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

[rbethman]

  No question regarding the "screen"!

The AN/URM-25(X) leaks horrendously!  I used one for a sand state 
receiver.  Even six feet away, I dialed the attenuator down to absolute 
ZERO!  I was STILL receiving a carrier at over S-4/5.

I use Chuck Rippel's suggestion of grounding one side of the balanced 
input and feed the center conductor of the RG-8/RG-58 to the open side.

I was "attempting" to get this across, however - I FAILED MISERABLY!

Using the older test gear works fine when one takes in all the issues.

Line-up:

     AN/URM-25(D)
     TS-505D/U (VTVM)
     TS-585D/U (Audio load/meter)

Nothing fancy here.  Except the Tek 5440 scope, and a B&K frequency counter.

Bob - N0DGN

On 9/21/2010 7:48 PM, Flowertime01 at wmconnect.com wrote:
> 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>
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|               AM Amateur Radio Operator    NØDGN                 |
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| Bob Bethman                \\\|///     " The absence of a danger |
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