Topband: Balloon vertical antenna
k6se@juno.com
k6se@juno.com
Thu, 18 Jul 2002 09:03:29 -0700
Mac, PY2CW wrote:
"I am consideering a balloon for a 160 vertical. Questions... copperweld
wire? thickness? balloon volume to 1/2 or 5/8 wave vertical? What about
4~8 elevated radials in 30 ft tower?"
==========
For a balloon vertical you want to consider the weight of the wire. I
use 1/4 wave of #22 AWG stranded wire with a braided cloth insulation,
which is lightweight and quite strong and has no problem handling 1 kw.
Using helium, the lift of the balloon is about one ounce per cubic foot
of the gas. Hydrogen has slightly more lift capability. Weigh
everything the balloon must lift, including the wire, and insulators,
etc., and the weight of the balloon itself. For the 15 ounces my balloon
must lift, I inflate it to about 3.5 feet in diameter, which is about 23
cubic feet, and it has no problem flying the antenna.
I advise against using a 1/2-wave or 5/8-wave vertical for at least three
reasons:
1) Complexity of matching and feeding compare to 1/4-wave
2) 1/4-wave has the best 2:1 SWR bandwidth.
3) Unless the antenna is over "very good" or better soil conductivity,
1/4-wave is the best overall choice, per this table:
OVER "VERY POOR" GROUND:
VERTICAL dBi GAIN @TOA -3 dB POINTS
-------- -------- ---- ------------
1/8-WAVE -0.61 30.0 10.8 57.5
1/4-WAVE -0.98 27.6 10.1 54.1
3/8-WAVE -1.30 23.9 9.0 47.0
1/2-WAVE -1.19 19.9 7.8 38.3
5/8-WAVE 0.04 17.4 6.9 63.0
OVER "POOR" GROUND:
VERTICAL dBi GAIN @TOA -3 dB POINTS
-------- -------- ---- ------------
1/8-WAVE 0.53 27.5 9.0 55.9
1/4-WAVE 0.32 25.3 8.5 52.4
3/8-WAVE 0.16 21.9 7.7 45.3
1/2-WAVE 0.26 18.1 6.7 35.8
5/8-WAVE 0.76 14.6 5.7 29.5
OVER "GOOD" GROUND:
VERTICAL dBi GAIN @TOA -3 dB POINTS
-------- -------- ---- ------------
1/8-WAVE 1.57 24.6 7.3 54.0
1/4-WAVE 1.42 22.7 6.9 50.3
3/8-WAVE 1.31 19.6 6.3 43.1
1/2-WAVE 1.36 16.1 5.6 33.4
5/8-WAVE 1.35 12.6 4.7 24.7
OVER "VERY GOOD" GROUND:
VERTICAL dBi GAIN @TOA -3 dB POINTS
-------- -------- ---- ------------
1/8-WAVE 3.11 18.7 4.0 50.4
1/4-WAVE 3.16 17.2 3.9 46.4
3/8-WAVE 3.36 14.9 3.6 39.2
1/2-WAVE 3.31 12.3 3.3 30.2
5/8-WAVE 4.21 9.7 2.9 21.4
OVER SALT WATER:
VERTICAL dBi GAIN @TOA -3 dB POINTS
-------- -------- ---- ------------
1/8-WAVE 4.54 8.0 0.5 45.1
1/4-WAVE 4.81 7.4 0.5 40.8
3/8-WAVE 5.33 6.4 0.4 33.8
1/2-WAVE 6.26 5.4 0.4 25.8
5/8-WAVE 7.41 4.3 0.4 18.2
Even at my 160m contest portable location, where the ground conductivity
is better than salt water, I still use a 1/4-wave balloon-supported
vertical which gets out extremely well. I simply series feed the antenna
at the base with 50-ohm coax and have a 100 kHz bandwidth under 2:1 SWR.
No matter what size vertical you use, you *must* have a good ground
radial system. Elevated radials 30 feet high are only slightly better
than the same radials lying on the ground. If you have the base of the
antenna at ground level, with on-ground radials, launching, tuning, and
using the antenna will be much easier for you.
73, de Earl, K6SE