Unusual capacitor Vertical Antenna (Compact)


This version provides enhanced performance over the 40M antenna described
in an earlier post
(Googlemoregroupsrec.radio.amateur.antennasear ch-160M small city
lot. I used the same "capacitive hat" at the top, and insulated
aluminum sheet at the bottom . This design provides essentially
constant-current to be distributed over the entire 35 feet of vertical
antenna wire (hanging from a pulley and nylon rope to a high tree branch)
on 40M, (and current in a conductor is what does the radiating!).
I modeled this theory on EZNEC thusly:
A vertical wire 0.1" diameter, 0.05' off the ground at the bottom to 35'
high:
At the top end (35'), selected 30 radials of 5' x 0.25" diameter:
At the bottom (.05'), selected 30 radials of 6' x 0.25" diameter:
Placed a Load (capacitor, Xc= -585) at 40% up the vertical wi
Placed a Source (I) at 1% up the vertical wi
Used Real/High Accuracy Ground, and 7.2 MHz.

Results showed that current in the entire 35' remained fairly constant
(plus or minus 8%), and phase varied (plus or minus 4-1/2 degrees).

The omni-directional maximum lobe was 15-30 deg at -.86 dbi which is down
about 3 or 4 db compared to a half-wave dipole at 1/2 wl high, but would
equal or exceed the dipole in some other directions.

EZNEC showed a feed impedance of 98 ohms, but in my real world
construction, it measured 1.1 to 1 SWR at 50 ohms. (The Halo loop that
I used for a capacitive hat probably added some linear length).

I had a modest size dual-space, split-stator capacitor that I placed in a
half-gallon plastic jug for weather protection. EZNEC indicated a
capacitance of 35-40 pf, but mine was considerably less than
that, probably about 20 pf. You have to run the pulley up and down to
tune the capacitor by trial and error. Setting the center frequency
seems quite critical, but after that, it offers a wide band
match, remains 1:1 to 1 over the entire 40M band.

On-the-air tests with 100 watts PEP gave very satisfactory results. I
also can run the power up to 1 Kw without my capacitor arcing over (I
connect stator to stator, and leave the rotor floating, for low loss and
High voltage spacing), and it performs Very well at 1Kw (Competes well in
pileups).

This antenna requires no ground (or radials in the usual sense).
Requires only about 4' x 8' of real estate (such as a 4x8 sytrofoam sheet
with aluminum foil).

I used RG-58 coax to feed the antenna, and looped 3 or 4 turns thru a
toroid to act as current balun, although this did not seem to be necessary.
(It made no difference in the SWR, with the coax laying on the grass).