[Milsurplus] BC-1000 (WS31) Squelch Capture Problem

[Brooke Clarke]

Hi:

In the article on the Wireless Set No. 31at 
http://www.vmarsmanuals.co.uk/newsletter_articles/ws31.pdf  it mentions 
that all of the original WS31 sets were upgraded to Mk 1/1 status which 
was functionally the same as the newer Mk 2 and the article has 
speculations about why this was done.  It appears that the BC-1000 was 
the first FM squad radio to see real operational service and so was 
first to encounter jamming based on the FM capture effect.

An enemy can jam an FM receiver by sending an unmodulated carrier that's 
a little stronger than the friendly signal.  The receiver will break 
squelch but the audio output will be silence.  In later squad radios 
that use the 150 Hz PL tone the radio never hears the tone and so does 
not break squelch.  The modern squad radios deal with this by having a 
Squelch Disable function.  When activated the operator should hear noise 
when no signal is being received.  If the operator activates Squelch 
Disable and the radio is quiet then either the radio is not working or 
is being jammed.  A better solution is to use frequency hopping which 
greatly increases the power required to capture the receiver.  That's 
one of the reasons the SINCGARS and Have Quick radios use frequency hopping.

Note that AM radios are not susceptible to this type of jamming since 
they do not have the capture effect.  If two stations, even considerably 
different in power are on the same frequency the receiver will hear both 
of them.  That's why today aircraft radios use AM as a safety measure, 
not for jamming reasons.

73,

Brooke Clarke, N6GCE

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