problem with pre-amp

[wesct]

take a look at these sites
http://www.bsrg.org/aatn/hawgamp.html

http://www.repeater-builder.com/rbtip/preamps.html

the first site has a instructions on how to make a gasfet preamp with a typical gain of 13db. the second site talks about attenuation and how too much adds floor noise.

my boss says 13 db should be the max, and hes been in the business for a long time.
when a company called cpi made radios designed by motorola, their preamp had a maximum gain of 13db. it seemed to work great.
ofcourse, we are talking about equipment made 25+ years ago and alot has changed since then.
if i am wrong, then i am wrong. but it seems correct.

wesct

 

[xusmarine1979]

Now is that 13db for all bands, or just 800Mhz?

 

[N_Jay]

Its probably an OK rule-of-thumb, but it's better to learn how to count with all your fingers.

 

[Al42]

take a look at these sites
http://www.bsrg.org/aatn/hawgamp.html

http://www.repeater-builder.com/rbtip/preamps.html

the first site has a instructions on how to make a gasfet preamp with a typical gain of 13db. the second site talks about attenuation and how too much adds floor noise.

Which has nothing to do with too much gain adding noise.

my boss says 13 db should be the max, and hes been in the business for a long time.

I've probably been in the business as long as he's been breathing, but tell him for me that there's no one gain that's best for all frequencies, all uses, all antennas and all receivers. Without knowing the gain and noise figure of the receiver, and the noise figure of the preamp, there's no way to determine optimum gain. Even then, "optimum" is a variable. Do you want maximum signal? Minimum noise? Maximum S/N ratio? Are we talking about cleaning up noisy signals, bringing a digital signal up close enough to the noise floor to be demodulated or trying to receive a low powered transmitter on the surface of the moon?

when a company called cpi made radios designed by motorola, their preamp had a maximum gain of 13db. it seemed to work great.

With those receivers for that usage. One of the best preamps ever "made" was the final stage of the transmitter of the Motorola U44, the one with the lighthouse tube. It was totally immune to overload and it had one heck of a lot more than 13 db gain.

if i am wrong, then i am wrong. but it seems correct.

You're not "wrong". You're not right either. No single "correct" gain figure for preamps is correct.

 

[wesct]

Which has nothing to do with too much gain adding noise.

I've probably been in the business as long as he's been breathing, but tell him for me that there's no one gain that's best for all frequencies, all uses, all antennas and all receivers. Without knowing the gain and noise figure of the receiver, and the noise figure of the preamp, there's no way to determine optimum gain. Even then, "optimum" is a variable. Do you want maximum signal? Minimum noise? Maximum S/N ratio? Are we talking about cleaning up noisy signals, bringing a digital signal up close enough to the noise floor to be demodulated or trying to receive a low powered transmitter on the surface of the moon?

With those receivers for that usage. One of the best preamps ever "made" was the final stage of the transmitter of the Motorola U44, the one with the lighthouse tube. It was totally immune to overload and it had one heck of a lot more than 13 db gain.

You're not "wrong". You're not right either. No single "correct" gain figure for preamps is correct.

you have valid points-nothing is perfect and too much gain adds noise depending on the receiver and other factors.

wesct

 

[N_Jay]

Gain and noise-figure are not the only issues.

Pre-amps are also rated by "Third-Order Intercept Point" to indicate what level of signal they can pass without generating IM, and their "compression point" that indicates the level of signal that can be passed without a reduction in gain.

The overall effective sensitivity of the system (what you REALLY care about) is affected by all these factors.

 

[Al42]

too much gain adds noise depending on the receiver and other factors.

Gain doesn't add noise. That must be on the same page as the 13db gain figure. (Gain may actually decrease noise. The only way gain can "add" noise is if the device creating the gain is generating noise.)

 

[Al42]

Gain and noise-figure are not the only issues.

Pre-amps are also rated by "Third-Order Intercept Point" to indicate what level of signal they can pass without generating IM, and their "compression point" that indicates the level of signal that can be passed without a reduction in gain.

The overall effective sensitivity of the system (what you REALLY care about) is affected by all these factors.

And by the receiver's first stage. If that has better numbers than the preamp, the preamp will degrade performance.

 

[par38lamp]

I had a 800Mhz system I was trying to receive using both ath RS 800Mhz rubber duck, and the RS centerl loaded adjustable whip. No luck.

I then purchased a used 800 Mhz yagi off eBay, and WOW. Comes in perfect. Now all I have to do is mount the thing in my attic.

BTW, I am in Fenton, MO (63026), and trying to monitor St. Louis City public safety.

 

[Al42]

I had a 800Mhz system I was trying to receive using both ath RS 800Mhz rubber duck, and the RS centerl loaded adjustable whip. No luck.

I then purchased a used 800 Mhz yagi off eBay, and WOW. Comes in perfect. Now all I have to do is mount the thing in my attic.

BTW, I am in Fenton, MO (63026), and trying to monitor St. Louis City public safety.

You'll receive 800 MHz stuff from SL, but if you draw an X, with the front legs at 45 degree angles to the front direction of the Yagi, the legs of the X will be pointing in the 4 directions you'll receive almost no signal from - so plan accordingly. Sometimes you have to turn the antenna for a little less than maximum signal from one source in order to not completely lose the signal from another source.

 

[par38lamp]

You'll receive 800 MHz stuff from SL, but if you draw an X, with the front legs at 45 degree angles to the front direction of the Yagi, the legs of the X will be pointing in the 4 directions you'll receive almost no signal from - so plan accordingly. Sometimes you have to turn the antenna for a little less than maximum signal from one source in order to not completely lose the signal from another source.

Yeah, but I am only trying to monitor one system. From what I can tell, the OP only mentioned one system, so my meager suggestion would work for him, too.