[ARC5] ATD/ARB on the air

[Dennis Monticelli]

It may not be necessary to reverse the input/output connections to the
tuner in order to make the match.  It depends on the tuner's input
impedance range.  It is true that most commercial tuners optimize for
a higher Z on the input side vs the output (nominally 50 ohm), but
there is not law saying that tuners need to be designed that way.

Dennis AE6C

2008/10/7 Meir-WF2U <wf2u at ws19ops.com>:
> 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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