Hi Ray, these are all very deep and difficult topics. The only matching I
have is between coax patch cord that plugs into rig and mfj989d tuner. It
matches coax to the ladder line. But between the ladder line and the
antenna I have no matching device, because the ladder line is simply cut
and conductors spread into a T shape. Yes, this is a mismatch, but most
dipoles are constructed that way. It does bother me, but I would need to
study very hard what can be done at that location and I don't yet have a
full grasp of these tough electromagnetic concepts. While VSWR is constant,
the impedance is a function of position. Certain points are beyond the
tuner's ability to form a match. Now it is true there can still be a high
SWR in the ladder line, (just that transmitter doesn't see it) because of
antenna mismatch. This will lead to power being reflected back and forth up
and down the line which can lead to resistive (heating) loss. But a big
advantage of ladder line is it is very low loss so every round trip only
dissipates a small amount of power. I don't feel any heating when I touch
the line. It is not ideal, but until I figure out a better option I am
stuck with what I have. I read that a folded dipole in which the antenna
has two conductors instead of one has 4 times the impedance. This would
mean almost a 300 ohm driving impedance very close to the rated 450 ohm
ladder line. I probably should have done that, but too late now. Yet,
simple dipoles are frequently used. I like the idea of a perfectly balanced
antenna. No RF burns in shack. When I was a kid I had a Hygain 18AVT-WB
vertical on my roof. I followed all the instructions. I had radials,
lightning arrestor, ground rod. Yet if I touched outside metal conductor on
coax I got a zap. As a kid I had no clue why this was happening.
Electromagnetics is a very tough subject with a lot of advanced math. Need
to really plug away to get every nuance and really understand it
intuitively. I hope someday to master it.
All the best,
Barry WA2VIU
--
Barry Jacobson
WA2VIU
bdj@alum.mit.edu
@bdj_phd
On Mon, Feb 6, 2023, 8:59 PM Gmail <anyone1545@gmail.com> wrote:
> SWR is defined as ratio of maximum to minimum voltage / current on a
> transmission line. I don’t see how changing the line length will affect
> that ratio.
> The dipole has a 72 ohm impedance at the center and ladder line has a
> theoretical impedance of 250 - 350 ohms. Just want to confirm there is a
> matching device between line and antenna.
>
> Ray
> W8LYJ
> _______________________________________________
> CQ-Contest mailing list
> CQ-Contest@contesting.com
> http://lists.contesting.com/mailman/listinfo/cq-contest
>
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|