Hal Rosser wrote:
If you have a MFJ-259, you can get the VF of the wire easily
or
1. put up a 10-meter dipole using bare wire - note the resonant freq
2. duplicate exactly using 14 THHN Str - note the NEW resonant freq.

divide the resonant freq from step 1 by the resonant freq from step
2. (or
is it the other way around)

I wish. Finding the resoant frequecies is my fundamental problem.

Anyway, I don't think you have to add your social security number.

Heh.

w3rv

build a scale model with insulation at 900 MHz and test it on a network
analyzer in the lab

remove insulation and retest?


Mark

The question that comes first to my mind is, "Why do you care?"
Certainly an antenna does not need to be resonant to work well. I can
imagine you'd like a reasonably low indicated SWR, just so your
transmitter has a reasonable load to drive.

If you really want to know what's going on at the antenna feedpoint,
you'll need to back the effects of the feedline out of your antenna
analyzer readings, or use an analyzer that does it for you. If you
have a reasonable estimate of the feedline loss and know its electrical
length (easy to find if you put a short at the end of the line and look
at the resulting impedances read on the analyzer), then you should be
able to translate your analyzer readings to actual feedpoint impedance.
Do you have the feedline properly decoupled from the antenna so it's
not a significant part of the radiating system? If not, there seems
little reason to bother making the measurements.

I'd expect half-wave dipole resonance to result in lowest SWR on a
50-ohm feedline, but it won't be a very sharp minimum. So is it worth
worrying about?

Another 'speriment to try: build a fairly wide-spaced two wire
transmission line from your wire. Short it at one end, open at the
other, and look for quarter-wave resonance; or short both ends and look
for half-wave resonance. Measure the resonant frequency, which will be
a pretty sharp resonance (much sharper than the dipole). Remove the
insulation and see how much the resonance changes. Try for various
spacings to see what effect the spacing has. (Expect that close
spacings will show more effect than wide.)

Cheers,
Tom

K7ITM wrote:
The question that comes first to my mind is, "Why do you care?"
Certainly an antenna does not need to be resonant to work well. I
can
imagine you'd like a reasonably low indicated SWR, just so your
transmitter has a reasonable load to drive.

If you really want to know what's going on at the antenna feedpoint,
you'll need to back the effects of the feedline out of your antenna
analyzer readings, or use an analyzer that does it for you. If you
have a reasonable estimate of the feedline loss and know its
electrical
length (easy to find if you put a short at the end of the line and
look
at the resulting impedances read on the analyzer), then you should be
able to translate your analyzer readings to actual feedpoint
impedance.
Do you have the feedline properly decoupled from the antenna so it's
not a significant part of the radiating system? If not, there seems
little reason to bother making the measurements.

I'd expect half-wave dipole resonance to result in lowest SWR on a
50-ohm feedline, but it won't be a very sharp minimum. So is it
worth
worrying about?

I agree 100%, it's not worth worrying about - if all I wanted is to get
a 20M dipole running on the air. But that's not my point. I'm trying to
use dipoles to determine the velocity factors of insulated wires used
for the radiators.

Another 'speriment to try: build a fairly wide-spaced two wire
transmission line from your wire. Short it at one end, open at the
other, and look for quarter-wave resonance; or short both ends and
look
for half-wave resonance. Measure the resonant frequency, which will
be
a pretty sharp resonance (much sharper than the dipole). Remove the
insulation and see how much the resonance changes. Try for various
spacings to see what effect the spacing has. (Expect that close
spacings will show more effect than wide.)

Now we're cookin', I like it, this approach has definite appeal and I
need to explore it for several reasons. First because of the sharp
nulls, it takes out the coax and it's a simple sort of "bench test" I
can do at ground level. I'll build three identical shorted 20M or 30M
close-spaced quarter wave lines. One with bare #14 stranded wire, one
with the insulated #14 THHN wire I usually use for quick & dirty
dipoles and one with The Wireman's very flexible #544 or #546 #14
insulated PVC jacketed wire I'd use to build a hex wire beam or a quad.


Then I'd use a grid dip meter to find the resonant frequencies of all
three. I'd use an HF rcvr with a digital freq display to listen to the
GDO rather than trust the GDO dial calibration and resolution. Yes?

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?

Thanks,

Cheers,
Tom

w3rv

Brian Kelly wrote:

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the
quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?

