[R-390] R-390 alignment questions

[Craig McCartney]

Bob et al,

In some situations, like a large receive site, matching the impedance
presented by the receiver to the antenna is not so important.  The antennas
all terminate in a set of distribution amplifiers.  The output of these amps
is what drives the receiver.  The amps are nominally rated at "50 ohms"
which typically means a low impedance cathode follower (or emitter follower
depending on vintage) output with several 50 ohm resistors, one in series
with each output connector.  This is the setup at all the Globe Wireless and
AIRINC HF sites (and also in my shack).  

In this case I think it is best to align (and measure) the receivers on the
bench with the 50 ohm termination or matching pad.  Then they will be
optimized for a 50 ohm system.

By the way, most antennas are not 50 ohms resistive over much of a frequency
range (if any).

Just some rambling thoughts.

Craig

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Bob Camp
Sent: Thursday, October 14, 2004 3:16 PM
To: K2CBY at aol.com; r-390 at mailman.qth.net
Subject: Re: [R-390] R-390 alignment questions

Hi

In just about every other measurement I know of what you say is exactly
true.

For a variety of reasons receiver sensitivity is not measured that way. In
the case of a receiver you do not know the impedance the receiver presents
to the antenna. It may be high and it may be low, who knows. It is rarely 50
ohms.

One solution would be to measure the actual impedance that the radio
presents to the antenna. With a reasonable amount of gear you could do that.

Once you know the radio's impedance you could come up with a combination of
coils, capacitors, and resistors that converted that impedance to 50 ohms. 
In doing that you would have to make some decisions about the impedance
transform you applied in the process. Different individuals could make
different assumptions and their data would be significantly different as a
result.

Since that's more than anybody wants to go through they came up with a
different approach to this particular measurement:

1) Make sure the *source* impedance of the generator is correct. The old
Measurements generators came with an attenuator to do this. Most modern
generators have a number of fancy attenuators in them that do this quite
nicely.

2) Calibrate the generator into a 50 ohm load or into what ever impedance
you are running.

3) Hook the generator straight into the receiver. Use the "calibrated" 
numbers on the generator to report the data.

Obviously this means that a radio *could* have twice the calibrated voltage
at it's input. In fact this is exactly what most radios do. You could look
at this as cheating but that's how it's done.

    Enjoy!

        Bob Camp
        KB8TQ




----- Original Message -----
From: <K2CBY at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, October 13, 2004 8:50 AM
Subject: [R-390] R-390 alignment questions


> You may be deceiving yourself with respect to the sensitivity measurement.
>
> You make no mention of terminating the 8640B signal generator.
>
> To get an accurate reading, the output attenuator of the 8640B (and any
> other signal generator) has to be terminated with a specific load 
> resistance -- 
> in this case, 50 ohms. If it is terminated with a high impedance  -- i.e.,

> the
> unbalanced input of the R-390A -- less current is going to be  pulled 
> through
> the series element of the attenuator, and the "multiply by"  marked on the
> skirt of the output attenuator dial isn't going to be correct.  What 
> appears to
> be 0.2 or 0.3 uV is actually going to be higher.
>
> The BALANCED input is nominally rated at 125 ohms (though this is probably
> far from constant either from one band to another or across a signle 
> band). A
> 50-ohm signal generator connected to the balanced input should therefore
> terminated by an 82 ohm resistor (with short leads) connected across the 
> antenna
> input terminals.
>
> The impedance of the UNBALANCED input is not indicated in any of the
> documentation I have, but it is supposed to be "high." I would therefore 
> terminate
> the signal generator with a 51 ohm resistor so that it sees a proper 
> load.
>
> Only when the signal generator is properly terminated do the output 
> signal
> readings shown by the panel meter and the switch  attenuator correspond 
> with
> reality.
>
> Miles, K2CBY
>
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