I'm only going to add,..
Using any premade amp - you have two things to worry about - both are reciprocal - meaning they can be inverted - they are the baluns - matching the impedance of one section - to another.
Unfortunately the Amplifier Input and Output impedances are not the same - they cannot be inverted.
The input is one known set of values (Input admittance)
- the output is another set of values (Output admittance - see explanation)
- all based upon a Bias and working range of input (Class of Operation)...
To simply expect the inputs and outputs to be "matched impedance of 50 ohms" to their ends to the environment and properly matched to the working level at the ends where the amplifier is doing it's job - requires some complex understanding of how systems can amplify and TRANSFORM their output result into a power that can be used.
It's not as simple as Balun down the input (thinking it's originally 50 ohms) - and then amplify the signal - then Balun up the output - from the impedance the amplifier is idealized for - into the 50 ohms we think the amp originally balun'd to meet the input of that amplifier stage as the output.
- Think about that for a moment - before processing the threaded notions of amplifiers working in-line as amplifiers and not something else.
The Low-pass filter - like any other accessory - IS NOT SUPPOSED to be the matching 50 ohm balun the amp designer didn't fix, repair or properly incorporate into the design - in the design and tuning of the amplifier. It is simply used for a Filter - if it is used to help the transformation - then you have another problem - the amplifiers output is not properly set to 50 ohms - and because of this - SWR reflections
DO occur. The output cannot be made balanced - it is an unbalanced output at a known impedance - to use the low-pass as a means to fix the 50 ohms, then realize that you have to design a matching network - a Balun to fix this as well as then apply a low-pass filter - you will need to know what the unknown realm of values that "Balun-ized" low-pass filter will need to fix.
In a typical CB output, you have the Final - into a Tank circuit (Transformation) - then into a multi-stage PI filter that not only filters but then also TERMINATES (acceptance/absorption then dissapation) of power not used
- you might recognize this as what a Balun would do
- - only thru it's winds as a ratio
- - the CB does thru this complex network matching scheme of coils and caps.
- You really can't call a CB Output Network a Balun for the Tank Circuit is supposed to provide the coupling and transfer
- - but to make it appear as 50 ohms for a given range of frequencies
- - it's why they use the values they do in circuits like this
- - to make the circuit appear as 50 ohms to that which it was designed for
- - to pass CB frequencies - not much else.
Funny, you look at the filter - take it apart and...
Gosh Darn It, it looks a lot like what that (Insert favorite Radio Brand Here or) Cobra has inside of it ...
The CB's Filter is this same process in multiple stages designed to look as a 50 ohm unbalanced load from one of it's sections into the other - the only changes in the winds and capacitance are for the variables of frequency that are not part of it's pass band. It's designed to appear as a 50 ohm to both the Final and the Antenna's
System (Includes Coax, connectors antenna mount) as long as everything is 50 ohms, your sitting pretty in the rocking chair watching the world go by.
The frequencies it is designed to pass, get thru - back and forth - the other stuff shouldn't - or at least not a enough of it to make the others complain.
- Sidebar for a moment: This is why there are wars that erupt over the SELECTION of a transistor over another because of power delivery seems to favor one type of part over another.
- Why?
- Many factors including the amount of LOSS in power the Transformation (Balun) does to the complex output (Conjugate) of one part REQUIRES over another (one fits the circuit better than another) at least not without a lot of retuning or offsetting a power coupling complex impedance problem.
When you have to use a Low-pass filter as the means to fix an SWR problem - sometimes it works, but for nearly every time - it's a gamble on how well or how bad - the filter can present itself to the system and yet work as a 50 ohms pass thru without attenuating the signals you're trying to amplify, let alone receive.
It a h*ll of a lot more complex than this, but I'm only trying to tell you to look out for those Icebergs - they are only the tip of a much bigger mess underneath.
