Hi TTians,
Recently, there was a spate of discussion on the TowerTalk
reflector about the test methodology used in the triband HF
antenna comparison done by Ward Silver (N0AX) and Steve Morris
(K7LXC). Many valid points were raised in the discussion. Some
relevant to the actual tests and some not. One that I thought
might be relevant was the "the patterns are all different so you
can't directly compare them near ground" argument. I thought it
might be useful to see what NEC2 could tell us about any relative
dipole comparison differences due to differing ground effects on
differing patterns. NEC2 handles horizontal antennas not very
near ground quite well in my experience. So I was motivated to
do a "comparison of models" test to attempt to get a sense of the
magnitude and direction of the effect.
Basically, I modeled three different antennas over three
different types of ground at two different heights. The antenna
gains ranged from 7.39 to 8.97 dBi. The F/R ratios ranged from
25 dB to 9 dB. The half power beamwidths ranged from 80 to 112
degrees in elevation and 57 to 66 degrees in azimuth. One was a
Log Periodic and two were three element Yagis with 26 foot booms.
I evaluated the gain at 0.1 degree TOA which was approximately 40
dB down in the ground effects null. The reference antenna was a
resonant dipole of identical construction to the DE of the Yagis.
I evaluated the gain of each antenna relative to that dipole in
the same environment.
The reason I chose the antennas I did for the initial study was
beacuse they represented something close to what _should_
theoretically be possible for a three element tribander on a
similar boom length to the largest antennas in the report. Only
one of the antennas in Ward and Steve's initial study had a boom
length longer than this.
Here is what I found:
1. At the lower height (33.75 feet) over real ground (both
"good" and "very poor" from the EZNEC selection) there is
indeed a slight gain or pattern related skew in reported dBd
numbers for the various antennas. The skew does in fact
slightly penalize the higher gain antennas. However, the
magnitude of this effect is quite small. It amounted to a
maximum of 0.19 dB across the entire range of gains studied.
See Table 2 below.
2. At the upper height (50.00 feet) over real ground (both
"good" and "very poor" from the EZNEC selection) There is no
detectable gain or pattern related skew in the reported dBd
numbers. See Table 1 below.
3. Amazingly, the dBd numbers found by doing the comparison at
low takeoff angles over real ground were _VERY_ close to
those obtained by doing the comparison in free space. In
fact, the worst case difference between free space dBd and
ground efects dBd was only -0.35 dB !! At the 50 foot
height, the difference over "good" ground was only -0.04 dB.
And the difference over "very poor" ground was only -0.02
dB!! See Table 2 below.
4. The maximum error in F/R ratio was a bit over 3 dB. This
magnitude of error was seen only on antennas that had high
F/R ratios in free space. The gain optimized antennas with 9
dB F/R showed changes in the +/- 1 dB (usually less) range.
It is clear that NEC2 using the high accuracy ground model in
EZNEC V2 doesn't think that the dipole comparison will be
compromised by differing ground effects of antennas with
significantly different patterns. It shows no effect whatsoever
so long as the antennas are operated at 0.7 wavelengths or more
above real ground.
Table 1. Relative differences due to "Ground Effects" on dipole
comparison gains for differing pattern characteristics
33.75 Foot Height | 50.00 Foot Height
------- --------------------- | ---------------------
Antenna => A B C | A B C
------- ----- ----- ----- | ----- ----- -----
G | |
R | |
O | Perfect -0.02 0.00 -0.05 | 0.00 -0.03 -0.01
U | |
N | |
D | "Good" 0.00 -0.05 -0.18 | 0.00 0.00 0.00
| |
T | |
Y |"Very Poor" 0.00 -0.06 -0.19 | 0.00 0.00 0.00
P | |
E | |
Table 2. Difference between dBd from Free Space comparison and
dBd from 0.1 degree takeoff angle comparison
Antenna "A" (Free Space Gain = 5.28 dBd, F/R = 24.61 dB)
Height Perfect Good Very Poor
(feet) Ground Ground Ground
------ ------ ------ ------
33.75 -0.09 -0.19 -0.16
50.00 -0.19 -0.04 -0.02
Antenna "B" (Free Space Gain = 6.09 dBd, F/R = 25.20 dB)
Height Perfect Good Very Poor
(feet) Ground Ground Ground
------ ------ ------ ------
33.75 -0.07 -0.24 -0.22
50.00 -0.22 -0.04 -0.02
Antenna "C" (Free Space Gain = 6.86 dBd, F/R = 9.18 dB)
Height Perfect Good Very Poor
(feet) Ground Ground Ground
------ ------ ------ ------
33.75 -0.12 -0.37 -0.35
50.00 -0.20 -0.04 -0.02
The small magnitude of the effect causes me to want to add more
antennas to the study. One is a higher gain (9.41 bBi - about all
you can get from 3 elements on a 26 foot boom at 14 HMz) lower
F/R antenna and the other is a much lower gain type. I think
I'll make a 3 or 4 dBd two element model. I may also want to add
a stacked pair with their phase center at the test heights. If I
do this, I'll report the findings here.