OK, I admit to being a spoiled brat when it comes to making
measurements like this. The easiest for me would be to set up a
network analyzer, with the analyzer's source and receiver each coupled
lightly to the line. But there are other ways. You may not even need
a signal generator. You could loosely couple a receiver to the line,
and loosely couple an antenna to it on the other side, and as you tune
the receiver across the resonance, you should notice a sharp peak in
atmospheric noise. With a loaded Q of a few hundred, the peak at 10MHz
would be a very few tens of kHz wide. You could also build an
oscillator which uses the tuned line as the frequency-determining
element, and just count the frequency of the oscillator. A simple
version of a network analyzer could be done by lightly coupling an RF
generator into the line, and putting an RF detector across the line a
small distance up from the shorted end. I'd use either a simple diode
detector, which can have pretty high input impedance, or one of the
Linear Technology or Analog devices RF detectors, but since those are
lower impedance, tap them down very far on the line. -- I'd expect a
10MHz quarter-wave resonator made from 600 ohm line using AWG14 wires
to have an unloaded Q around 350.

You can achieve that loose coupling by calling the center of the short
across the end "ground" and tapping up just one or two percent of the
length of the line from that for the "hot" connection. Or you can
couple in with a loop, say of a diameter about equal to the line
spacing, held next to the shorted end of the line. "Reference Data for
Radio Engineers" shows various coupling schemes in the "Transmission
Lines" chapter. As long as you keep the coupling light (to keep the
loaded Q high), and are consistent in the way you arrange things, you
should be able to measure the resonance to within a fraction of a
percent repeatability, if not absolute accuracy. Relative measurements
should be all you need in this case. And I assume it's obvious that
though the resonant line will not have exactly the same VF as an
antenna, as you increase the wire spacing, it should approach the same
effect as you'll see in the antenna. Then compare with the formulas
Reg posted, and if you see significant differences, try to resolve
what's causing them.

By the way, one place where the "velocity factor" effect might be
noticable is in parasitic elements of an array in which you're trying
to achieve maximum gain. The element tuning will affect phasing among
the elements and therefore gain. If the design is narrow-band,
high-gain, you might actually notice some effect from the insulation.

Cheers,
Tom

Tom

Someone might refer to you as a "spoiled brat" regarding your interest in
the details. I consider you to be a carefull thinker.

Jerry


"K7ITM" wrote in message
oups.com...
Brian Kelly wrote:

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the
quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?

OK, I admit to being a spoiled brat when it comes to making
measurements like this. The easiest for me would be to set up a
network analyzer, with the analyzer's source and receiver each coupled
lightly to the line. But there are other ways. You may not even need
a signal generator. You could loosely couple a receiver to the line,
and loosely couple an antenna to it on the other side, and as you tune
the receiver across the resonance, you should notice a sharp peak in
atmospheric noise. With a loaded Q of a few hundred, the peak at 10MHz
would be a very few tens of kHz wide. You could also build an
oscillator which uses the tuned line as the frequency-determining
element, and just count the frequency of the oscillator. A simple
version of a network analyzer could be done by lightly coupling an RF
generator into the line, and putting an RF detector across the line a
small distance up from the shorted end. I'd use either a simple diode
detector, which can have pretty high input impedance, or one of the
Linear Technology or Analog devices RF detectors, but since those are
lower impedance, tap them down very far on the line. -- I'd expect a
10MHz quarter-wave resonator made from 600 ohm line using AWG14 wires
to have an unloaded Q around 350.

You can achieve that loose coupling by calling the center of the short
across the end "ground" and tapping up just one or two percent of the
length of the line from that for the "hot" connection. Or you can
couple in with a loop, say of a diameter about equal to the line
spacing, held next to the shorted end of the line. "Reference Data for
Radio Engineers" shows various coupling schemes in the "Transmission
Lines" chapter. As long as you keep the coupling light (to keep the
loaded Q high), and are consistent in the way you arrange things, you
should be able to measure the resonance to within a fraction of a
percent repeatability, if not absolute accuracy. Relative measurements
should be all you need in this case. And I assume it's obvious that
though the resonant line will not have exactly the same VF as an
antenna, as you increase the wire spacing, it should approach the same
effect as you'll see in the antenna. Then compare with the formulas
Reg posted, and if you see significant differences, try to resolve
what's causing them.

By the way, one place where the "velocity factor" effect might be
noticable is in parasitic elements of an array in which you're trying
to achieve maximum gain. The element tuning will affect phasing among
the elements and therefore gain. If the design is narrow-band,
high-gain, you might actually notice some effect from the insulation.

Cheers,
Tom