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