Join Date: Jul 2009
Location: Eastern Ontario (Ottawa/Kingston)
That is a very interesting point you mention in the previous post.
Re: Try to keep line length between Antenna and pre-amp to a minimum.
Yes, makes sense.
There's two parts of THEORY I remember from school.
1. About RF transmission lines. Mismatches cause higher VSWR's on a transmision line, leading to signal reflections, and to standing waves on the line, and to larger MAX's and Min's in voltage along the length of the transmission line. Points of high and low voltage along the TL - transmission line lead to higher losses.
And also, less power is tranferred to the load at the other end of the line.
(and they mentioned and showed us that energy is consumed and stored along the length of a transmission line - so keeping that first, critical TL, between your antenna and your pre-amp, as short as possible must be a good advantage)
2. About getting the maximum signal down the line - towards your receiver.
To transfer maximum energy (and signal) to the load - the characteristic impeadance of the load must be the same as the line ( ie. they must be MATCHED).
Actually, they showed us, with complex numbers, vectors/phasors, impeadance, reactance and reluctance ... that the load impeadance should be the conjugate of the source impeadance. (same resistance, opposite reactance ... something like that).
These impeadances vary with frequency. The antenna, the pre-amp, the transmission line.
So your point makes sense to me - and has possibilities for improvements in system efficiency - i.e. getting fewer losses, and more signal down the line.
That means, as you mention - keep the line short between the antenna and the pre-amp.
But it also means that that the input stage of the pre-amplifier (the "LOAD" in this case) has to match as closely as possible the impeadance of the TL and the ANTENNA - across the frequency band you're working.
So this all suggests a PRE-AMP design that could mount right on the antenna, right at the terminals, practically right beside the driven element, and match it as closely as possible - over the band.
So now you're looking at a pre-amp that's custom built for a given antenna - mechanically (mounting and position - and designed so that it does not interfere with the signals around the elements of the antenna - since it it so close) and electrically (to match close as possible).
If this can be accomplished - you can get some great performance out of your antenna / pre-amp combo .
The same holds after the pre-amp, down the line to the receiver ... but I am guessing it becomes less important, because after the pre-amp, you have already amplified the signal up enough that those losses and in-efficiencies become less important. You still want a match here .. but the one with the antenna is most important.
Just some ideas and thoughts.
It's all theory that I remember from E-SKOOL ... but it all makes sense.