Topband: Input Impedance of the FT1000/S-Meter Calibration issues

[Tod - ID]

Sinisa:

I think this summarizes things to this point:

> 
> Therefore, S meter reading can be used to indicate signal 
> level trends, but they are hardly usable for real measurements.

The following generated a couple of questions.


> Effect of SWR at RX input is the same as in any other case:
> a part of the available power is reflected back towards the 
> source, and therefore not used to drive the receiver.
> 
> When measuring sensitivity of a high SWR RX, such as 
> FT1000MP, (if no correction is made) the receiver will appear 
> as less sensitive than it really is, i.e. requiring more 
> power from the source for the nominal output.

>Actually, the source will deliver more power, but only a part of it will 
> enter RX, rest of it will be reflected back to the source.
> 

The first question is directed to the last paragraph. I had the impression
that the reflected wave would move back down the line to the feed point of
the antenna and then reflect from there back to the RX. Presumably each pass
and reflection will leave a small amount of energy in the resistive part of
the impedance at the reflection points and some in the resistance of the
feed line and some in the dielectric losses of the system. Except for the
feed line losses I would have thought that all of the power would have been
dissipated in the RX resistance. Maybe I have the wrong concept of such
reflected waves.

The second question was suggested by all the paragraphs. There is an
implication that if one were able to cancel the reactance at the RX input
the signal 'heard' would be louder. Presumably if we had an unemployed
"Maxwell Demon" we could enlist his aid to constantly adjust a picoscopic
tuner to provide an exact 50 impedance for every frequency we tune and we
would have the best 'hearing' of a signal that would be possible with the
RX. 

That said, one wonders just what the improvement would be if we had that
'magic' 50 ohm load to capture the signal power. Would the improvement be
significant enough so that we should pursue it, or is the improvement so
small that it is of no consequence. 

Except for 80 meters the input impedance is +j and I suppose we could put a
capacitor in series with the input to cancel those reactances. Then, the
input resistance would be between about 40 ohms and 65 ohms (except for 40
meters). The result is a rather small SWR, less than 1.5:1. I would expect
that to be close enough to not need additional adjustment. 

Someone else who knows what they are doing might wish to calculate the
resultant signal difference between the resistive input and the resistive
plus reactive input. My SWAG is that the voltage change due to an all
resistive load at 40 or 75 ohms would be about +/- 1 dB. Not enough for me
to hear the difference.

Tod, KØTO