[Premium-Rx] Screened rooms

[Cecil Acuff]

Hey Folks,

        Another good choice which is more flexible than LDF but still 
maintains the leakage characteristics of LDF is another product made by 
Andrew called SuperFlex. (FSJ)  It's available in 1/4" and 1/2".  The 
connectors are a little tedious to put on but it makes nice jumper cables 
for use on the bench etc...much easier to work with.

Not as durable so they are not to be used between tower mounted antennas and 
main feedline runs.  The wind will fatigue the outer conductor.

Cecil Acuff
WB5VCE
----- Original Message ----- 
From: "Michael O'Beirne" <michaelob at tiscali.co.uk>
To: "Premium-Radio" <premium-rx at ml.skirrow.org>
Sent: Saturday, January 08, 2005 11:48 AM
Subject: [Premium-Rx] Screened rooms


Hi everyone,

While previous postings have considered the pros and cons of a screened 
room, it might be as well to consider additionally the effectiveness of the 
screening of standard coax.  Para 3.14 of the manual for the HP606B signal 
generator states that single braid shield types are suitable for use from 
max output (about +20dBm) to approximately -80dBm.  For better shielding you 
need to use double screened cable such as Suhner's RG223 or solid 
outer/flexible microwave cable such as LDF.

I first experienced this when using the Marconi Instruments TF801/D VHF 
signal generator to measure the sensitivity of a Yaesu FT221R two metre 
transceiver.  This genie is old but has superb screening and hardly anything 
gets out.  When I connected the counter output to a Racal-Dana 9916 counter 
with a short length of RG58, substantial QRM at S9 level (5 microvolts PD) 
showed up.  This is very loud when you have a front end with a noise figure 
of just under 2dB!  This QRM vanished when the counter's cable was unplugged 
from the genie but left the problem of measuring the frequency with 
sufficient accuracy because of slow drift at 145MHz with the free-running 
oscillator in the genie.  Then Pat McAlister, G3YFK, kindly supplied me with 
a short length of LDF cable with BNCs on the end and the radiation did not 
show up any more.  I also use short patching leads of RG223 which I obtained 
from government surplus, unused condition, for intermod and blocking 
measurements to interconnect the rig, hybrid combiner and genies.  You need 
these when the genies' output is at -7dBm and you are listening for an SSB 
MDS level of -130dBm for HF receivers.

73s
Michael
G8MOB






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