[Milsurplus] Re: TA2J High Power Tx in Catalina

[Mike Hanz]

We're probably beating this subject to death, but I'll hang on for one 
more round...

>----- Original Message ----- 
>From: "Mike Hanz" <AAF-Radio-1@cox.net>  
>
>>>I ask, does it use simply a tapped coil as the output, like the BC-230
>>>or the GF, for example?
>>>
>>Well, in a way.  It has eight detachable roller coils similar to those 
>>in a command transmitter, but with *two* roller taps (plus a grounded 
>>end)...  
>>    
>>
>Hue Miller wrote:
>Then it uses the same primitive harmonic-prone circuit as much earlier
>transmitters, yes? 
>

Well, I don't know how primitive it is - it's pretty ingenious in some 
ways.  Your observation about the harmonic content is correct, but I 
don't believe they were too concerned about spectral purity in a rig 
that was essentially designed in the mid-1930s for comparatively low 
power outputs.

>Do you have any comment on the actual harmonic
>levels to be expected? I 'believe' the situation is only really bad when
>using a high impedance antenna, one that is tapped well up on the
>coil, and the antenna also happens to be high impedance resonant
>at some odd harmonic.
>

It's hard to say, Hue.  Using the antenna as a distributed capacitive 
part of the tank circuit makes any serious analysis quite a bit more 
complex, and it all depends on what antenna parameters you select.  
Clearly, if you are using it near a point where the antenna impedance 
approaches a parallel resonance discontinuity, then harmonics may well 
increase.  But that's true of just about *any* of the tank designs, not 
just this one in particular.  The TA-2 is obviously a lot more trouble 
to get set up correctly, which in my mind is why they abandoned the 
approach in the later Bendix transmitters.  Anyway, your question is 
more than I would care to comment on *quantitatively* without a lot of 
thought and clearly specified assumptions on the antenna characteristics 
and frequencies being used.  Perhaps someone else on the list would care 
to get involved...?

>>>Also- Mike or anyone else - the RDF-1, the HF predecessor to the DU,
>>>which tuned to 8000 kcs. - do you have any idea how far down the
>>>reception with it would be, from a standard wire antenna of say, 30 ft. ?
>>>I mean in the B position, receiving totally on the loop alone, say on
>>>80 or 40 ?
>>>
>>Sure!  Just as soon as you define what "how far down"; "reception with 
>>it"; and associated receiver actually means. <grin>  I have no clue what 
>>you are talking about in technical terms.
>>    
>>
>
>I meant, when receiving solely on the loop, i.e. "B" "bearing" function, as
>compared to receiving solely on a fixed wire antenna, how many S or db
>down might you expect the reception to be. 
>

Another tighar debate question?  <grin>  The issue effectively resolves 
into RDF-1 loop versus 30' wire antenna gain [actually, a 30' increasing 
impedance transmission line when installed with the aircraft skin as a 
counterpoise] at 8000kHz.   I don't know the gain of either one with any 
certainty.  If you have the diameter and number of turns in the RDF-1 
loop then an approximation of its gain can be made.  With a decent 
antenna program, the 30' antenna above the fuselage can be modeled.  
Whether you would be within 10dB of reality with calculations like this 
is problematical.  Best way to answer your question is by direct 
experiment, but I don't have an RDF-1 nor a 30' aircraft hull 
handy...heh, heh...

73,
Mike