Topband: dynamic range

[Richard Karlquist]

> When calibrating an S-meter for 50uV equal to S9 the signal
> generator should
> be terminated into a 50 Ohm load, for best accuracy . However, when a
> receiver input is driven it would seem that the reflected impedance is
> likely something other than simply 50 Ohms resistive. Therefore, the
> generator is no longer precisely terminated and the output voltage will be
> something other than 50uV. This would create an error.
>
> My thought is you start with a generator with an output much greater than
> 50uV, then follow it with an attentuator connected between the generator
> output and the receiver input. This pads the generator and it sees a load

Any decent signal generator will have a built in output attenuator providing
a good 50 ohm source impedance.  You shouldn't need to pad the output
externally.

The mismatch error due to the receiver input impedance is
a dB or two at the most.  Thus if you calibrate the receiver
with a 50 ohm source, and then connect it to a transmit antenna,
any error due to change in match will be negligible compared to
the basic accuracy of the S-meter in the first place.

I found on my FT-1000D, the meter was pretty good from S-9 and up.
But S0 on the meter was really S4.  It was like 2 dB per S unit
until you got up to S7 or so.

Rick N6RK