On Tue, 26 Jul 2005 17:01:17 -0500, "Ken Bessler"
wrote:

"Ken Bessler" wrote in message news:...

My current antenna is a modified Van Gordon "All Bander".
For reference, the stock antenna is 134' of 14g stranded
bare copper fed with 100' of 450 ohm ladder line. I cut off
half of the ladder line and replaced it with a 12 turn, 5-1/4"
rg58 coil. 20 feet of rg58 goes from the coil into the shack.
I then trimmed the antenna to 7.175 mhz.

The antenna is a flat dipole up 20'. Seems to work OK but
I've got the itch to improve it. The feed point is 12" above
the peak of my roof. I was thinking about installing a 5' tripod
and 2 ten foot masts to elevate the antenna's feed point and
convert it into an inverted V. There will be no need to add to
the feedline.


snip


Well, it's up guys! The apex is about 40' and the ends are 28'.
Comparing it to my other antennas I'm seeing about a 1-1/2 to
2 S unit improvement on 40m and more on the higher bands.

I did a SWR plot before and after changing it and here's the
sticking point: Before, I had a 2:1 range from 6.900-7.280 and
now I've got 7.000-7.300 but the minimum SWR went from
1.10:1 to 1.55:1. My Elecraft KAT2 autotuner handles that very
easily but I'm wondering what would happen if I added 12 feet
to my 50' of 450 ohm ladder line? Would the minimum SWR
get better or just move? Or both? Does it really matter? (I don't
think it matters based on what I've read here).

Ken, this is all a bit scant on detail, and can't be interpreted with
certainty, so i am probably wasting my time running the numbers.

All the following in premised on my interpretation of your rather
loose description, and is only applicable to your situation if what
you have done is *very* close to my interpretation.

I think you have said you have cut a 135' dipole down to 7Mhz, so if
that means it is a half wave long at 7MHz, the feedpoint impedance at
resonance mounted low as you have described is probably somewhere
about 60+j0 ohms.

If you transform that with 50' of ladder line (I have modelled Wireman
554), the Zin to the ladder is around 110-j310 and the ladder line has
about 0.25dB of loss. You describe a coil of RG58 that seems to be 15'
in length and a further 20' of the same. That will transform 110-j310
ohms to 5+j14 at the ATU end with about 2.5dB of loss. The RG58
operates at high VSWR (~12), and although relatively short, the loss
is IMHO unacceptable.

There will be a little tuner loss, but it will be insignificant
relative to the 2.7dB of feedline loss.

If you want to improve the configuration, the RG58 has to be a prime
target of your attention. Most of the real power you deliver to the
feedpoint will be radiated (ie conductor losses in the dipole are
relatively insignificant), the challenge is to get the power to the
feedpoint. In your case you are probably getting around 50% of the
transmitter power to the feedpoint.

The closer the dipole is to ground, the more radiation is concentrated
upwards, so if you want low angle radiation, raising the dipole height
helps.

Owen
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