[ARC5] More Piipsqueak

[Mike Hanz]

Jack Antonio wrote:

>I was laboring under the impression that the Pipsqueak system
>transmitted a tone modulated signal on channel one  for the entire
>15 second transmit period. This is not the case
>
>Relay 23 in the BC-616 is a time delay that controls
>the frequency selection, and kills the MCW tone oscillator
>in the modulator.
>
>Question one is, how long is the time delay supposed to be?
>(someone will have to read the manual for this one). In my
>BC-616 this is about a second, but the time delay cap
>may have lost capacity.
>  
>

That's about right, given the charging time constants and pull-in 
voltage of the relay (which can vary considerably, by the way.)  The 
lack of any adjustable delay suggests it wasn't critical.

>Question two is simple, but with probably a complex answer.
>
>How did all this work in practice? 
>
>Was channel one setup as an emergency frequency and
>channel two the working frequency? 
>  
>

If you're defining "emergency frequency" as the Pipsqueak frequency, I 
think that's right.  Channel one gives the DF station a nice clean CW 
signal to DF on.  If you are using channel 2, 3, or 4, then the 
one-second tone lets everyone know the set is switching to Pipsqueak 
mode on channel 1 and won't be available for reception for the next 14 
seconds.

>Did the pilot request a fix on channel one or two?
>  
>

I'd say Channel 1 for the reasons above.  There's an excellent 
explanation of its use during the Battle of Britain at 
http://users.bigpond.net.au/mantis/FW/Bob/Fat.htm .  Here's a quote from it:

"The RAF during the Battle had only HF sets (although it was in the 
process of switching to VHF sets). Apart from poor receiving qualities 
and a woefully short range, these had only two channels. One was a 
pre-set frequency, meaning transmissions between squadrons were 
impossible, since each had a different frequency. The other was for 
"High Frequency Direction Finding", known officially by the code-name 
"Cockerel", or unofficially as "Huff-Duff" from its initials, or 
"Pipsqueak" from the noise it generated. Every aircraft's radio was set 
to automatically transmit a 1,000-cycle note for 14 seconds each minute. 
Sources vary on the exact application of the timing; some say that 
different a/c in the same squadron transmitted for different 
1/4-minutes, producing a constant note from the squadron. Others say 
that different squadrons were assigned different quadrants, allowing 
each of four squadrons to be located by this method, identified by the 
timing of their "squeak". Any more than four squadrons in the air 
required the extras to report their position verbally. Either way, each 
sector had three detection stations, and by taking a triangulation fix 
on this signal from their sector's a/c (a/c in other sectors generally 
being out of transmission range), the position of friendly aircraft 
could be accurately plotted by the controllers. Unfortunately for the 
pilots, their voice channel would not work during the 14 second 
transmission, although it could be switched off or over-ridden by the 
pilot when necessary."

You'll note the mention of an MCW tone for the entire period of 
transmission.  Not sure whether that is from British practice, which was 
then obviously modified for later AAF applications, or just an oversight 
in the original description.

>And,  is it true that the Pipsqueak went into
>fighters and not larger crew served aircraft?
>  
>

Not sure at this point, Jack, and I suspect it depends on whether you 
are talking British or American application, and the particular dates 
involved.  There are references to it being used by British multiplace 
bombers to find their way back in foggy old England long after the 
Battle of Britain.  Radio silence being the norm, you called up a 
certain frequency and asked for a location, then activated the Pipsqueak 
timer briefly.  There are also the references to it being used primarily 
on American fighters for air-sea rescue operations.  To complicate 
matters, Gordon White records , "About 500 BC-608 units were built 
before radar transponders outmoded the system (for IFF)."  (The words in 
parentheses are mine and leave the door open for later purchases, 
because that low number seems to be at odds with the number of these NIB 
that seem to still pop up in various places, including ebay.)  Its 
shortcomings as a sort of primitive IFF for defensive fighter control 
made that particular application fairly short - it was rapidly replaced 
by the more sophisticated radar based IFF as quickly as that became 
available.  The stories of getting a Pipsqueak position fix later on 
evidently grew from its use on those aircraft which hadn't yet received 
the more modern capability - there doesn't seem to be any other 
plausible explanation.

Best 73,
Mike