thank you all of you for your answers.
So more radials, means lower swr?
Do you think that the variable capacitor is all i need for low swr? If
i will not achieve swr<1.5 what do you think about to try to use a
variable coil, which will be connected between the common point (of
capacitor and 50ohm coax from transmitter) and ground. It will help to
achieve low swr?
How many uH must be this coil?
i mean something like this http://tzitzikas.webs.com/COUPLER.JPG
(without optional turns which are in series with capacitor).
--- On Sun, 5/30/10, Charles Moizeau <w2sh@msn.com> wrote:
From: Charles Moizeau <w2sh@msn.com>
Subject: Re: Topband: inverted L antenna match system
To: topband@contesting.com
Date: Sunday, May 30, 2010, 12:57 PM
Here is how I did it, along with my reasoning
behind the
design.
I have a tall ash tree and was able to achieve an 85' vertical run with
an easy
bow-and-arrow shot. I elected to build
an inverted L antenna
I knew that the current maximum in any antenna is where maximal
radiation
occurs, and that the current maximum in any antenna occurs a
quarter-wavelength
back from the wire's open end(-s). I
wanted as much radiation as possible to occur from the vertical portion
of my
antenna. With an antenna's sinusoidal
current distribution, that meant that I should make the length of the
antenna's
horizontal portion such that the current maximum would be positioned at
the
center of the vertical section.
I reckoned a quarter wavelength of #12 gauge solid copper insulated
house wire
to be about 128 feet. So with 85' up I
made the horizontal leg 85' out. Do the
arithmetic and you'll see that the current maximum occurs where I wanted
it.
I could point the horizontal leg in almost any direction, but I chose to
run it
towards the northeast. I did this so
that the antenna's horizontal radiation would be favored by a dB or so
from the
"elbow" of the inverted L towards the southwest.
I ran my 52-Ohm coax in a buried pvc conduit to the vertical portion's
base. With the antenna’s overall length of somewhat greater than 1/4
wavelength (in fact about 1/3 wavelength), there is an inductive
reactance at
the base feedpoint, so all I needed for matching was just some
capacitance
between the feedpoint and the 50-Ohm coax.
I used a husky transmitting variable capacitor. I
haven’t pulled it to measure its value,
but my eyeball guess is about 300 pF.
I have thus avoided a lossy inductor anywhere
within the
antenna, and at the shack there is no balun or antenna tuner with their
losses.
With eight in-ground radials I could adjust the capacitor to give me a
1.5:1
SWR at 1.810 mHz. I added six more
radials, readjusted the capacitor to get a 1.35:1 SWR, and then brought
that
down to 1.2:1 with 22 radials.
The antenna performs well on 160m.
Without any real effort, this past winter I worked several
European
countries with five Watts, and no QSOs occurred during contests.
If I had had only, say, a 40' vertical run, I would have adjusted the
length of
the horizontal leg accordingly. And if
I were space limited for the horizontal leg, I would have used linear
loading
to reduce the needed length by a third or so, or would have made a bend
in the
horizontal portion to fit it in the space available; i.e., do whatever
is
needed to locate the antenna's current maximum at the vertical leg's
center.
I do worry about the emerald ash borer, imported from China in the
wooden
packing crates used to pack stuff coming from there. Nothing
has yet been discovered to eradicate this pest that has
killed many thousands of ash trees in this country (including four of
mine so
far). Also, laying in radials that
range from 70 to 160 feet in length is very slow and tedious work when
extracting lots of rocks and going around tree trunks and under their
roots
near the surface.
72,
Charles, W2SH
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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