[ARC5] Tower Comm and VHF Homing

[Jack Antonio]

On 4/30/2011 4:11 PM, Robert Eleazer wrote:
> Late in the war some USAAF fighter aircraft, especially the P-51's on Iwo Jima, were equipped with the ARA-8 homing system, which apparently worked in conjunction with the ARC-3 comm sets.  It consisted of a couple of what I suppose were AN-104 antennas on the rear fuselage, side by side and an adapter that seemed to be some sort of a switch.

The antennas were not AN-104s.  They were AS-148/ARA-8. They are a 
little shorter, somewhat skinnier, and have sharper
leading and trailing edges. Viewed from the side, they have a different 
shape,  they are a symmetrical tall isosceles triangle,
rather than the vertical leading edge of the AN-104.  The book 
recommends limiting homing operations to between 120 and 140
mc.
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>
> I would guess that the adapter switched the receiver input between the two antennas to detect direction.

Not exactlly.  The base of each antenna had a "tee" connector on it, and 
a short length of coax connected the two antennas
together using one side of the tee.  The other side of each tee went 
through a length of coax to the switch.  The switch was
a motorized coax switch that connected one input to the output in a D 
pattern, and the other  in a U pattern.  The output
of this switch went through another relay that selected either the comm 
antenna or the homing system. The end result was
a cardioid pattern that was switched to favor left or right.

>   Not sure how well this could have worked, given that the distance between the antennas looked to be no more that a couple of feet at most.
IIRC the spacing was 8 inches.  The coax lengths were specified exactly 
as well. If you like I can dig up the exact lengths.


>   I have never heard any description of how this system worked but you can see the antennas on pictures of the Mustangs that escorted the B-29's to Japan.

Google "Uncle Dog P-51" and you can find some information on usage. The 
system was nicknamed this due
to the U and D characters.
>    And I have no idea how the directional info was displayed to the pilot.

There was no display.  This operated very similar to the AN radio range. 
The pilot heard a morse D or U or steady tone, and
acted accordingly. The only cockpit presence was a small panel that 
selected comm, homing or lock-keyed the transmitter
(using the comm antenna) so that others could home on it.

I put this system together with an SCR-522 a couple years ago.  I set it 
up on the front lawn of the house and tested it,
not minding the neighbors thoughts of my sanity, and could tell that it 
worked, but the book recommends a minimum of 50'
distance to test, and I didn't have that much space.  I was getting some 
good indications, but a few false nulls as well.

Jack Antonio WA7DIA/4
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