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

[mac]

The combinations of ART-13 front panel adjustments that hams use on  
160, 80 and 40 meters basically configure the PA tank circuit as an  
"L" network with a shunt variable capacitor (knob E)  at the plate  
(high Z) end  and a series inductive circuit made up of a tapped coil  
(knob C) in series with a variometer (knob D).  This configuration  
works FB for feeding short aircraft antennas with low values of  
radiation resistance but not with the 50 - 75 ohm resistive loads hams  
employ.  The addition of a shunt capacitor on the antenna output  
terminal serves to  reconfigure the ART-13 tank circuit as a "Pi"  
network, the same a that used in many ham transmitters and linear  
amps.  The inductive component in the ART-13 (see above) can be varied  
over a very wide range and so with a large inductance the amount of  
capacity (external fixed cap in series with the internal variable  
behind knob E) required to resonate the circuit will be relatively  
small and, conversely, with a smaller value of inductance MORE total  
capacitance would be required.  This accounts for the wide range of  
shunt capacitor values reported over the years, most probably  
determined by "cut-and-try" methods.  There is, of course, an optimum  
LC ratio for the tank circuit depending on plate current an voltage.   
Apparently not critical, though, given the wide range of values  
reported and used successfully.

However, 2000pf for 75M as reported by one author sounds like far too  
much capacity for that band.

Dennis D. W7QHO
Glendale, CA



On Sep 8, 2010, at 2:58 PM, <arc5 at ix.netcom.com> <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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