[Premium-Rx] Screened rooms

[Michael O'Beirne]

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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