I have noticed a strange phenomenon when tuning up on different bands. On
40 meters, using a nearly resonant dipole, the SWR drops as I increase power
and the tuner responds much better, i.e. tunes out all the reactance. On 15
meters, using a nearly resonant dipole, the opposite occurs. At low power,
SWR is well behaved, and it is easy to get a match using the tuner. As you
increase power past a certain point, about 30-40 watts, SWR shoots up, and
the best the tuner can do is about 1.5 to 1. or so. I am using #18 guage
insulated wire for the 15 meter dipole and #14 guage bare wire for the 40
meter dipole. My feedline in both cases is 50 feet RG-8X, a .25 inch d. low
loss (@2 db/100 ft at 30 MHZ) coax to a coax balun at the antenna feedpoint.


Thank you, and 73

Lee
KI6AZS
--

Hi Lee

I'd suggest its a non linear frequency response of the devices (diodes?)
that are used in the SWR measurement circuit.. At the higher freq you
also get the other variable of a higher sense voltage for the same Po.
(larger fraction of a wavelength)

And depending on your configuration what you read as SWR will vary
depending where you are on the feeldine. No doubt others that know all
the theory on this will jump in and comment on this.

Do you know if the balun really is leaving the feedline current balanced?

ie quite a few variables to contend with.

If you really want to find the reason I would suggest using a resistive
load for an experiment. I'd suggest that the behaviour would change..

Cheers Bob VK2YQA

lkokot wrote:
I have noticed a strange phenomenon when tuning up on different bands.

Hi Lee

Well the first thing is to pull the tuner out and try;

rig - swr meter - dummy load

Assuming you have the meter as a separate entity. This will verify the
dummy load. If not at least check its resistance with an ohm meter. Not
a perfect check but if its open you know there is a problem there as
well! Your test against the load then antenna though implies the antenna
is close to perfect without the tuner!

I'd suggest though that the tuner has a major problem that needs a look.
Why not open it and have a poke around? Bypass mode should be a direct
connection to the output but I'll admit I dont know the model. Is it a
coaxial antenna connection thing or open wire feeder? You could argue
that anything that detunes a almost perfect match to a dummy load
implies it is broken!

What are the specs/power handling of the tuner?

Bob VK2YQA (getting use to the accents in East Texas)

Rockinghorse Winner wrote:
Tried as you suggested the dummy load.