Rick,
You got me interested, so I fired up EZNEC and tried analyzing an antenna
such as you describe.
I modeled it as having four sides, each 66 feet long, and 30 feet above a
"medium" ground (conductivity .005 ms, relative dielectric constant 13).
When I drove it in the center of one of the sides:
The pattern on 80 meters looks pretty much like a beach ball with the
maximum radiation straight up. The gain at 90 degrees elevation is 6 dBi.
At 3.6 mHz it's drive point impedance is 82.5 - j187 ohms. It is nearly
resonant at 3.81 mHz where its impedance is 97.3 +j1.7 ohms.
The pattern on 40 meters, at 7.1 mHz, looks sort of like a four leaf clover.
The maximum radiation is at about 44 degrees where it peaks at 5.3 and 5.9
dBi in the leaves. At 90 degrees take off angle (straight up), the pattern
is down to -5.3 dBi. The source impedance is 216 -j339 ohms. (The
inductive reactance suggests it is too short to be resonant at 7.1 mHz. I
didn't fool with the length.)
The pattern on 20 meters, at 14.1 mHz, looks like a flower with 6 petals.
There is a relatively deep node off the ends of the driven wire. At 0
degrees azmuth (off the end of the driven wire) the pattern peaks at 59
degrees elevation where it is -.87 dBi. The maximum radiation is at about
40 degrees azmuth and 29 degrees elevation where it is 7.8 dBi. The source
impedance is 471 -j936 ohms.
When I drove it almost at a corner (6 inches off the corner was as close as
I could get):
The pattern on 80 meters looks again like a beach ball with a maximum
straight up at 90 degrees of 5.9 dBi. Its source impedance is 82 - j191
ohms at 3.6 mHz.
On 40 meters the pattern looks like a ball of clay with maxima more or less
off the two corners of the square adjacent to the corner being fed. At 52
degrees elevation which appears to be about the optimum "take-off angle,"
the gain is 5.5 dBi at 52 degrees azmuth (the maximum), 4.1 dBi at 163
degrees (a relative minimum), 5.4 dBi at 217 degrees (a second maximum), and
1.7 dBi at 298 degrees (another relative minimum). The 7.1 mHz source
impedance is 93.5 -j477 ohms.
I strongly suspect if I lengthened the antenna a bit to get rid of the
inductive reactance, it would be a decent match to 75 ohm coax on both 80
and 40 if driven at a corner.
On 20 meters the pattern looks like a flower with 4 petals. The largest one
is off the corner of the array opposite the driven corner. The max
radiation there is 9.8 dBi at 31 degrees elevation. There are three other
relative maxima at this elevation, 8 dBi at 43 degrees and 223 degrees and 7
dBi at 313 degrees. There are minima at the 31 degree elevation: -6.2 dBi
at 86 degrees; -7.9 dBi at 181 degrees; -10.5 dBi at 266 degrees; and -11.3
dBi at 1 degree. The source impedance at 14.1 mHz is 454 -j765 ohms.
In comparison, a 130 foot center fed dipole at 3.6 mHz, 30 feet high, has
8.8 dBi of "gain" peaking at 90 degrees elevation with maxima broadside to
the wire, also a "cloud burner." At 7.1 mHz it has a peak of 8.5 dBi again
at 90 degrees with maxima broadside to the wire. At 14.1 mHz it has a peak
of 7.8 dBi at an elevation of 31 degrees and at azmuth angles of 42, 138,
222, and 318 degrees. There is a sharp null off the ends of the wire on 20
and broadside to the null, at 90 degrees, the gain is 0,5 dBi.
The quad loop is a very interesting antenna!
Jim, W8KGI
Rick Westerman wrote:
(Re. Horizontal Loops).
>
> Typically, I had the loop 10m up (30 ft.) and it was a full
> wavelength for 80m. I usually fed it with open wire to enable
> multiple band usage. Occasionally I built one for either 40 or
> 80m and then fed it with 50 Ohm coax without any sort of matching
> at all.
>
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