Re: NEC-LIST: More on the ground stake effect.

At 12:27 PM -0800 12/10/03, D. B. Miron wrote:
>...I conclude that having a ground stake might be a good idea
>for lightning protection, but it should be rf-decoupled from
>the antenna.

I'm sure the answer is different if your "ground" medium is seawater,
but in my experience with New England so-called soil, a ground stake
is another name for a dummy load. Much better is an above-ground
counterpoise, no matter how humble.

The following little report of a few NEC-4 simulations that I wrote
for my Wireless-Set-No19 group
<http://groups.yahoo.com/group/Wireless-Set-No19/> illustrates how
much better a counterpoise of only three or four thin (18-ga.) wires
works than a hefty (2 m long, 1 cm diameter) ground stake.

-Chuck, W1HIS

BTW: The Wireless Set No. 19 was a low-power HF transceiver used
extensively by the Allied armies in WW II. (It also included a VHF
transceiver and an intercom.)

--------------------------------------------------

Subject: Efficiencies of traditional WS 19 antennas

_Wireless_for_the_Warrior_ vol. 2 describes various antenna
configurations used with the WS 19. I have modeled several of these
configurations using the Numerical Electromagnetics Code (NEC-4D),
for the purpose of comparing their radiation efficiencies.

I assumed ground parameters typical of New England, where I live:
dielectric constant epsilon/epsilonzero = 5 and conductivity sigma =
0.001 S/m. I calculated the radiation field of each antenna at
elevation angles of 10 and 30 degrees above the horizon. For
reference, I also calculated the radiation field of a 34-foot (10.3
m) vertical antenna on perfectly electrically conducting (PEC)
ground. All calculations were done for a frequency of f = 3.7 MHz.

I neglected impedance-matching, transmission-line, and wire losses.
Of these losses, only impedance-matching is important in practice.
This loss can be substantial, e.g., more than 10 dB for a whip
antenna on a tank, because both the radiation resistance and the
ground loss resistance of this antenna are much much smaller than the
effective source resistance of a WS 19, and a 19 Set has no means for
impedance transformation. (The WS 19 variometer is not a
transformer; it is only a series inductor whose positive reactance
cancels the negative reactance of a short antenna.)

The WS 19 antenna configurations that I considered were:

"AFV" -- a 12-foot vertical whip on a conducting box 6 m long, 3 m wide,
and 2 m high, crudely representing a large armored fighting vehicle.
The bottom of this box was 0.5 m above the ground. (Yet TBD is the
effect of conductive contact with the ground, as from tank treads.)

"Ground Station" [Wireless for the Warrior, vol. 2, page W.S.19 (Cdn)-30,
Figure C19-42] -- a 34-foot (10.3 m) high, 0.8-inch (2 cm) diameter,
vertical antenna base-fed with respect to a 2-m long, 0.4-inch (1 cm)
diameter "ground spike".

"Trench" -- a 25-foot horizontal insulated wire laid on the ground, toward
the desired direction or radiation. I used the 2-m in-ground spike
with this antenna.

"Long-range" [Wireless for the Warrior, vol. 2, page W.S.19 (Cdn)-41,
Figure 19-57 and Table 4] -- a 45-ft. wire going vertically 25 ft., then
horizontally 20 ft., fed against a counterpoise. I assumed the
counterpoise to be a fan of three 20-ft. wires centered below the 20-ft.
horizontal portion of the elevated antenna and fanning + and -15 deg, one
foot above ground.

The calculated radiation field strengths are given in the following table
in deciBels with respect to the reference antenna (34-ft. vertical with
PEC).

                | Elevation angle |
Antenna config.| 10 deg | 30 deg | Comment
---------------+----------+----------+--------------------------------------
AFV | -15.6 | -10.7 | The AFV body is a good counterpoise.
Ground Station | -24.1 | -19.1 | A ground spike is a rotten "ground".
Trench | -33.9 | -27.8 | 10 dB worse than the 34-ft. vert.
Long-range | -17.6 | -12.7 | Almost as good as the AFV. Probably
                                         would have been better if I'd beefed
                                         up the counterpoise.
Notes:

1. The gain at 10 deg is consistently 5 dB worse than at 30 deg elevation.

2. I did not calculate ground-wave fields (although I could have easily).
However, I recall from calculations I've done for MW broadcast
antennas, that comparing sky-wave field-strengths at 10 deg elevation was
equivalent to comparing ground-wave field-strengths.

3. Although I didn't see it mentioned in Wireless for the Warrior, I
thought that the 34-ft. "ground station" mast working against a radial wire
counterpoise (rather than a ground spike) might be fairly efficient, so I
modeled this configuration, assuming four radial wires of 18 gauge, 20 m
long, 90 deg apart. I'm aware that many more radials are required for
really good efficiency, but I wanted to be practical. The radials were one
foot above ground. The results:

                | Elevation angle |
Antenna config.| 10 deg | 30 deg | Comment
---------------+----------+----------+--------------------------------------
Ground station | | |
mast w/ radials| -12.0 | -7.0 | Best of all, so far.

4. For comparison I also modeled a center-fed 22-m horizontal wire (a
"doublet") 8 m above ground:

                | Elevation angle |
Antenna config.| 10 deg | 30 deg | Comment
---------------+----------+----------+--------------------------------------
Doublet | -13.5 | -4.3 | Best yet, at 30 deg elevation,
                                         but horizontally polarized. (Good
                                         for skywave, but not for talking to
                                         tanks via ground-wave.)

Note that the difference between 10 and 30 deg elevation here is 9 dB --
breaking the "5 dB" rule mentioned above. In other words, the gain
of this horizontal antenna drops faster as elevation goes to zero.

Conclusion:

For low HF such as 3.7 MHz, if you need a vertically polarized antenna (for
ground-wave), then use an above-ground counterpoise rather than a ground
spike. A tank or large truck body is a good counterpoise, but as few as
three or four wires close to ground level are good and possibly better.

73 de Chuck W1HIS

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