[R-390] Measuring Sensitivity

[Barry]

> Here is what I suggest, if you want to "Make sure you can reasonably
expect
> the actual RF voltage at the antenna terminals to be what you think it
is."
>
> Make up a voltage divider with a 50 ohm resistor and a one-half ohm
> resistor in series from the input to ground.  The output is taken off the
> one-half ohm resistor.  This will do a number of things:
> - It will present a 50.5 ohm load to the end of the cable from the
generator.
> - It will divide the voltage at it's input by 100.
> - It will let the cable operate at 50 ohms, so there will be no or little
> standing waves to bother anything.
> - It will reduce the effect of any possible leakage into the cable or
> connections up to the antenna connector, either from the generator or from
> broadcast band signals or whatever.
>
> I suggest you make this thing up inside a twinax connector with a BNC
> connector fitted into the back end.  This will provide a convenient
> divider/load for use with all your R-390 radios.  The half ohm resistor
> (two one-ohm units in parallel perhaps) go from twinax pin to pin.  Ground
> one of them (there is a correct one to be grounded). Feed the other from
> the center pin of the BNC connector via the 50 ohm resistor.  The BNC
> connector can be mounted into the cable clamping nut of the twinax
> connector either by filing the threads and force fitting or soldering, or
> by threading the clamping nut.  (I think it's a 3/8 by 32 thread.)  Allow
a
> flexible wire from BNC center connector to the 50 ohm resistor to allow
for
> threading the clamping nut into the connector body.
>
> Now, if you have a GR 1001A, use the 100 micro volt setting of the
> attenuator with no series resistor.  If you use other settings of the
> attenuator, use the 40  ohm series unit. You have a generator system that
> delivers to the antenna terminals one percent of the voltage indicated on
> the panel.  If you have a generator that operates correctly with the cable
> terminated in 50 ohms,  just use the cable from generator to voltage
> divider unit.
>
> The voltage at the antenna terminals will be one hundredth of the panel
> indication.  The input impedance of the radio will not matter enough to
> bother.  (The "output" impedance of the signal source is about one half
> ohm, and the input impedance of the radio is from 50 to 700 ohms.)
<snip>
> Roy
> Who needs an output attenuator dual pot for his 1001A

Roy,

I'm curious about this voltage divider.  If the voltage at the output end of
the cable when terminated in a 50-ohm load is 1/2 the value set on the
generator's controls, then is it correct to say that the voltage at the
connection between the 50-ohm and 1/2-ohm resistors is actually
one-two-hundredth the value shown on the controls rather than
one-one-hundredth?

For example, it I set the generator to output 1mV (0.0001V), then would the
voltage the radio sees be 5uV (0.000005V) and not 10uV (0.00001V)?

Thanks,

Barry - N4BUQ