it means that it has to be powered. The one that is powered with the coax is done through a power inserter which is the coax being the one providing power and a signal carrier.
The other is powered using a seperate power cable. So you have a cable for signal and a second cable for power. I use these http://electroline.com/en/products/drop_amplifiers/eda/specifications.html
I am making sense
Take care

 

[kmacka]

Hey Kmacka
Thats exactly what it means that it has to be powered. The one that is powered with the coax is done through a power inserter which is the coax being the one providing power and a signal carrier.
The other is powered using a seperate power cable. So you have a cable for signal and a second cable for power. I use these http://electroline.com/en/products/drop_amplifiers/eda/specifications.html
I am making sense
Take care

What would you recommend for a first time user, which one is the easiest to use.

 

[kmacka]

I was looking at those products also. Are they both a pre-amp and also a multi-coupler? I was also a little confused on if they would work with scanners being as they are designed for cable tv.

 

[ka3jjz]

You may not know this, but many of the carrier freqs used by cable TV fall in ranges receivable by scanners. A caution however- some of them don't cover the VHF lo band, so be sure to keep this in mind.

Our milcom wiki has links for other preamps; however, a drop amp (which is likely what these units are) are a much better bet. A drop amp is used to compensate for losses incurred when running RF through cable; a couple of db gain is all that would be necessary. A drop amp would be much less susceptible to causing problems higher-gain preamps can, and often do, particularly in urban or semi urban environments. Depending on their design, they may well have the ability to feed multiple devices (multi couple) at the same time.

The problem can show up in a couple of ways; you start hearing pagers and other junk in places you didn't before, you lose sensitivity on one or more bands, or a combination of other factors, including these. Losing sensitivity is a very loose description of what is referred to as 'desense'.

If you're in an urban or semi urban area, with lots of pagers, taxi drivers (notorious in my neck of the woods for dirty transmitters....) and other common carrier junk, a drop amp is preferable to a high gain broadband preamp. Get a good antenna, feed it with the right coax, then if you find you need a small bump in the sensitivity department, a drop amp would be right up your alley.

73s Mike

 

[zz0468]

Done properly, inserting power via the coax doesn't have to carry a performance hit with it. What I would question is, was it done properly. Running power through the coax is easier to install - one cable. Otherwise, you run the coax and the power. Simple enough...

Preamps are a touchy subject here, and there is a lot of misinformation. I have built receivers that can hear the black body radiation off the moon, and I roll my own preamps, so I have been quite successful in their application.

I would caution you on the drop amps. Unless the noise figure of the preamp is less than the noise figure of the receiver, no amount of gain will help you. Higher s meter readings DOES NOT directly translate into greater ability to hear weak signals. Drop amps are intended to take an already relatively stong signal, and make it stronger to overcome cable and splitter losses. They are also designed to operate in an environment where most or all of the signals (cable tv) are about the same level. This is far from the environment that a scanner lives in. They are not designed for low noise reception of weak signals. Period. Some do pretty good, others are horrible. And the noise figure is frequently not listed in the specifications, and even more frequently, it's not understood by the buyer.

Beware if you go that route. It may or may not work for you. Don't be surprised if there are times when it seems like the preamp is hurting reception.

 

[ind224]

I was an amp skeptic myself. 2045 on LMR 400 and a discone at 35'.
Tried to make a "threeway" using chassis mounts and coax to run the 2045, 96 and 2006 at the same time. Nope. Each radio emits its own garbage and it tends to affect the other radios. Always stopping or requiring squelch adjustment.

Now I have the Electroline EDA 4-G and I love it. It does have F connectors and it must be powered to work i.e. no passive for fast comparisons. Yes, you could put a switch and two more connectors coax etc but I'm anal about that. I hear the difference.
Anyway, this thing rocks. Already good reception is now awesome. Full quieting on the crappy Meca 1 in Indy. Now I can run four scanners from one antenna with 25db isolation port to port and 7db boost from the amp (just an added bonus as I was really looking for the splitter with the isolation)
Passband is 54 to 1000 so you get more than the ones you had listed. I scan everything and it is great from air VHF to 900 confirmed. Digital, Mot II, EDACS (14 miles away) ,conventional I get it all. I don't hear hum or noise added but have not put it on an analyzer nor do it think it is needed.
Downside is it has to be powered so another wall wart or if you already have 12v going no problem.

As the others mentioned, noise, no matter the source can make amplifiers annoying. I suggest getting the antenna coax etc to the preferred location for "pre preamp" testing. And even that is no promise.

My computers/ modem / real CRT monitors play havoc with my HF listening but I can surf no problem with the scanners on the desk.
Good luck and happy scanning.
Stu

 

[kb2vxa]

Hi kmacka and all,

Like the man said, preamps here or anywhere is a touchy subject. As for those air band units in your original post, if it's only air you want air you'll get and lots of it. Being tuned for the band they reject signals outside the specified range so you'll get little to no overload and associated interference. The one powered through the coax I would recommend since you don't need another wire so power it. It has a coupler that sends the DC through the coax and another in the preamp itself to keep signals where they belong, out of the power supply. At the antenna end a preamp is far more effective since shoving a stronger signal down the pipe is better than trying to recover weak signals after losing something in the coax. That's why the pump is at the well head and not at the faucet.

Wide band "DC to light" preamps often cause more problems than they solve, the reasons have been discussed to death here on the forums. CATV amplifiers are NOT preamps, they operate at much higher signal levels and are entirely unsuited for scanner applications. A TV preamp designed for boosting signals from an antenna is pretty much the same as one designed for scanners, I'll bet dollars to donuts only the connectors are different, it's the same circuit board inside the box.

When all is said and done most listeners really don't need a preamp because 90% of your monitoring post is on the roof. Preamps are useful but only in specific applications such as pulling weak signals out of the mud on a specific band. We hams use them for terrestrial weak signal work using CW and SSB and moon bounce (EME) where they shine but frankly they suck on FM and what do you listen to most? Yeah, you get the picture.

Back to those in the first post, air band is AM (with a small sliver of FM for corporate comms) so one would be of great benefit to you. A while back I built one into an old 2M AM rig and I started hearing stations I never knew existed before.

One last word of caution, put the bits of those other posts together and you have a consensus, Frankenstein lashups produce Frankenstein monsters. This public service message brought to you by Igor and the gang down at Frankenstein Laboratories, a division of Altec Communications. http://www.w2sjw.com