[R-390] R-390A sensitivity measurements

[Michael Murphy]

A lot of these RF guys on the R390 net will attest that leakage can surely
fool you. Some have probably solved the leakage isue by building something
or using  some clever technique. The basic symptom of RF leakage is that the
receiver seems miraculously sensitive - "what a good tuneup job did I!"  You
keep reducing the RF input signal (throwing in more attenuation) and the
receiver just keeps hanging in. Wow is this baby hot!

I have been burned by measurements done much below half a microvolt on the
bench. We have problems at work obtaining accurate measurements outside a
screenroom or screenbox environment. One typical (difficult) test is to
determine end to end system gain with a high power transmitter and a
sensitive receiver. The RF just seems to be able to go around to the back
door! This is a bad example since our test generator is not a 2 Watt
transmitter, but the ideas still apply.

Let's look at an approach.. Good results are obtained by placing the RF
generator on the outside of the screen room. This may not be ideal, but it
keeps the receiver from picking up extraneous signals and keeps the higher
level RF on the outside (the dirty side). Terminate the generator if that is
required. Connect the generator to the screenroom bulkhead via double
shielded coax like RG-223. Adding an inline pad of say 10 dB on EACH SIDE of
the bulkhead is another good trick. Just inside is also a good spot to put a
precision variable attenuator inline. Next connect to the receiver with more
RG-223 using one of the matching networks described. Of course one will have
to know the total loss of the cable attenuator system. Our three Screen
Rooms at work are double shield Faraday types with a single point ground and
they claim better than 100 dB of isolation. Solid construction screen rooms
are actually far better than this.... but it gets so lonely.

I would say that a more practical home version of this approach might be to
make a simple screen box to house the generator side. This could be made of
copper screen and wood. Making the inside and outside of the box insulated
from one another using square dowel or lumber should enable you to build a
tight little Faraday screenbox. The bulkhead can serve as the single point
ground. Ground the box. A clever door with RF gasketing fingers would allow
access to the generator for setting adjustment. Think about it - the door
must also be two metal doors without inside touching outside to maintain the
cage integrity. Again, double shielded cable is a must.

Mike Murphy WB2UID

----- Original Message ----- 
From: "Roy Morgan" <roy.morgan at nist.gov>
To: "Cecil Acuff" <chacuff at cableone.net>; <DJED1 at aol.com>;
<ghayward at uoguelph.ca>; <r-390 at mailman.qth.net>
Sent: Wednesday, March 02, 2005 4:22 PM
Subject: Re: [R-390] R-390A sensitivity measurements


> At 11:48 AM 3/2/2005, Cecil Acuff wrote:
> >I've been contemplating a balun to go from the balanced input to the
> >antenna/generator anyway.  Question is do I do a 2:1 or a 1:1.
>
> Cecil,
>
> Try both. And let us know what happens.
>
> I plan to install a little toroid core inside a twinax connector with a
BNC
> female connector mounted in the rear of the twinax.  there's plenty of
room
> for a small toroid.  I'll be glad to hear how you make out.
>
> The TMC company in their GPR-90 receivers used a toroid input transformer
> inside at the antenna connector.  I don't know it's turns ratio.  The
input
> RF amplifier tube is a 6AB4 triode in grounded grid configuration. The
> transformer is tapped for both 300 and 75 ohms input, and the crystal
> calibrator signal is fed to the 300 ohm input point.
>
> Roy
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
>
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