I'm attaching the results of the modeling data below so that
those who want to pick through it and find all my errors may do
so. I seem to have a mild case of calculator dyslexia these
days.
73, Eric N7CL
------------------------------
Antenna Modeling Data
Reference Antenna (Dipole)
Free Space
Gain Delta Delta
(dBi) (FS) (opTOA)
----- ----- -------
2.11 N/A N/A
33.75 Ft, Perfect GND, TOA = 31 degrees
Gain Delta Delta
(dBi) (FS) (opTOA)
----- ----- -------
8.10 5.99 N/A
33.765 Ft., Perfect GND TOA = 0.1 degree
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
-37.48 -39.59 -45.58
33.75 Ft., "Good" GND, TOA = 29 degrees
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
7.16 5.05 N/A
33.75 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
-37.14 -39.25 -42.19
33.75 Ft., "Very Poor" GND, TOA = 28 degrees
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
6.41 4.3 N/A
33.75 Ft., Very Poor GND, TOA = 0.1 degree
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
-37.06 39.17 -43.47
50.00 Ft., Perfect GND, TOA = 21 degrees
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
8.38 6.27 N/A
50.0 Ft., "Perfect" GND, TOA = 0.1 degree
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
-33.77 -35.88 -42.15
50.00 Ft., "Good" GND, TOA = 20 degrees
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
7.45 5.34 N/A
50.00 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
-33.82 -35.93 -41.27
50.00 Ft., "Very Poor" GND, TOA = 19 degrees
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
6.88 4.77 N/A
50.00 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta
(dBi) (FS) (opTOA)
------ ------ -------
-33.80 -35.91 -40.68
Antenna "A" (9 element Log Periodic)
Free Space Gain = 7.39 dBi (5.28 dBd), F/R = 24.61 dB
Vertical Beamwidth = 112.8 Degrees
Horizontal Beramwidth = 66 Degrees
33.75 Ft., Perfect GND, TOA = 28 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
12.46 5.07 N/A N/A 4.36
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
19.02 -5.59 N/A
33.75 Ft., Perfect GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-32.29 -40.68 -44.75 5.19 -40.39
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
19.97 -4.64 0.95
33.75 Ft., "Good" GND, TOA = 27 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
11.56 4.17 N/A N/A 4.40
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
22.53 -2.08 N/A
33.75 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-32.05 -39.44 -43.61 5.09 -39.21
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
24.42 -0.19 1.89
33.75 Ft., "Very Poor" GND, TOA = 26 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
10.90 3.51 N/A N/A 4.49
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
21.42 -3.19 N/A
33.75 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-31.94 -39.33 -42.84 5.12 -38.35
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
28.05 3.44 6.63
50.00 Ft., Perfect GND, TOA = 20 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.15 5.76 N/A N/A 5.05
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
22.73 -1.88 N/A
50.00 Ft., Perfect GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-28.68 -36.07 -41.83 5.09 -36.78
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
22.77 -1.84 -0.04
50.00 Ft., "Good" GND, TOA = 19 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
12.39 5.00 N/A N/A 4.94
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
22.33 -2.28 N/A
50.00 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-28.58 -35.97 -40.97 5.24 -36.03
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
23.09 -1.52 0.76
50.00 Ft., "Very Poor" GND, TOA = 18 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
11.85 4.46 N/A N/A 4.97
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
22.93 -1.68 N/A
50.00 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-28.54 -35.93 -40.39 5.26 -35.42
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
23.35 -1.26 0.42
Antenna "B" (3 element Yagi on 26 foot boom optimized for F/R
performance)
Free Space Gain = 8.20 dBi (6.09 dBd), F/R = 25.20 dB
Vertical Beamwidth = 98.2 Degrees
Horizontal Beamwidth = 62.6 Degrees
33.75 Ft., Perfect GND, TOA = 28 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.05 4.85 N/A N/A 4.85
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
27.69 2.49 N/A
33.75 Ft., Perfect GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-31.46 -39.66 -44.51 6.02 -39.56
