Do some antennas work better w/o a pre amp ?

[GKolo]

Diamond Discone with 50' of RG-6 into the pre amp, Short jumper into the scanner i lose my Chattanooga trunked system, If i come right from the antenna into the radio i get Chattanogga back.
I tried 2 Pre Amps, A 11db and a 15 db with the same results....

[gonzalu]

You may be overdriving the front end of the receiver wit hthe pre-amp. Can you measure the signal strength at the antenna lead for a particular frequency? Perhaps you already have a good enough signal.

What does the signal / transmission sound like when on the antenna directly without the amp? And BTW, most amps have a bypass switch so you can keep them in-line and introduce them or take them out of the picture without having to re-connect them.

[popnokick]

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And the reason you're using the preamp at all is?....

[GKolo]

The Chatt Trunked sytem has static without the PreAmp but for the most part you can hear what they are saying. I have no way of measuring signal strength at the antenna. I use the Pre Amp to try and bring in those distant VHF signals better.

[gonzalu]

What Pre-amp are you using? Is it rated for your band? does it have any limitations? Does it have band-pass filtering of any kind?

[popnokick]

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If your preamp is at the radio end of your 50' coax then you are amplifying all the noise in your coax run... Perhaps just enough more noise to kill Chattanooga. Can you move the preamp so it is right next to the antenna on the other end of the coax?

[GKolo]

The first amp is a Scientific Atlanta 51/1000 mhz 11db Drop Amp
The second is a CommScope Amp 53/1000mhz 15db amp

Neither would bring in the Chatt system. I dont have what it takes to mount the Amp up on the Antenna.

[zz0468]

How do the amplifiers behave with stronger signals?

There is a parameter in amplifiers known as noise figure. It's one of the single most important specifications a preamp would have. In order for it to actually make an improvement to reception, a preamp MUST have a lower noise figure specification than the receiver. Typical scanner noise figure is 4-6 db. Many cable drop amps have a noise figure around 8 db. The symptom you would see is that the amplifier gives "more bars" on strong signals, and weak signals would get worse, or go away completely.

Without actually knowing the noise figure of a preamp, you run the risk of actually harming reception. Another characteristic you need to know is the gain. Too much gain in front of an already sensitive receiver will destroy weak signal reception.

Getting real improvement from a preamp can sometimes take a bit of work.

[GKolo]

HMMMMM
As soon as it quits raining, I am going to put my homebrew antenna up on the mast and see what it does with the pre amp, It used to work really good with the same radio and preamp, Just wondering if the antenna makes a difference.

[zz0468]

Originally Posted by GKolo

...Just wondering if the antenna makes a difference.

It can make a difference, based on how much signal the antenna is delivering to the preamp. But it's not so much a matter of some antennas work with preamps and other's don't.

It can depend on particular circumstances peculiar with YOUR specific installation. For example, if you are using a yagi, and there's a strong cell site off the back, you might have enough rejection that you can get away with a preamp to help whatever is in front of the yagi. But replace that antenna with an omni, and all of a sudden that cell site is now strong enough to overload the preamp.

Or maybe you have an omni, and it works fine with your preamp, you switch to a yagi and that troublesome cell site is in the direction your yagi is aimed, and starts causing problems.

See what I mean? No antenna is "better" or "worse" for use with a preamp, but the specific circumstances can make it seem like it is.

And in your case, we don't have enough information to help you.

[GKolo]

So far the Diamond discone sucks with the PreAmp, The other antenna is a homebrew that started life as a 20-176 Radioshack, I cut and added elements for the frequencies i wanted to hear and it seemed to work good, Its going back up on the mast here in the next hour or so.

[gonzalu]

Yeah the performance would be best at the radiator end of the feed line to minimize amplification of noise induced by the cable length :-(

[benbenrf]

Originally Posted by GKolo

Diamond Discone with 50' of RG-6 into the pre amp, Short jumper into the scanner i lose my Chattanooga trunked system, If i come right from the antenna into the radio i get Chattanogga back.
I tried 2 Pre Amps, A 11db and a 15 db with the same results....

Do some antennas work better w/o a pre-amp?

A pre-amp will make no difference to how an antenna performs - but it can make a ton of difference to how the receiver performs.

In short a pre-amp will:

- raise a signal level that is to low to a level that your scanner/receiver can work with it (i.e. demodulate)
- raise a signal so much it saturates the receiver front end - and then you'll hear nothing or just noise.

Interestingly research shows that in something like 65% - 85% of cases where folk think they need a pre-amp, the 2nd point above turns out to be the result. What is actualy needed is a filter.

Remember antennas receive everything - not only the desired signal. They present your receiver front-end with a whole bunch of frequencies and it is up to the receiver front-end mixers to tune into and isolate the frequency you want to demodulate and listen to, and block out the rest. If the signal you are tuning into does not have a sufficient strength in relation to signals around it, or harmonics of the tuned frequency are above a certain level (thats your Signal to Noise Ratio), so that it is unable to stand-out from all the other signals, then the front-end is not going to be able to extract it from everything else and will land up mixing the whole lot together: end result - no demodulation, just silence or noise.

Solution: filter - not pre-amp.

Problem with filters: good filters can cost quite a bit of money - in some cases as much as an average scanner! - which discourages folk from buying them. But properly chosen and setup they will turn the cheapest far east scanner/receiver into a world class performer.

[GKolo]

Hmmm
My main interest is bringing in VHF high and low bands.
When i first added the pre amp it seemed to do pretty good, Maybe its just on the 800 trunked system it did poorly.