Mike Coslo wrote:
As a G5RV is to a dipole, is your modification *really* a G5RV?
Seems to me that you have a ladder line and capacitor tuned dipole, not
a G5RV.
The cap is only on 75m to lower the 50 ohm SWR. It doesn't have to be
installed if one can tolerate a 3:1 SWR.
Does the 102 foot dipole have to be called a G5RV? Heck, Cecil - you
might just be able to call this the W5DXP antenna. Seems different
enough to me! 8^)
The physical difference is negligible. It is still a 102 ft dipole fed
with a series section transformer made out of ladder-line and fed with
coax from that point to the shack.
Do You switch the capacitance in and out, or use variables? And if
you switch, do you have the caps at the coax ladderline junction or in
the shack?
As I said earlier, if you can tolerate a 3:1 SWR, you don't need the cap.
Presently I switch the cap in and out by hand. My G5RV is at 40 ft. on
the ends and I just grab the feedline and pull it down to access the
ladder-line/coax junction where the cap is installed for 75m - thus
eliminating the need for a tuner. It only takes a minute or two to pull
on the feedline and plug or unplug the cap. I hope that's clear. Here's
a diagram for 75m operation:
XMTR---any length 50 ohm coax---+cap+---25 ft. 450 ohm feedline---102' dipole
The parallel 1000 pf cap is across the two feedlines at the twinlead/coax
junction.
Possible really stupid question alert!: When you speak of the ladder
line being 20-36 feet, and or 25 feet for 3.8 mHz (with cap) you mean
that the total height from the coax/ladder line junction to the antenna
itself must be the length of the shortest piece of tuning ladder line?
Maybe a confusing question? I have 8, 4, 2, 1 ft pieces of ladder-line
that I insert between the 21 ft. minimum length of ladder-line and the coax.
That allows me to vary the length of the series section transformer from 21
ft minimum to 36 feet maximum. Obviously, I could use one more length to
increase it by X amount.
IOW, If I run my coax to the roof of my house to the box I would
construct to hold the different lengths of tuning line and switching
electronics, I could only have the antenna roughly 25 feet above this
box? If so, I can still probably get the antenna 50 feet above the ground.
I had envisioned mounting the box on a 2x4 sticking straight up at the
middle of the dipole. But I could probably mount the box on my roof.
Any problems with rolling the ladder line?
DON'T ROLL THE LADDER-LINE! Keep adjacent loops at least six inches apart.
Here's an idea that I have been toying with. It involves varying the length
of the coax as well as the length of the ladder-line to maintain a constant
total length of feedline. This is a physical solution to the mechanical
problem and has virtually no electrical effect.
XMTR-----coax------------ ----coax---- ------coax------
A B C
==twinlead== ====twinlead==== ==twinlead==dipole
This is for illustration only. Not all the switchable lengths of twinlead
are shown. Only two are shown.
We need a DPDT relay at A and C. We need a 4PST relay at B and other points
like B. For minimum twinlead, we could have a coax to coax connection at
points A and B and a coax to twinlead connection at point C. For maximum
twinlead, we would have a coax to twinlead connection at A and a twinlead
to twinlead connection at points B and C. The physically parallel coax
and twin lead would have to be physically separated, maybe one on each side
of a 2x4 about a foot apart.
An SWR sensing circuit could be added to turn this thing into an autotuner
for a G5RV. Hope I didn't get carried away. These are just some random
thoughts toward perfection of the G5RV.
--
73, Cecil, W5DXP
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