James, you are kind of close in your thinking, but wrong in how you
explained it.
Openwire does not make it easier to tune than coax would.
The feedline has nothing to do with the ability to match the antenna.
That is purely a function of the matching range of the matchbox, and the 238
has a broad enough range.
Jim was correct, he is just tuning past the match point with out recognizing
it.
The problem with using coax is, loss in the coax, AND in the balun if you
use one.
You will have no problem matching it.
Second point, due to the fractional bandwidth on 80m, the tuning is usually
NOT broad.
It is typical that you will have to re-adjust the settings slightly, several
times when trying to cover the entire 80/75m band.
More on this can be found in Joel's Antenna Tuner Guide on page 2-5.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Richards
Sent: Monday, July 08, 2013 9:43 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Tuner
What type and length of transmission line are you using? I presume this is
a regular old half wave dipole, right? If so, I believe the text book
suggests that a regular old half wave dipole tends to work on its ODD
harmonics, which would exclude 40 m on an 80 m antenna. Not that the tuner
may not be robust enough to force the matter, but typically it is difficult
to tune a coax fed half wave dipole on the first higher
EVEN harmonic. (I use Off Center Fed (OCF) antennas because they tend
to work on their EVEN harmonics...)
Now the textbook also suggests that 80 m dipole is a "doublet" when fed with
open transmission line and used with an appropriate tuner and it is
purportedly easier to match on any band using a tuner and open line.
So... what are you using?
I agree with JB's comment it may be you are just not seeing where it
does tune on 80m. I recently acquired a Model 248C tuner and was
struck by how narrow the tuning range is. Not that it does not match a wide
range of loads, but that the sweet spot is very narrow, and requires careful
handling or you can easily miss finding the sweet spot.
First thing to do is test the antenna with your antenna analyzer to see if
impedance is hither or lower than 50 ohms at the point where the tuner is.
You use information that to select either a LowZ or HighZ fixed capacitor
setting. Set the Capacitor Knob to around the |3| position, and then work
the inductor starting from 0 upward - and go slowly, it won't take much.
The sweet spot is a narrow, almost pin-point range, so watch the cross
nettle meter carefully and when you are close to the sweet spot, the
Reflected Power needle will suddenly and sharply drop. Tune for the lowest
reading, and then mess with the Capacitor Knob left and right until you find
the lowest possible reflected power meter reading.
Do this for each of the five fixed capacitor settings, although after a bit
you will determine the best or most likely capacitor choice 1 through 5, and
repeat the procedure described in the paragraph above.
Make an charter of the settings that work for each frequency band on each
antenna, and you will find the best setting one in short order.
Because the sweet spot for each band is fairly narrow and sharply defined,
it is easy to miss it. On the other hand, the best setting will be sorta
obvious and dramatic, and the best bit is that setting should work for
nearly the entire band, which means it is a sharp tuning point, but that
point will cover all or most of the band, obviating the need to keep
retuning as you QSY up and down the band.
Once you get this zeroed in, you will really like this tuner because it is
either tuned or not, with little ambiguity, and once tuned, works a wide
operating segment of the band for which it is tuned.
Hopefully this helps you find that sweet spot.
================ K8JHR ==================
On 7/8/2013 12:46 PM, Walt Stewart wrote:
> I recently purchased a used 238 tuner. It will give me a perfect match on
40 meters with my 80 meter dipole, but I cannot get it to match the 80 meter
dipole cut for 3800 down to the CW area. What am I doing wrong?
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