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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 |
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