[R-390] AN/URM-25s

Thu Mar 1 19:43:19 EST 2012

I've worked with the guys performing repairs on radar sets and the like, 
all the the same time the massive search radar was just above us in 
Western PA.  It saw out into the Atlantic.

We certainly didn't have all this isolation that has been described.  
Yet - we were able to get the MTRs, (Missile Tracking Radars), repaired 
and tested.  Sorry, but these were antiques in the early '70s.  Some of 
the circuit boards had discrete components such as 2N110s on one of the 
little boards.

One of the MTRs requirements was to lock on the top 12 inches of the 
Nike Hercules, AND stay locked for the entire launch until intercept.

So such extreme measures really aren't all that necessary to perform a 
high degree of adjustment and tweaking of equipment.  That is a simple 
reality that some folks do not grasp despite their intellectual acumen.

BTW - We were required to take an entire Missile battery to Ft. Sill, OK 
annually, and perform actual firings against drone aircraft such as T-33 
and/or F-89s.  So I know from experience the ability to achieve this level.

Bob - N0DGN

On 3/1/2012 6:25 PM, 2002tii wrote:
> It can be done, but it is a challenge.  I generally find that most
> hams' measurements of sensitivity at HF are optimistic by 10 dB or
> more due to signal leakage.  For all of their good qualities, R390As
> are particularly egregious offenders -- they leak RF like a sieve, so
> ANY leakage from your sig gen will make them appear to have very high
> sensitivity.
>
> Note that there are two issues -- signal from the generator leaking
> out of the cabinet and into the DUT, and external QRM and QRN leaking
> into the DUT (possibly including RF that leaks into the sig gen and
> then appears at its output jack, or is induced into the cable
> connecting the sig gen to the DUT).
>
> It helps greatly to use a well-shielded external coaxial attenuator
> (e.g., HP355A + 355B, or JFW adjustable attenuators, or Mini-Circuits
> in-line attenuators), placed right at the input of the DUT, rather
> than using the internal attenuator in the sig gen.  (By "right at," I
> mean connected using just a coax adapter, no cable at all -- prop the
> attenuator up on a stack of books or something so it isn't just
> hanging there.)  That way, the level in the connecting cable is much
> higher and any QRM/QRN introduced before the input jack of the DUT is
> attenuated by many dB.  Ideally, you would move all of the
> attenuation to the DUT end of the cable and run the sig gen at 0 dBm
> or more.  If nothing else, get three or four Mini-Circuits HAT-30
> fixed attenuators (http://www.mini-circuits.com/pdfs/HAT-30+.pdf)
> ($10 each) and put them right at the input of the DUT to do 30, 60,
> 90, or 120 dB of your attenuation there, and do the rest (always less
> than 30 dB) at the sig gen.
>
> Preventing leakage into the DUT other than through the antenna jack
> can be harder (and, of course, to the extent that you reduce it, your
> results will not represent how an off-the-shelf unit works).  A GOOD
> power line filter is mandatory, on both the DUT and the sig gen.  Use
> copper mesh gaskets at all cabinet joints (or these days, conductive
> polymer shield gaskets).  One of the greatest sources of this ingress
> (or egress, in the case of signal generators) is the chassis openings
> for controls.  One thing that helps is to put mesh grounding collars
> on the shafts of rotary switches, capacitors, and potentiometers to
> ground the shafts to the cabinet (look inside the front panel of an
> HP8640B for an example).  These days, you will probably have to make
> your own -- coarse stainless-steel wool works OK (do NOT use regular
> steel wool).
>
> If the input jack of the DUT is floating (not galvanically connected
> to the chassis right there at the jack), make sure the body of the
> connector is RF-grounded using a 0.01-0.1 uF capacitor (best practice
> is to use the cap in parallel with a 10-100 ohm resistor).  This
> applies to any other floating connectors, as well, not just the input
> jack.  You also may want to wind 10 turns or so of the cable
> connecting the sig gen to the DUT around a ferrite toroid core (Type
> 43, say 2.4" OD) to keep any QRM/QRN riding on the shield from
> getting to the DUT input.  Put this choke immediately before the
> external attenuator (or the DUT input, if you are not using an
> external attenuator -- but understand that if you are putting -130
> dBm and lower signal levels into the sig gen end of the cable, you
> are unlikely to get good results no matter what else you do).
>
> If you have a chance, taks a Harris 590 apart and observe what they
> did -- the 590 is the best-RF-sealed receiver I have seen
> (interestingly, the 590As I have seen are not as good by 15-20 dB at
> some frequencies).
>
> Best regards,
>
> Don
>
>
> Copyright (c) 2012.  Not for redistribution
>
>
>
>
>
>
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