Bill:
Ah, when in doubt, actually measure it!
What a novel idea to use a simple home
brew reflectometer. If you have an SWR
bridge or antenna analyzer, you might also
check if that pot is purely resistive. I would
guess that any reactive effects are quite small
at 1 MHz anyway.
73, Gary K5AMH
On 12/23/2015 10:09, Bill Crowell via BVARC wrote:
Hi Gary:
I don’t mind your 25 cents.
What I figured out was a way to actually measure the Zo. I use an
Oscope and a signal generator and about 3m of line under test. I
terminate the line with a 10-turn pot.
I bang a 1MHz square wave into one end and watch the scope while
carefully adjusting the pot. When it’s matched, the square wave is not
distorted and the amplitude is at the same ratio as the change in
impedance from 50 ohms! The scope is In high-Z mode.
The interesting thing is how the values are close to the calculated.
The lines in the 9:1 are calculated roughly 200 ohms, but measure 190.
------------------------------------------------------------------------
Jerry Sevick told us his experimental results, but didn’t tell us how
to make a balun. Yes, he added a tremendous amount of knowledge to the
art, but I’m making up the backend of how one actually builds it.
Sadly, the photographs in the publications are such poor quality as to
only give an idea of the fabrication of the test jigs he made. Lots of
really good analysis, but I’d love to see his raw data.
B
Bill Crowell, N4HPG
Pearland, TX
Text messaging one-handed since 1982
From: BVARC <[email protected] <mailto:[email protected]>>
on behalf of BVARC <[email protected] <mailto:[email protected]>>
Reply-To: BVARC <[email protected] <mailto:[email protected]>>
Date: Wednesday, December 23, 2015 at 9:59 AM
To: BVARC <[email protected] <mailto:[email protected]>>
Cc: Gary Sitton <[email protected] <mailto:[email protected]>>
Subject: Re: [BVARC] TTFD Part Deux
Bill:
I'm sticking my neck out here, but here's an
idea for window line transmission lines. Use
the weighted average of the dielectric constant
(dc) for air and the feed line spacer/insulator. In
the case of real ladder line, the dc is very close
to that of air or approximately a vacuum.
The window line I use has 14 AWG wire and is
actually spected at closer to 400 Ohms than the
450 Ohms usually associated with window lines
using 18 AWG wire. Line losses go up as the
ohmic dielectric losses in the separator material
is more dense, e.g. solid twin lead is the worst.
Another consideration is velocity factor. True
air spaced ladder line is about 0.99 whereas
ladder lines are more like 0.95. This is all mainly
dependent on the dc as the permeability of most
dielectric materials is that of air or a vacuum.
Again, pardon my 1.5 cents worth on this Bill.
73, Gary K5AMH
On 12/23/2015 08:55, Bill Crowell via BVARC wrote:
All,
I’ve received a number of emails from you and I’ll send out a reply
to the group.
This is an on-going experiment that I hope to have wrapped up in a
few weeks. I’m waiting on some material to arrive from eBay.
Action items:
1. Using the PVC box instead of an aluminum box necessitates using a
feed line choke. The material is coax to wind to make the choke.
2. Testing with a 500 ohm terminator. I’m fabricating a test jig
that will let me use jumpers to select the values for further
testing.
3. Some have asked for detailed build instructions instead of paying
me <grin>. Of course. I dislike it when people hide the designs
as patented and proprietary – when it comes to ham radio.
4. Some have asked me to do a presentation at a club meeting or 3.
My health has been slowly improving so that’s possible after the
holidaze.
Since the article was written, I’ve developed a way to actually test
the characteristic impedance of the flat transmission lines. If you
look at the formula, it is a function of the diameter of the wires,
the spacing and the dielectric constant of the material – fine when
you have a solid piece of material such as a TV twin-lead, less so
when you have some other material.
One very important item was left out of the article and will be in
the revised article. Grounding. There is provision for a ground lead
on the balun; however, I’ve not installed a ground yet. The antenna,
being balanced, does NOT require a ground for operation. This is
especially important on 160-40m. A vertical needs a lot of ground
plane for efficiency.
Grounding at the balun is important for SAFETY. With a wire up 50’ in
the air, lightning happens.
Any volunteer to help me install 2 ground rods? One at the antenna
and one for the main station ground.
73!
Bill
Bill Crowell, N4HPG
Pearland, TX
Text messaging one-handed since 1982
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