[R-390] DA-121 adapter in MK–288 for R-390A, URM-25

[Larry H]

When connecting an R-390A balanced antenna to an AN/URM-25 Signal Generator
for testing, most of the documentation says to use the DA-121 impedance
adapter.  But, only very little documentation says that you can find it in
the MK–288/URM  Electronic Equipment Maintenance Kit, or where the
documentation
can be found for the kit.  Well, a few years ago, David Hallam asked about
the documentation and Manfred Antar provided it through the R-390 List
forum.  Thank you Manfred.

Then, a few weeks ago, I realized that it would be beneficial to have a
link to the online version of it on our website for easy access to it.
Since the documentation is an Army document, the link to it (TB SIG 319) is
at the bottom of the Army section of our 'Reference' page.  The MK–288/URM
(TB SIG 319) doc is on 'radionerds.com' at: nfe82b2.tmp (radionerds.com)
<https://radionerds.com/images/b/bd/TB_SIG_319.pdf>.

Because the DA-121 impedance adapter is a resistance 'L' pad and as such
the output voltage is a percentage of the input  voltage (45% of input, 9.1
db loss, or 4/9 of the input) for this adapter (each adapter is
different).  EG: if the sig gen reads 1 uv out, the voltage seen at the rx
will be .45 uv.  Of course, this is only true when the actual input
impedance of the R-390A Balanced antenna connection is 125 ohms.  Well, as
you probably know, the impedance varies from about 50 ohms to about 200
ohms, depending on the received frequency.  Although I have not calculated
it, I believe the variance is small enough to not make much difference.

Regards, Larry