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
21.63 -3.57 -6.33
33.75 Ft., "Good" GND, TOA = 26 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
12.12 3.92 N/A N/A 4.02
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
25.92 0.72 N/A
33.75 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-31.29 -39.49 -43.41 5.85 -38.45
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
21.64 -3.56 -4.28
33.75 Ft., "Very Poor" GND, TOA = 25 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
11.47 3.27 N/A N/A 5.06
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
26.05 0.85 N/A
33.75 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-31.19 -39.39 -42.66 5.87 -37.60
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
22.17 -3.03 -3.88
50.00 Ft., "Perfect" GND, TOA = 20 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.82 5.62 N/A N/A 5.44
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
21.55 -3.65 N/A
50.00 Ft., "Perfect" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-27.90 -36.10 -41.72 5.87 -36.28
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
23.28 -1.92 1.73
50.00 Ft., "Good" GND, TOA = 19 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.10 4.90 N/A N/A 5.65
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
23.70 -1.50 N/A
50.00 Ft., "Good" GND, TOA = 0.1 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-27.77 -35.97 -40.87 6.05 -35.22
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
24.72 -0.48 1.02
50.00 Ft., "Very Poor" GND, TOA = 18 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
12.56 4.36 N/A N/A 5.68
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
24.71 -0.49 N/A
50.00 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-27.73 -35.93 -40.29 6.07 -34.61
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
25.52 0.32 0.81
Antenna "C" (3 element Yagi on 26 foot boom (not fully) optimized
for Gain performance)
Free Space Gain = 8.97 dBi (6.86 dBd), F/R = 9.18 dB
Vertical Beamwidth = 80.6 Degrees
Horizontal Beamwidth = 57.2 Degrees
33.75 Ft., "Perfect" GND, TOA = 26 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.37 4.40 N/A N/A 5.27
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
10.97 1.79 N/A
33.75 Ft., "Perfect" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-30.74 -39.71 -44.11 6.74 -38.84
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
10.2 1.02 -0.77
33.75 Ft., "Good" GND, TOA = 25 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
12.39 3.42 N/A N/A 5.23
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
10.29 1.11 N/A
33.75 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-30.65 -39.62 -43.04 6.49 -37.81
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
9.69 0.51 -0.60
33.75 Ft., "Very Poor" GND, TOA = 24 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
11.77 2.8 N/A N/A 5.36
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
10.01 0.83 N/A
33.75 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-30.55 -39.52 -42.32 6.51 -36.96
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
9.47 0.29 -0.54
50.00 Ft., "Perfect" GND, TOA = 19 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
14.39 5.42 N/A N/A 6.01
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
9.04 -0.14 N/A
50.00 Ft., "Perfect" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-27.11 -36.08 -41.50 6.66 -35.49
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
8.78 -0.40 -0.26
50.00 Ft., "Good" GND, TOA = 18 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.68 4.71 N/A N/A 5.30
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
9.15 -0.03 N/A
50.00 Ft., "Good" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-27.00 -35.97 -40.68 6.82 -34.45
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
8.89 -0.29 -0.26
50.00 Ft., "Very Poor" GND, TOA = 18 degrees
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
13.15 4.18 N/A N/A 6.27
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
9.25 0.07 N/A
50.00 Ft., "Very Poor" GND, TOA = 0.1 degree
Gain Delta Delta Delta Ref Delta Ref
(dBi) (FS) (opTOA) (opTOA)
------ ------ ------- -------- ---------
-26.96 -35.93 -40.11 6.84 -33.84
F/R Delta Delta
(dB) (FS) (opTOA)
------ ----- -------
8.98 -0.20 -0.27
-----------------------------------
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