Dave,
Your data are quite interesting. Can you tell me what the
configuration of the other elements was while you were measuring
the self impedance of each one? Were they open circuited,
grounded, removed from the array, terminated in a load, or
connected to the phasing harness?
With your excellent ground system, it is extremely unlikely that
local ground conditions are the entire reason for the high
feedpoint resistances. Particularly on 40 meters where it
extends far enough to cover virtually all of the near field zone.
Please keep us updated as you proceed.
73, Eric N7CL
>From: dbowker@shire.sjv.net (Dave Bowker)
To: <antennaware@contesting.com>
>Date: Wed, 30 Sep 1998 01:57:36 -0400
>
>Reference Gary Breeds note of 23 September.
>
>The following is actual data which may be of interest to the
>discussion relating to 1/2 or 5/8 wavelength radials versus
>1/4-wavelength raidators.
>
>I recently completed Phase-1 of a 2-phase project to build a
>dual-band 4-element cardioid array (similar to the classic
>4-square except for the phasing and direction of firing).
>Phase-1 provides basic 40M operation and phase-2 will add the 80M
>capabilities next spring. The primary objective of this project
>is to collect actual data on the typical 1/4-wave spaced array
>and on 1/8-wave spaced arrays (which this array will become on
>80M).
>
>The array presently consists of four self-supporting 1/4-wave
>elements of identical physical length and cut to be as close to
>self-resonant at 7.010 as possible and spaced 1/4-wave per side
>on 40M. The radial system consists of 120 1/2-wave radials, each
>70-feet long under under each element. The radials are layed on
>top of the sod by cutting grass short, laying in the wire, and
>fertilizing and watering to restore the grass. The radials
>extending outward from each element and from the geometrical
>center of the array are full-length 1/2-wave long. Where radials
>extend inwards towards the geometrical center of the array and
>where radials of one element would "cross" those of another
>element, they were terminated in a common bus which crosses
>through the geometrical center of the array (typical of broadcast
>directional array installations...see Jasik chapter 20 and papers
>by Smith Electronics, circa '70's).
>
>Estimated soil constants are for very poor ground, as soil
>content consists of approximately 2 feet of sandy loam over
>gravel and shale base river bed.
>
>The array is positioned on 1-acre of clear land with tall popular
>and spruce trees on one side (the side adjacent to elements 2 and
>4) of the array and spaced approximately 3/4-wavelength from it.
>Nearest structure is 1-wavelength distant (at 40M).
>
>Measurements of self-impedance and coupled impedances were made
>using a GR-916A RF impedance bridge with calibration traceable to
>NIST. The self-impedances measured at 7.010 MHz were as follows:
>Z11 = 45 + j4.64, Z22 = 43 + j5.0, Z33 = 45 + j6, Z44 = 43 +
>j3.3.
>
>Note that these self-impedances are not the classical 36 +j0 (38
>+ j0 if one were to include the typically accepted loss
>resistance of 2 Ohms for such a radial system over average
>ground...see Brown, Lewis, Epstein). I have not thoroughly
>investigated and analyzed all the factors which may be causing
>these self-impedances to be considerably higher, however, I
>believe the ground characteristics are certainly a factor. At
>the time this data was taken, the soil was extremely dry and the
>grass had not yet over grown the radials. I will obtain another
>data point in next few weeks as the radials are now completely
>overgrown and soil is moist down through the loam top layer.
>
>As a point of interest, I extended each of the self-supporting
>elements to be resonant at 4.6 MHz (approximately 53 feet) and
>took self-impedance data as follows (same radial system, same
>soil conditions). At 4.6 MHz the radials are approximately 0.31
>wavelengths. The following self-impedances were measured. Z11 =
>43 + j1, Z22 = 46 + j0, Z33 = 44 + j0, Z44 = 45 + j1.5.
>
>Comparing the self-impedance's of the 1/4-wave radiators over
>1/2-wave radials at 7 MHz to the 1/4-wave radiators over
>0.31-wave radials at 4.6 MHz shows little difference in the
>measured data.
>
>Measured performance data of the array on 40M is 21 dB F/B and
>3.7 dB gain (as compared to a single resonant element), based
>upon actual FS data taken at ~1/2-mile distance from one face of
>the array (using calibrated HP attenuators - not S-meters!).
>Front/side is 4.9 dB down from main lobe (obtained by switching
>direction of array with measurement equipment at same location as
>for F/B and gain measurements)
>
>Photos of the array, radial system installation, element
>mechanical design and construction, phasing network, measurement
>setup, and data will be available to subscribers via email in a
>month or so (I'll post to reflector when available).
>
>73, Dave, K1FK
>Fort Kent, ME
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