[ARC5] Newbe Question

[Mike Hanz]

Your ultimate goal is to match the existing transmitter terminal 
impedance to a nominally 50 ohm resistive load (this assumes you've 
matched the antenna on the other end of the coax to look like a 50 ohm 
resistive load to the coax as well...if you haven't done that, you have 
a real hodge podge with additional coax losses on your hands.)

The HF command transmitters are looking for somewhere around a 100pF 
capacitive component and a 5 ohm resistive component in order for the 
tank circuit to resonate at a full 40 watt output on CW.  Any deviations 
from that will result in lower power output.  However, any flim-flam 
between the actual radiating element and the output terminal of the 
transmitter to permit resonance reduces power available to the radiating 
element, so there is no free lunch.  Simply paralleling capacitance 
drops output voltage dramatically, despite the fact that it may allow 
you to tune to resonance.  Sometimes that's a good thing with 
electrically short antennas to reduce corona in a high altitude mission, 
but at your QTH?  Naaah - not so great with a decent ham antenna.  You 
may think you are putting out a nice signal, but a field strength meter 
near the antenna will tell you the truth.  Even use of a toroid UNUN, 
while making the match much more closely, has some losses in the 
transformer that you can't get around.  In order of pure effectiveness, 
  a broadband transformer to match the 5 ohms to 50 ohms is the first 
choice - more complex networks of inductance and capacitance can do it 
too, but can be a pain to keep tuned if you use more than one frequency, 
and they can interact with the tank of the command transmitter in 
unexpected ways.

The bottom line is that the tank and following matching circuit isn't 
designed from the start to match the 1625 plate impedance to a 50 ohm 
ham antenna.  If power is your only goal, then that tank needs to be 
redesigned or you need to bring in an old B-29 fuselage into your back 
yard and string an antenna up on it.  :-)   On the other hand, if you 
can get it to resonate, why worry about a few watts lost?

If you're interested in some of the more arcane aspects of the antennas 
the command transmitters were designed to feed, I put an old 1942 
article up at
http://members.cox.net/mymhh/Aircraft%20antenna%20design%201.GIF
http://members.cox.net/mymhh/Aircraft%20antenna%20design%202.GIF
http://members.cox.net/mymhh/Aircraft%20antenna%20design%203.GIF.
(If you have a browser with variable magnification like Opera, set the 
zoom value to 20% - the pages are fairly large to allow high resolution.)

73,
Mike

Tbs50A@aol.com wrote:
> Hi,
>  I'm new to the list and had a question about using the BC-696 and 459 on the 
> air. I follow Dave Stenton's recommendations for cleaning and using a small 
> series capacitor to get it to load into a 50 ohm load. When I do that I can 
> only get about 95 or 100ma plate current. I found by paralleling a capacitor 
> from non antenna post side of the series capacitor to ground I can easily get 
> it to load up 150 to 200ma with about 50 watts output. The grid drive seems 
> to be good as well as the 1625's Is this a good way to do it or am I missing 
> something?
> Terry N3GTE