In a message dated 7/24/01 10:49:43 PM Pacific Daylight Time,
Bgsalesmel@cs.com writes:
Hello,
Name is Mel, KD7DCR, here and I am new to this ham radio gig. I am just
getting my first station set up, tower up, etc. this summer.
You seem to "flat know" your stuff about ladder line applications and I
have a basic question: i.e., How do you install your 123' or 135' of line
when the center connector is only +/- 45 ft from your operating
position/single point entry? This is a dipole design from CQ mag of about 3
or 4 years ago and in it they suggested using 66' to 125' of 450 ohm line. It
is about 140ft long made out of house grade Romex, center at 70ft naturally,
with the 3-cond "dasiy chained" for an effective length of +/- 210ft per
side. I personally think this will "look" like a 3/8" OD single conductor due
to induction - but, who knows: If it plays well and works for me here, I
don't care if it thinks it is 10ft long! Tuner will be a MFJ-949E that
accepts and switches 450 ohm line directly.
I can only put up a 40ft tower here to start - will fight the permit battle
when I can afford a self-supporting one in about 2 years. In the mean time,
make do with what I can afford and do now. Dipole will hang off of this tower
at the 32 to 34ft level, with a Unistrut/pulley assembly, and the other end
will be at about 22ft on a pole in my garden. Side lobe will be centered at
about 020 deg (true - not mag) by 200 deg.
I have an XYL that objects to "wires hanging all over the place", etc. and
need to keep this "looking" very sanitary. I have a blank wall of the house,
about 9ft tall x 14ft long, T-111 wood siding, right beside my entry to the
basement operating position. This is behind a green bush right now, it would
be in the open during winter months.
Is there any way to "loop" or "zig-zag" this excess ladder line on this
wall before going down into the shack? The eve has a metal rain gutter,
everything else is wood. Will be using one of the Wireman's QST grounding
spark plug devices before heading into the shack, and a knife switch inside
as well. I intend to disconnect from all ant and power when not working.
Top ant on the tower is Force12 C-4.
Thanks if you can shed some light on my question.
Regards & 73's,
Mel Frost
Coeur d'Alene, ID >>
Mel: Looping and zig zagging is about the only way to use up the excessive
open wire length or live with the higher reactive loads of short length
feedlines if the tuner will match them. If you can get 1:1 SWR on the coax
to the rig and no arcing or warming of the tuner, you have maximum transfer
of energy into the input of the feedline and into the antenna minus the loss
in the feedline what ever it is. Matching at the input of the feedline is
the most important match.
I'm going to make a "Ferris Wheel Spiro" to use up excesive length. The
spacing will be about 2' between the line. Right angle bends are not
recommended. I'll grid dip it before and after. The Ferris Wheel can be on
a post or on the side of the house.
I have in the past had a 2 ganged 2 pole switchs so I could swtich in
certain lengths of open wire line so as to obtain a Hi-Z or a Lo-Z for the
tuner. This is practical for the higher frequencies. You don't always need
a tuner with typical Lo-Z's around 50 ohms reflected to the end of a 1/2 wave
multiple feedline. I have found you can feed and open wire line directly
into the unbalanced 50 ohm output of a rig or in my case a coax switch
without RF in the shack. I add a series BC 3 gang variable in the grounded
or shield lead. Connect the rotor to the shield or ground connection.
Resonate a dipole or vertical antenna low in the band and using a feedline
that is a 1/2 wave multiple, you can often obtain a low SWR and/or resistive
load easy over the whole band to match by even the 50 ohm rigs in many cases.
In the "Old Days" of "Link Coupling," one could have a balanced or
unbalanced output with a series variable to obtain a resistive load to the
final even with SWR. Band edge operation was not a problem it is now with 50
ohm output finals. With a resistive load one could swing the link in an out
for any power level desired without having to redip the final. With the
right number of turns in the link one could reflect the optimum load to the
final for max output for loads of 30 to 1000 ohms also--no tuner. You just
had the inconvenience of changing tank coils. So all Art Collins did for you
with the band switching Pi is to eliminate plug in coils, lower the Z
matching range and allow only an unbalanced load. You could match higher
SWR's on coax but at a lower final efficiency. I have built a blanced Pi
output final for push-pull tubes and it's great for open wire line of Hi/Lo-Z
loads. I still have my old Pr-813's final and will perhaps just add 2 more
813's.
With open wire line you don't have to worry about your feedline losses going
up in time and feedline damage is easily seen and repaired. It is not with
coax. I also still have and open wire line I made in the 30's. That's great
feedline economy. k7gco
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