Andy Cowley wrote:
I think you'd be better of with 102' or 135' rather
than a resonant 80 m aerial as these are easier to match on some of the other
bands.

If he were using 300 ohm or lower Z0 feedline, you might have a
point, but there's nothing to worry about with 600 ohm feedline.

A resonant 80m dipole may have a feedpoint impedance of 60 ohms.
An 80m dipole used on 40m may have a feedpoint impedance of 6000
ohms. In both cases, the SWR on the 600 ohm open-wire line is 10:1.
A 60 ohm antenna fed with 600 ohm feedline and a 6000 ohm antenna
fed with 600 ohm line are equally mismatched. If one doesn't know
the length of the feedline, one can't tell the difference at the
transmitter.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Andy Cowley wrote:
I think you'd be better of with 102' or 135' rather
than a resonant 80 m aerial as these are easier to match on some of the other
bands.

If he were using 300 ohm or lower Z0 feedline, you might have a
point, but there's nothing to worry about with 600 ohm feedline.

A resonant 80m dipole may have a feedpoint impedance of 60 ohms.
An 80m dipole used on 40m may have a feedpoint impedance of 6000
ohms. In both cases, the SWR on the 600 ohm open-wire line is 10:1.
A 60 ohm antenna fed with 600 ohm feedline and a 6000 ohm antenna
fed with 600 ohm line are equally mismatched. If one doesn't know
the length of the feedline, one can't tell the difference at the
transmitter.


All true for the two bands. But - I did know the length of the feedline.
I did do the EZNEC simulations for the impedance seen looking into the
feedline for all bands and the lengths chosen do result in the 'easiest'
tuning solutions. The SGC is a good bit of kit but it won't tune just
any old thing, there are limits to the amount of L and C it can supply.
AFAIK it uses a simple 'L' solution always. If you get the wrong feedline
length and aerial length combination you can quickly get solutions which,
even when in range, produce excessive voltages at 100W. The SWR on open
wire line is not the significant factor here, losses are very low even at
very high SWRs, but the impedance transformation required in the tuner. The
SGC has fairly low voltage limits so I think 'tis best to treat it as
gently as possible.

I hope you don't think I'm recommending a G5RV here. The only good G5RV
is one without the coax, in which case I believe the design with those
lengths is due to Arthur Collins, (W9CXX?) in a prewar QST.

Thanks for the input anyway, Cecil.

vy 73

Andy, M1EBV

Andy Cowley wrote:
If you get the wrong feedline
length and aerial length combination you can quickly get solutions which,
even when in range, produce excessive voltages at 100W.

My point was that the wrong feedline length yields the same results for
a 60 ohm antenna as it does for a 6000 ohm antenna. If you feed that
60 ohm antenna with an odd multiple of 1/4WL of feedline, the impedance
seen at the tuner will be 6000 ohms. If you feed the 6000 ohm antenna
with a multiple of 1/2WL, the impedance seen at the tuner will be 6000
ohms. The impedance of the antenna doesn't really matter as long as it
is 1/2WL on the lowest frequency of interest. What matters is the SWR
on the feedline and the length of the feedline. The feedpoint should be
in the ballpark location of the current maximum point, not the voltage
maximum point.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Andy Cowley wrote:

If you get the wrong feedline length and aerial length combination you
can quickly get solutions which, even when in range, produce excessive
voltages at 100W.

My point was that the wrong feedline length yields the same results for
a 60 ohm antenna as it does for a 6000 ohm antenna. If you feed that
60 ohm antenna with an odd multiple of 1/4WL of feedline, the impedance
seen at the tuner will be 6000 ohms. If you feed the 6000 ohm antenna
with a multiple of 1/2WL, the impedance seen at the tuner will be 6000
ohms. The impedance of the antenna doesn't really matter as long as it
is 1/2WL on the lowest frequency of interest. What matters is the SWR
on the feedline and the length of the feedline. The feedpoint should be
in the ballpark location of the current maximum point, not the voltage
maximum point.

My statement is assuming 600 ohm open-wire feedline as specified earlier.
--
73, Cecil http://www.qsl.net/w5dxp



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