[ARC5] ATD/ARB on the air

[Meir-WF2U]

Robert,

 

Thanks for answering Neil’s question. Indeed, the ATD antenna output is
described in the manual as “ loading into a 25 ft. center fed wire” which
presents a “1.2 ohm and 50 pf load at 3 MHz” . At the 80 M ham band, around
center frequency of 3700 KHz, it is probably around 1.5 ohm, interpolating
from the next higher given frequency load as shown in the manual.

 

The MFJ antenna tuner I happen to be using has a cross-needle SWR/power
meter. The scales are of course now reversed.

Just for reference, I had a 100 pf  “doorknob” capacitor which I used
initially in series with the antenna output of the ATD into the 50 ohm
system. I managed to get around 25 W out of it, running out of tuning range
on the ATD output tuning controls. With nothing at all between the ATD and
the 50 ohm power meter (just to illustrate the large mismatch), maximum
output I could get was 6 W.

 

73, Meir WF2U

Landrum, SC

 

   _____  

From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of WA5CAB at cs.com
Sent: Monday, October 06, 2008 10:32 PM
To: neilb at ihug.co.nz; ARC5 at mailman.qth.net
Subject: Re: [ARC5] ATD/ARB on the air

 

Meir's idea is a good one as it works, and in retrospect, it's obvious that
it would work although I can't recall anyone else ever writing that they had
figured that out.  But the load resistance of a short whip or wire is low,
not high.  I've seen the load resistance of the standard 15' whip quoted as
5 to 20 ohms, with an average of 13 over the HF range of 2-12 MC..  This has
been pointed out here and elsewhere dating back to at least the early 40's.
Recall that the majority of the WW-II aircraft sets only went up to 9 MC.
The BC-191 and 375 went up to 12.  Only the ATC went to 18,  Of the tactical
ground sets, only the SCR-399, SCR-499 and modified SCR-299 went to 18.  The
others, except for the BC-191, topped out at 6 or less.

Anyway, because the expected antenna lengths were electrically short, the
output impedance of the WW-II vintage aircraft and most of the ground
transmitters was low, not high.

In a message dated 10/6/2008 8:56:49 PM Central Daylight Time,
neilb at ihug.co.nz writes: 



>The antenna matching on the ATD is very tricky.
>I used a quick and dirty method: I connected the ATD output into the  
>antenna output of a manual MFJ antenna tuner, while the 50 ohm input  
>was used for the output.  Of course I pretuned the ATD/tuner into a 50  
>ohm dummy load.

I had often thought that this might work well for hi-impedance milsurplus
transmitters. Are the forward and reflected power readings on the meter
transposed?



Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480 

 


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