amplifier

[friedthat]

howdee folks,

i need to find out if i can use an old tv amplifier to boost the signal for my scanner.
here are the specs: UHF/VHF/FM distribution amplifier, 2 outputs, 75 OHM Coax with gain control and switchable FM trap.
Bandwidth: VHF 50-400MHz UHF 470-890MHz
Gain : 25 DB 23DB
Response flatness +/- 2 DB
Max Output : 40 dBmV
Noise figure : 4 DB 7 DB
impedance: Input 75 OHM -1
Output 75 OHM -2
FM Trap attenuation:
(switchable) -20 dB 88-108 MHz

now i find it works well with some of trunked Motorola BMR Fleetnet 140- 145 MHz range.

However when i tried to use it on an EDACS Standard system (863MHz) i can see the signal on the scanner go from 0-1 bar to 3-4 bars BUT i do not hear anything and it does not stop on the control channel.. i don't understand . so after given the following info can some one help me out.

 

[zz0468]

It's easy. The noise figure for the 470-890 range is higher than the noise figure to your receiver. The amplifier is creating a worse signal to noise ration than you have without the preamp.

Look at the specs you posted. The noise figure is the measure of the amount of noise tha amplifier adds. That number MUST be lower than the noise figure of the receiver it's feeding, or it won't actually improve things. S meter readings are completely meaninless.

 

[friedthat]

hmm indeed. well thank you kind sir for pointing that out. now what noise figure should i look for so i will get better results? or for that matter what should i be looking at when i go buy a amp for TV etc?

It's easy. The noise figure for the 470-890 range is higher than the noise figure to your receiver. The amplifier is creating a worse signal to noise ration than you have without the preamp.

Look at the specs you posted. The noise figure is the measure of the amount of noise tha amplifier adds. That number MUST be lower than the noise figure of the receiver it's feeding, or it won't actually improve things. S meter readings are completely meaninless.

 

[zz0468]

You're actually better off without a preamp most of the time. If you HAVE to have a really long feedline, then can be useful, though. You can get some decent wideband preamps with a noise figure around 3 db. If you get one that's band specific, you can get them with a nf of around 1 db. Higher than 4 or so, and you're actually destroying your ability to hear weak signals, even though the s meter may read higher. It's all just noise.

Also, you'd do well to avoid anything marketed for TV reception. TV sets have 10 or 12 db noise figures, so those 7 db amps actually help. Your scanner is probably around 4 or 5.

 

[friedthat]

okay well for the VHF band BMR Fleetnet i am good without amp but i want to really be able to pick up the waterloo EDACS system and since i have moved out side the city all i hear is static and the odd time i will hear some voice but it is very broken.
i currently have a makeshift antenna ( an off centre dipole) in which i use 12 awg wire for both ends and hooks up to a 300- 75 Ohm thing that connects to regual TV coax.. sooooo is there a away i can reconfigure the antenna to make it more compatible with the UHF?

 

[zz0468]

Start with a decent antenna and coax, and then evaluate whether or not you need a preamp. Even a good preamp won't fix the problems caused by a cruddy antenna.