|To:||"K0PYK" <firstname.lastname@example.org>,"Tower Talk" <email@example.com>|
|Subject:||Re: [TowerTalk] 80 Meter Hybrid Triangle Loop|
|From:||"Guy Olinger, K2AV" <firstname.lastname@example.org>|
|Date:||Sat, 6 Nov 2004 13:26:50 -0500|
----- Original Message -----
From: "K0PYK" <email@example.com>|
The configuration is, from the connection point...40 feet up vertically...120 feet cross the top horizontally...them angle back down to the connection point.
* I have seen different formulas thrown around...Is 1050/f the correct one for this loop? * What is the best way to feed this loop? Coax, ladder line, 1/4 wave 72 ohm...etc. I have a tuner in both rigs.
I modeled this (only three wires, so no big sweat). Observations:
1) Shows a feed Z of 30 ohms at 3.6 MHz. Close enough to the resonance point but I'm not sure what Z you were expecting
2) Overall pattern gain (perfect = 1.0) is 0.23 or -6.47 db, rather like a vertical with a poor radial field. The losses are almost entirely vertically polarized ground losses within two or three wavelengths.
3) With that in mind, the gain of the main lobes is only 0.65 dbi.
4) The radiation pattern is irregular, a) with moderate cancellation of high angles, b) main lobes broadside to antenna are vertically polarized because the long horizontal runs have current distributions nearly equal and out of phase broadside and cancel both out and up.
5) With the specified feed point, the two horizontal runs do little other than act as a device to provide a non-ground current sink for the coax shield, and a top hat for the short vertical section.
6) if you want to match specified impedances with coax, close the loop at the bottom and feed the sloping section at various distances from the bottom. In the model,
50 ohms is found out approx 23 feet. (1:1 balun)
100 ohms is found out 37 or 38 feet. (1:1 balun and electrical 1/4 wave of 75 ohm coax)
Either of these matching points is narrow (because the vertical section is short) with a 2:1 bandwidth of only 75 khz or so. You will need to shorten or lengthen the horizontal sections to tune to desired center. You should keep the vertical section as long as possible.
The 200 ohm point is touchy and problematic. (4:1 balun)
Moving the feed and therefore also the current max out the sloping section starts to modify the pattern and allow some high angle radiation. It also mildly reduces the ground loss.
The 50 and 100 feed points produce a hemispheric pattern broadside that will fill in local coverage without any reduction of the low angles.
If one wants to put up antennas of this sort, it really is time to get a copy of EZNEC (or ....) and start modeling. Without it, it's just throwing up wires blind and hoping.
There is an adjustment to this design that produces a far more satisfactory result. I will post that later.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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