[Milsurplus] RBM - MF question(RU-19 selectivity)

[Jack Antonio]

On 10/31/2017 3:37 AM, Hubert Miller wrote:
> I wanted to know the -60 db selectivity of some receivers, since, AFAIK, 
> that's a good judgment point for overall selectivity.

> 
> RAX  IF=160 kHz   -60 db  -6.8 kHz  +6.8 kHz
> 
> RBM IF=112 kHz  -60 db  -7.5 kHz   +8  kHz   ( switch= Narrow )


I've been playing with an RU-19 recently, and trying to figure out
how to use one on the air with my GP-7.

I tried checking into the OMRN net on 3570 with the G range
coil set. which was very frustrating. The digital signals
at 3576 simply overwhelmed the weaker CW signals. And on AM,
it always seems there are strong SSB stations 10 kHz up that
wipe out the Sat morning net.

One thing I tried, which helped a little, is to put the
receiver in MCW and use the LM as an external BFO. Then
tune the receiver down in frequency, to put the QRM down
on the upper side of the curve. The CW stations lower
in frequency weren't as much of a problem as the digital
signals. But still, it wasn't all that successful.

So now, I am playing with a crystal controlled converter
using the low range coil set for the RU-19. The
converter is a rough equivalent to the Novice Q-5er,
which Ken Gordon has the article for on his website.

In doing so, I made some simple selectivity
measurements using the O/P coil set for the RU (roughly 200-400 kHz).

The quick and dirty measurements I came up with are

200kc    -6db  2.1 kHz         -20db  3.8 kHz
400kc    -6db  6.8 kHz         -20db 13.0 kHz

Still a bit broad, but much better. The RU-19 only has
3 tuned circuits in the signal path, do I didn't expect
it to be as good as a BC-453.

Results so far are promising. Right now I'm seeing what
happens if I feed the converter into the loop input, and
using the front panel switch to switch between antenna
direct and the converter.

If all this is successful, I may try building the converter
into a derelict ZB housing to make it look "proper".

One other rather extreme experiment I tried, is to feed
the 455kHz IF out of the R-390A into the RU. But that is
bordering on the ridiculous.

Jack Antonio
WA7DIA