[R-390] rf alignment vs. sensitivity [a/k/a saga pt 8]

[Flowertime01 at wmconnect.com]

Getting ready to do the rf alignment.  Haven't found a published spec on how
many dB I can expect to gain/lose going to/from the unbalanced input to the
balanced input.  Observations, anyone?
73, Brian KA9EGW

Brian,

I was introduced to the R390 and R390/A back in 68. To this day I have no 
idea why I would try to do a signal to noise ratio on the unbalanced input.

If the only connectors you have will mate to the unbalanced input, then 
move a balanced line over to the unbalanced connector so you can get connected. 
Then just short the other balanced input wire to ground.

On the bench where you only have one signal coming in to the first RF mixer 
from a generator, it makes no difference how you get it to the grid. Either 
path will have about the same small loss. Then you measure noise and signal 
plus noise. You still have no idea of the absolute sensitivity of the 
receiver. You have a signal you can detect or you do not detect the signal. There 
are no absolute values. 

The balanced or unbalanced path of the single signal from the generator 
will have the same ratio of signal to noise. You got to get low to detect the 
difference in the noise path between the antenna connectors and the RF tube 
grid. Either of those entrance ramps are in good shape and have low noise 
additions and low signal path loss.

At the level of the receiver's sensitivity, you need a very good screen 
room to get any useful values.

In the real world where the antenna brings a spectrum of signals into the 
grid of the RF tube, the balanced input "filters" "most" of the out of band 
signals from the antenna to ground. Thus providing much more "selectivity" 
than the unballanced input. As out of band mixing products are deducted at the 
output of the RF tube the "noise" is reduced compared to the signal of 
choice. Thus a smaller/weaker signal of choice can be detected from amongst all 
the signals available to the antenna when the filter path of the balanced 
input is used.

Thousands of these receivers were run with one pin of the balanced input 
grounded and the center conductor of a coax line shoved into the other pin of 
the balanced input. There is a $25.00 plus right angle connector that was 
made just to get the job done. It accepts a "C" connector just like the 
unbalanced input uses.

Antenna farms larger than some small counties, were coupled to bunches of 
R390's that way and no one through their was a better passive way to get the 
job done.

Roger AI4NI</HTML>