[Milsurplus] ART-13 Transmitter and BC-348 Receiver info

[Mike Hanz]

  I suspect the conundrum lies in the definition of "50 ohm antenna", 
Dave.  Very few hams ever characterize their antenna impedance over the 
bands they operate in, and the tendency to feed it over 50 ohm coax 
seems to magically make it a "50 ohm antenna" in some minds.  Nothing 
could be further from the truth, of course.  The military got it right, 
at least in postwar aircraft, by conditioning the antenna impedance very 
close to 50 +jO ohms before it entered the transmission line.  That made 
it easy at the transmitter end.  But, throw some imaginary impedance in 
there and the results go all over the place, especially if you begin 
considering series versus parallel capacitance.  These are duck soup for 
an antenna engineer, but few of us are.

On 9/8/2010 5:58 PM, arc5 at ix.netcom.com wrote:
> ----- Original Message -----
> From:<k4pf at juno.com>
> Subject: ART-13 Transmitter and BC-348 Receiver info
>
> Tony Vernucci (I0JX) recently worked over an ART-13
> to get it operational, and wrote up his experiences.
> ....
>
> http://www.qsl.net/i0jx/ART-13_BC-348.pdf
>
> --------------------------------
>
> Something I just don't get....
> A lot of people have found that adding a capacitor between the ART-13's
> antenna output and ground lets it tune properly into a 50-ohm antenna.
> But everyone who writes about this quotes a different value-
> radically different.  For instance, in I0JX's article:
>
>      "4.2 Improvement of transmitter efficiency on 40- and 80-meters
>      I have experimentally determined that on 80 meters, and often also
>      on 40 meters, some capacitance must be put in parallel to the antenna.
>       More precisely, in my case:
>     · on 40 meters, an extra parallel capacitance (about 300 pF) is needed...
>     · on 80 meters an extra parallel capacitance (about 2,000 pF) was
>      found to be absolutely necessary, as the RF output power
>      would otherwise be very low......"
>
> I got 120+ watts out on 75 mtrs with the ATC I rebuilt.
> The cap I used was 500 pFd.  That's a long way from 3000.
> And I've heard quotes all up and down.
>
> What gives?
>
> 73 Dave S.
>
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