> On 80 using the Gap, you have to pick your 50 kHz
>window, or use a tuner (low power only).
Why low power only, or is the Gap not able to withstand
higher voltages, say up to 800 volts or so? From what has
been written about the Gap, it sounds as if it uses some
sort of "linear loading" wires.
Perhaps with the "linear loading" wires, it is not possible
to get much over 50 to 60 kHz of the band below a 2:1
vswr on the feed line. But so what?
With a tuner, you still get the element to radiate all the
power that leaves your rig; just be sure your coax/antenna
are rated to handle the voltage increase at the max swr
voltage points, hi. Neither the antenna, nor your
tuner terminals will see the voltage max places;
just the coax dielectric. Has someone written that the
Gap has coax inside of some small size for ? If so,
that could be a limiting factor.
Even at a vswr of 2:1, you
have effectively cut in half the potential power
handling ability of your coax by the time you reach the
breakdown voltage of the line (Max voltage on the line
is the square root of the product of three values:
the forward power, the line characteristic impedance, and
the vswr)!
So, even with say an extreme line swr of 8:1, and
running a full 1500 W peak power, the maximum
voltage within a 50 ohm coax run will be about 775 volts;
way below the coax breakdown rating for typical RG8 coax,
but don't know about the Gap antenna. Now this is an
extreme example, just to illustrate the point.
Even here, with an antenna tuner properly adjusted, the
rig happily provides the power thanks to the tuner, the
impedance at the output of the tuner is such that all the
reflected power from the load, which is causing the 8:1 swr,
is totally reflected back up to the load and eventually
radiated, within nano/microseconds, way faster than the
20 millisecond rise time of, say a 40 wpm CW dit!! So,
all the power in a 1500 watt dit is radiated with no
one able to tell any difference from a perfectly
flat antenna/line 1:1 match !
Only loss is the above scenario would be from resistive
losses in coax experienced by the reflecting/re-reflecting
energy in the line. Or, for typical RG8, with a loss
attenuation of about 0.4 dB in a 100 foot length, the total
attenuation of our rig's output power in traversing the 8:1
mismatched line/load situation is about 1.4 dB in the
40 meter band; about 0.9 dB lost on 75/80 M.
And this is admittedly an extreme
load mismatch example. But it illustrates that vswr in
the feed line, caused by a mismatch, even an extreme
one, between the antenna and the feed line, does not
necessarily mean a disaster. Certainly less of a problem
for the efficiency of the antenna system than that from
having the antenna mounted over poor ground,
or otherwise improperly!
Hope this helps in understanding the situation faced.
73, Jim, KH7M
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