[ARC5] ZB discussion!

[Leslie Smith]

Hello Group,

This is a question I have about the design of the ARC-5 sets that arises from the current general discussion about navigation and the enemy intercepting homing signals.

I notice first that an earlier  posting referred to the possibility of enemy intercept from the oscillators in superhet radios.  

I wondered about this for a long time:  The oscillators in BC-453 (and similar) sets run at exceedingly low voltages.  The manual give the plate voltage for the mixer (the 12k8) as being around 15 volts, and on measuring the voltage on the plate it is at that value (even after 60 years).  The LO was operating perfectly at this voltage.

Similarly, I once replaced the switch on the BFO (which shorts the plate to common) with a transistor switch.  I used a schmidt trigger so that any RF voltages floating around from the transmitter's end fed aerial wouldn't turn the transistor on.  This meant that when the "shorting" transistor was "on" about 3 or 4 volts remained on the collector (and this means also on the BFO plate).

I was very surprised to discover that the BFO still oscillated (albeit weakly) at this plate voltage!  

I concluded this was to reduce the chance of tracing the signal back to it's source by the enemy (whoever that might be).  However, that is only my own "guess".

Can any-one comment on this?   

Les Smith
formerly VK2BCU

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