On 11/11/2010 12:51 AM, Richard Clark wrote:
On Thu, 11 Nov 2010 16:31:35 +1100, "VK2KC Gmail"
wrote:

But what I want to know is how the Arrow manages to match the antenna to the
50 ohm feed.

I am thinking its in the spacing of the two elements?

Hi OM,

Both styles are the same basic method. You are using a length of
transmission line (the paired section) to match one Z to another Z.
The line Z is part of the equation (the spacing, as you asked).

One method (most observed) uses a tapped feed which for the shorted
section is located at the line's translation to 50 Ohms, the open end
is a higher Z matching to the half-wave radiator (I won't go into the
difficulties and considerations of the matching section also
radiating).

The mystery feed point (do I connect my "hot" lead to the long wire,
or the short one?), its magic position, all contribute to a religious
lockstep of praise. In itself, it has virtually nothing to offer but
complexity.

The other method (infrequently observed) simply approaches the
situation in the classic application of a line transformer. Think of
the smith chart. It's amazing how often this method is used at HF,
but draws blank stares when we shift to UHF.

I have built, measured, and tested antennas employing the second
method because, frankly, it is far simpler to construct on a SO-239
connector: one wire in the center conductor, one wire from any of the
four holes - end of story (neglecting the precautionary tales of
decoupling everything).

The practical side of this home-brew is you build one "close enough,"
and then with the aid of a SWR meter, you clip off 1/8th inch portions
of wire (either side until you get the drift of it) until you have
flattened it out the pesky SWR - and then walk away.

Anyone versed in using the Smith Chart will immediately recognize that
there are many possible satisfactory solutions. As is common with
cautioning children not to run with scissors, not all Z-match
solutions are preferred radiators.

The free version of EZNEC is eminently suited to exploring this.

73's
Richard Clark, KB7QHC

Just as Richard has constructed several, so have I. I made a somewhat
frequency adjustable one for a friend that does audio for car shows here
in the Minneapolis area. He has dozens of remote speakers that run on
commercial RF distribution freqs, but also transmits low power FM
broadcast for those cruising the show. I used a base length for the 2
rods for the high end of the band plus 1 tip for the quarter wave
section and 2 for the 3/4 wave section. Gave him a chart that showed
what combinations covered what frequencies with roughly 1.5 to 1 or
better SWR. Works like a champ. used a simple 7 turn on 1.5 inch PVC
choke to _reduce_ feedline radiation. Told him he might want more, but
up to him.

Scaling the Arrow design to around 100 MHz involved increasing the
spacing between the 2 rods, as you might have suspected. The design
using changable/removable tips works, but the bandwidth got narrower
towards either end of the US FM band.

tom
K0TAR