5 July 2008
A continuation of my earlier spiral-coil-as-ground-plane test
A continuation of my earlier spiral-coil-as-ground-plane test
Test #1. 20 meters (results at bottom of page)
16.5' vertical wire and 7" diameter spiral coil:
16.5' vertical wire and 7" diameter spiral coil:
A 4-40 bolt holds the start of the coil and is also the feed point.
I was trying for a lightweight coil, so I chose aluminum wire and small
basswood sticks from Home Depot. There is 16.5 feet of 18ga aluminum
wire wound every 3/8" along the frame. Spacing of wire becomes
1/4" because of the angles. After I wound the coil, one side of
the frame broke due to weakening of the wood near the center. I
glued a
piece of plexiglass to the bottom for strength. Next time, I will use
1/2 x 1/2" wood for the frame.
Frame is 3/8" square basswood. Slots
for wire were hand cut with a miter saw every 3/8".
I made the frame larger than needed then trimmed the ends.
This makes the spacing wider than on my original 40-meter coil.
I made the frame larger than needed then trimmed the ends.
This makes the spacing wider than on my original 40-meter coil.
Small table holds FT-817, T1 tuner, and 7" coil 18" off the ground.
Cabelas 16.5' fishing pole and large PVC base
Results: I made one contact before a rainstorm came through town. On 20
meters, I worked Dave, KD7VA, in Las Vegas, NV. He was 58 to 59+ here
in Colorado. There was QSB, and we both reported S7 noise levels. Even
so, he said I was 58 at times and once peaked to S-9! As the rain began, I made some direct measurements (without tuner) on the MFJ analyzer. With the 16.5' vertical and 7" Petlowany coil, the best SWR was1.7:1, but was found at 14.980MHz, with R=78 and X=18. At 14.270MHz, the SWR was a dissapointing 6.9:1, but this only shows the need for greater than 1/4-wavelength of wire in the coils. Even so, the setup obviously worked thanks to the Elecraft TI autotuner finding a match.
Then I tried the same coil with a 12' 11" wire vertical on 17 meters. The best SWR was a nice 1.2:1, found at 15.380MHz. This shows the need to remove wire from the 20 meter coil to make it optimal for 17. By now, the rain drops were beading up on the gear and smudging the ink in my notebook, so further testing will have to wait. Also not tested was the 2:1 bandwidth.
Perhaps in the near future I'll try the setup on a backpack to see if it might efficiently eliminate the need for a dragging counterpoise when running 1/4-wave whips.
Jake, N0LX
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