[TowerTalk] Deltas and gain

[L. B. Cebik]

A couple of notes regarding getting more punch from a delta and feeding
it.

First, there are various figures given for where to side-feed a delta loop
to maximize vertically polarized radiation.  The differences in the
numbers folks use stems from 2 problems:  1.  People do use different
shaped triangles, ranging generally from equilateral to right angle (with
the right angle at the apex); and 2.  people insist on measuring from the
horizontal wire at the bottom upward.  The correct measurement is from the
apex downward, and that is always 1/4 wl, where the actual distance is 1/4
the loop diameter.  That will give you a different distance upward as you
change triangle shape, but will keep constant the 1/2 wl distance from the
feedpoint down to the horizontal wire, across and up the far side wire to
the corresponding point.  The SWR minimum is broad and allows movement up
and down the wire witout great changes in SWR, but this movement does
change the current distribution, whose minimum point should be centered
along the horizontal wire for minimum horizontally polarized radiation.

Incidentally, a delta resonant for side feed will not be resonant for
feeding either the upper apex or the bottom center, when the antenna is
used on other bands.  The different feedpoints yield different current
distributions, radiation polarization, and different size resonant loops.
Resonance, though, is rarely a problem for either of the horizontal modes,
since these are commonly fed with parallel transmission line for use on
several bands with an ATU.

Second, it is possible to derive well above 3 dB forward gain and a
front-to-back ratio of 10-15 dB by placing two vertical loops about 0.15
wl apart.  The result is a parasitical 2-element beam with the same low TO
angle.  The beamwidth will be fairly wide: 80-90 degrees, without the side
nulls we are used to with upper HF high altitude Yagis.  Feedpoint
impedance will be to the 60-65 ohms range at resonance.

Expect both the driven loop and the the reflector to be a bit shorter than
a resonated single loop, with the driven element shorter than the
reflector.

If you care to scale some numbers from 7.15 MHz, here is a right angle
delta loop and its 2-element counterpart.  Given are the baseline and
height (one is twice the other), and the sides are about 1.414 the height.
This model had a maximum height of 60.4' which was held constant for the
2-element version to achieve comparable TO angles (17 degrees for the
model)

Antenna               Baseline          Height           Spacing
single ra delta        60.8'            30.4'             ---
2-el ra delta
  driv. el.            59.3'            29.65'
  reflector            60.6'            30.3'             20.5'

These figures are consistent with other 2-element parasitical arrays
modeled from other members of the self-contained vertically polarized wire
antenna family (SLVs), including half squares and bobtail curtains.

A 2-element wire Yagi will give slightly more gain at 17 degrees elevation
when mounted at the apex height of the delta.  However, its elevation
angle of maximum radiation is about double that of the SLV group, and the
SLVs have higher gain below the 17 degree mark, with 1/2 power points
ranging from 7-10 degrees elevation, depending on the actual antenna
height.  Hence, the choice of antenna types depends on the user's 
operating goals.

When made into a parasitical beam, the SLVs show reduced 2:1 SWR bandwidth
(relative to their resonant impedance).  At 40 meters, both SWR and
pattern begin disintegrating somewhere around +/- 50 kHz from the design
point with #12 wire. Widening that bandwidth depends upon using truly fat
wires with equivalent diameters of about 6" at 40 meters for full band
coverage with reasonable gain and F-B (arbitrarily defined here as 3 dB
gain over a single loop, >10 dB F-B, and <2:1 SWR).  Fat wires can be
roughly simulated be using 2 wires spaced twice the diameter of the wire
to be simulated, with spacers and periodic shorts across the wires.  If
scaling the measurements above, be sure to scale the wire size as well for
full band coverage--or decide to prune the antenna for narrow band
operation.

Incidentally, models of the half square and the single loop DMS (otherwise
known as a side-fed rectangle) show about a dB gain advantage over the
delta loops in this application, and this gain also transfers to
parasitical arrays of them.

I hope these notes are useful to someone.  All the usual caveats about the
limitations of modeling apply to these notes, but perhaps they are a
start.

-73-

LB, W4RNL


L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
Knoxville, Tennessee     /\   \   \ \   /  / || /      (FAX)(423) 974-3509
37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
         URL:  http://funnelweb.utcc.utk.edu/~cebik/radio.html





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