David,
The Wireman has a lightning arrestor for use with ladder line - it looks
very much like a pair of spark plugs mounted on a piece of copper. It
does not ground the ladder line, but it does provide suppression for
lightning surge voltages.
You may have better luck with a 1:1 balun rather than the 4:1 - it all
depends on the electrical length of your ladder line and the antenna.
To check that, measure the input impedance of the antenna and feedline
with an antenna analyzer at the point where you are placing the balun.
If you read a relatively low impedance with the 4:1 balun, change it to
a 1:1. The L network tuners have more trouble matching a low impedance
load than a higher impedance, so if you can get the impedance in the 40
to 150 ohm range at the point where the coax connects to the balun, you
will have succeeded. A 3:1 SWR will not add a whole lot of extra loss
up through 30 MHz.
I assume you are using a low loss coax for that 50 ft run - if not, you
should be. RG213 as a minimum, but LMR400 would be better. If you can
find a length of the 1/2 inch data type (75 ohm) that the cable folks
use, that is a good low loss cable too. There was an article in QST a
month or two ago about using RG6 for transmitting, I was particularly
interested in the fact that the RG6 loss is about the same as that of
RG213 - the 75 ohm 1/2 inch data cable should be much lower loss than RG6.
73,
Don W3FPR
David Wilburn wrote:
A very interesting read, I have a couple of questions about practical
considerations.
The number one rule I look at all antennas with, is that all antennas
are a compromise. Let me know if I am getting off on the wrong foot here.
I like ladder line fed to various antennas (Horiz. Loops, Vert. Loops,
Doublets and such) but if I have more than one antenna, then I have
multiple sets of 450 ohm twin lead coming to the house. As I understand
things, I need to keep the ladder line away from metal as much as I can.
I have not found a way to safety ground ladder line and run it into the
house. After taking a lightning strike last year, I have no wish to
have ladder line and / or balun's inside the house.
*Is there a way to properly ground that ladder line, and then run it
into the house?
*If there is a practical way, I assume one uses a plastic box or no box
at all?
Based on all of these practical considerations, I am left with the
following current configuration.
K3/KAT3 -> Balun (4:1) === Ladder Line === Doublet
(In this case a http://www.k1jek.com/ 200' antenna)
The coax is about 50' long (to get to the shack in the house, I would
love to get this shorter, working on that) and the 450 ohm ladder line
is about 50' long.
Configuration I am working towards;
K3/KAT3 -> RF Switch -> Coax (trying to get this down to about 10') to
box mounted on side of house (metal) -> Lightning arrestors tied to good
ground rods -> short coax to balun mounted beside house -> 450 ohm
ladder line to antenna.
Would like to have a vertical loop and dipole or horizontal loop, fed
with ladder line.
The antenna I have now, gets out very well, and tunes pretty well from
160m -> 6m. Some bands like 40m, it does not tune as well, but has
performed well. I have heard very little on 6m, but did work Portugal.
The 40m issue is supposed to be helped by reducing the length of coax,
and lengthening the ladder line. That makes sense, to reduce the
resistive losses in the coax. Understanding that this is a low dipole
(about 40 - 50 feet) I'm wondering if changes to the configuration would
help the antenna's ability to hear. I'm not sure it hears as well as it
could.
Options to improve antenna, and possibly receive
================================================
*Get current antenna higher. With the trees that I have to use, this
could only be about 10'.
*Drop ends of antenna to 10', and raise center higher using a different
tree. This has possibilities of getting the center higher than 60'. I
am not sure this would help receive, but it is an option.
*Go to a different balun setup 1:1 versus 4:1.
*Not sure of a practical way to get multiple ladder lines safety
grounded and into the house. So I do not know how to get away from
having a balun.
Are there other practical options I am missing here? Yes, I would like
to have a short tower and a multi-band cubical quad. Maybe at some
point in the future, but not anytime soon.
David Wilburn
K4DGW
Ron D'Eau Claire wrote:
Also, the load has a huge effect on balance. Few wire antennas for HF
offer
decently balanced loads. Unless the wires are literally wavelengths
(usually
hundreds of feet) from the earth and other objects, those objects will
have
a strong effect on the currents on each side of the antenna. The
effect is
greatest near the ends of the wires, where they typically come close to
supports, trees, houses, etc. Unless both ends have identical
surroundings,
the antenna, and so the currents in the feedline, are unbalanced.
Ron AC7AC
-----Original Message-----
The question is, what is good enough? To minimize radiation from an open
wire tuned feeder requires, I believe, that the currents in the two
wires to
be equal in magnitude and have a phase difference of 180 degrees at the
feedpoint of the feedline. Feeding a slanted dipole, which is
certainly an
unbalanced antenna, is it practical to build a 1:1 balun on a ferrite
core
(core type choice?) that, when placed on the output of an unbalanced
tuner,
is good enough to force the desired currents from 40m thru 10m without
excesive losses? Using an LC inductively coupled balanced tuner on
such an
unbalanced antenna will not produce the desired results--deliberately
unbalancing the LC tuner by offsetting the taps on the coil will
sometimes
get close for me.
73 Paul W5DM
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