On Nov 27, 2:46*pm, Jeff Liebermann wrote:
....
Good luck, but first a little math. *What manner of tolerance do you
thing you need to cut your coax pieces? *Let's pretend you wanted to
get the center frequency accurate to 1Mhz. *At 418MHz, one wavelength
is:
* *wavelength(mm) = 300,000 / freq(mhz) * VF
* *wavelength = 3*10^5 / 418 * 0.83 = 596 mm
That works out to:
* *596 / 418 = 1.4 mm/MHz
So, if you want the center frequency accurate to within +/- 1MHz, you
gotta cut it to within +/- 1.4 mm. *Good luck. *Like I previously
ranted, you'll need a cutting fixture. *A steady hand, good eye,
quality coax, and plenty of patience are also helpful.

But why would you care to try to get it within 1MHz? With only four
radiating elements, the beam 3dB width will be roughly 8 degrees if
the bottom of the antenna is a wavelength above ground (30 degrees in
freespace...). There's not much point in putting a lot of effort into
get closer than perhaps 4 electrical degrees along the line, and I
don't believe even that is necessary to get good performance. That's
several mm, and should be easy with such short lengths. Using foam-
Teflon coax makes it easy to do: the insulation doesn't melt when you
solder things together. I cut the sections to matched lengths, use a
little jig to trim the layers to the same lengths on each, and then
put a wrapping of 30AWG or so silver plated wire (wire-wrap wire)
around each joint to hold it while soldering. That makes it easy to
adjust before soldering, and solid after.

Incidentally, since the top 1/4 wave element represents something
close to perhaps 50 ohms, it would be interesting to measure the
amount of RF that isn't radiated and actually gets to the top section
of the antenna. *If my analysis of the antenna is correct, the first
section (near the coax connector) radiates 1/2 the power. *The next
section 1/4th. *After that 1/8th, etc. *By the time it gets to the top
of the antenna, there won't be much left. *However, that's theory,
which often fails to resemble reality. *It would interesting if you
stuck a coax connector on the top, and measured what comes out.


There's very little loss in a half wave of decent coax at 450MHz.
That means that the voltage across the lowest junction between
sections is echoed up the antenna at each other junction. In
freespace, by symmetry, the currents will be very nearly the same
going down from the top as going up from the bottom. My model over
typical ground (bottom a wavelength above the ground) shows current
symmetry within a percent or so, assuming equal voltages driving each
of the three junctions. If you wish, you can use the parameters of
the line you're actually using to figure the differences among the
feedpoint voltages, based on the loads at each junction. When I've
done that in the past, the differences are practically negligible.
You can iterate, feeding those voltages back into the model to find
new load impedances, etc., repeating till you're happy that the models
have converged. Recent versions of EZNEC even let you put the
transmission line into the model, along with its loss.

The supporting tube certainly will affect the feedpoint impedance, but
in my experience, it does not materially affect the pattern. I deal
with the impedance through a matching network; it's no trouble to
adjust for a low enough reflection that I don't worry about it.
Decoupling is the more interesting problem, to me.

Cheers,
Tom