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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