WhiteStorm71
What coax to use and do you need a preamp?
By all means RG6 - but keep in mind a few things: attentuation per ft/per 100ft/per 100meters .. or whatever unit of lenth you are using, and the relationship between attentuation & gain and the pre-amp you decide to use.
Typical RG6 attentuation per 60ft at 100Mhz will be around 1.6 - 1.7 db. At 200Mhz it will be around 2dB - 2.1dB. At 400Mhz it goes up to around 2.5dB. At 600Mhz it's around 3.4dB (in other words your signal strength into the receiver has halved from the antenna!) and at 900Mhz it will be around 4dB (your signal strength from the antenna will have dropped to around 1/3).
Is this bad?
Nope, it's actually quite tolerable and so long as you're in a quiet area from an rf perspective you shouldn't have anything to worry about. But if its a noisy rf area, and you decide to use a preamp, things can get real noisy and complciated. I hate pre-amps as a rule. As most know they not only amplify the weak signal but they also amplify the noise - and now the quality of the coax does become an important issue.
Sadly, coax is usualy only thought of in terms of loss and mistmatch, and while loss and/or mismatch many will argue is not too big an issue in general VHF/UHF scanning scenarios, it can become a real impotant issue when a pre-amp is thrown into the equation.
Poor quality coax is plenty plenty more noisy than good quality coax - so you'll land up with the original noise picked up by the antenna, as well as the coax noise. In short: you get a poor SNR. I prefer to leave out the pre-amp where ever possible i.e. a weak signal with good SNR is almost always preferable to a strong signal with a poor SNR.
But there are cases where a pre-amp is unavodiable, and going to the effort of ensuring the best pre-amp for the job is used, requires a few poitns to be thought through.
Antenna Signal Strength
Having an idea of average signal strengths the antenna can provide you with in it's standard passive mode across the various frequencies it is going to be worked on, without the pre-amp in the link, is the starting point.
Receiver Sensitivity
What is the reciever sensitivity (i.e. min & max levels for various bandwidths).
Coax Loss
.. and of course the coax loss/attentuation at various frequencies.
Without this info, or at least an idea about it, you stand a chance of making things worse, not better, by introducing a preamp. Time & time again over the years I have seen amature and professional setups use the largest gain preamp that could be afforded - as if, "well I could afford the 20dB gain pre-amp - so I chose it" as if this was the best approach to take, or no attention was provided to the input level the pre-amp required to avoid its 1dB compression level, or no attention was proivded to it's gain levels across the bandwidth it was going to be used on.
Running pre-amps without at least some insight into the above, can fast makes things worse than they would, or could be withotu the pre-amp in the first place. Not only do they amplify basic noise levels (as everyone knows), but running any component in the link at at a higher gain level than that which is required or needed, will have output stages running most of their time, in, or very close to, their 1dB compression point. The result: poor signal levels, saturation and added distortion. The objective is defeated.
Read up on amp 1dB compression point - and just what impact it can have on received signal quality. All preamps & amps run best/most effiecently and at their quiestest when their input levels are in the reange they are designed to work with
By same token, be careful you don't run the preamp with too high a voltage. That too can quickly push a preamp into it's 1dB compression point and/or above - now you're going to have a real XXXX signal - loads of distrortion and loads of clipping. If the preamp is going to be run from dc over the coax, check out how much dc is lost over the length of coax used, and ensure the power supoply used can provide the extra voltage required - but not too much.
In summary: coax loss/attentuation over log runs at different frequencies should be quantified carefully if a pre-amp is going to be introduced to compensate for that loss. Knowing what the antenna gain is to start with, what signal strength the pre-amp needs at the diffrent frequencies to provide the amounts of gain required at those frequencies over the length of coax cable used wil ensure that the receiver gets the best possible signal avaliable.
What does it cost to keep in mind the above points when selecting a pre-amp? Nothing, just a bit of extra time.
