[ARC5] Interesting documents

[Leslie Smith]

Hello James/Jim,

You wrote:
     "Also some fighters had VHF and others HF. Why was that?
    All of a certain squadron had to have the same radios, so they
    couldn't swap 
   out planes between squadrons?  So much for flexibility... "

   I still wonder why the receivers were tuneable but the transmitters
    were set at a fixed frequency and in this case even inaccessible!"

Here's what I thought:

Like you, I saw many interesting things in this film.  Like you, I
always assumed the modulator would be near the transmitter.   (I was
wrong there!!)  On the other hand I'm not surprised to see the emphasis
on the mechanical aspect of maintenance.  My experience (in industry,
not aircraft communication) is that 95% of all failures are mechanical. 
Trades-people walk on cables, bend sensors out of place and so on.  In
aircraft vibration (and "G" forces) will be a common source of failure. 
  Over time even thermal heating/cooling might loosen locking nuts on
cable connects (and so on).  That's my guess.

The topic of HF vs VHF communication has been a topic of  speculation on
this list.

I know little about this topic.  That's why I'm writing this - I like to
expose my ignorance!  Some figures can help the speculation.  For a
start, I don't imagine the technical types that thought about air combat
in 1936 to 38 (when the specs were set) had ANY idea about the scale of
combat in 1943/44.  Did they imagine a 300 bomber raid?  How about a
1000 bomber raid?

Take a 300 bomber raid on Germany.  If we assume 7 bombers per
'unit/squadron/group' that means 42 "channels" -  (300/7).
We have 3100kHz available for 42 "channels" or a channel spacing of
(approx) 70kHz.  (I assume every bomber in the 7 bomber "unit"
communicates on the same frequency.)   All these aircraft fly in close
proximity.  I don't know how close, but I understood the separation
between bombers (wing-tip to wing-tip) in a unit might be 20 or 30
meters.  That's merely my  "guess", but they flew at in astonishingly
close proximity.  My figure of 42 channels makes no allowance for
fighter liaison communication.    I imagine a lot of cross-modulation at
that frequency.  And there is the problem of temperature-related drift
in the transmitters (or receivers)  I have no problem in imagining the
temperature variation (ground to 25,000 feet) 45C.  If the V.F.O. drifts
500Hz per degree - that's a 22kHz drift.  Is my figure of 500Hz per
degree too high?  I don't have any idea, but I've built VFO's that drift
more than that.   I measured the drift of a BC-221 at 166Hz per degree. 
I doubt any VFO will be more stable than that.

On top of this we know that the Brits "went" to the VHF spectrum because
the RAF communication types listened to the Nazi bombers "forming up"
during the battle of Britain.  Fighter Command used the fore-warning to
scramble fighters at the last moment.  In other words the RAF cut the
fuel and maintenance burden by keeping the fighters in the air for a
minimum time.   So the long-range capability of the HF sets was seen as
a distinct disadvantage by RAF fighter command.

So that's my novice understanding.  The Brits believed they got a double
advantage from using VHF comms.  (More 'channels'/a reliable shorter
range).   

I understand why the technical types would be reluctant to change the
basis for an entire communication system.  (HF > VHF)
For a start, lives depend on a reliable command system.  Then there was
politics.  Was VHF better?  The Brits thought "yes" - but was this idea
shared by the US technical types?    Lots of room for speculation here!!

We know that communications (or lost command communication) was a source
of trouble in the battle of Midway - but that's the extent of my
knowledge on this topic.  I would like to know about some book that
documented this problem.  (I'm sure some list reader will know the name
of this book).

Yes, Wayne's film was more than interesting!!

Les





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