described below Helical Antenna for the 20- meter band has small
sizes, as well it is very easy to tune and moreover the antenna
has high performance at the band.
see on the schematic of the Antenna (Figure 1A). Antenna consists of from a vertical
radiator V1, helical inductor L1, antistatic resistor R1, antenna
Antenna Tuning Unit. Figure 1B allows us understand the design of the
radiator V1 (item
1) is an automobile whip antenna in length 1.2- meter.
It was bought by me on a sale in Canadian Tire for $6. Helical
Inductor L1 is wound on a plastic form (item 3). The plastic form is a water tube in 5-feet
length and diameter 1-1/2 inch. I bought it in Canadian Tire for
$7. †Inductor L1 is wound by electrical copper wire in diameter 18- AWG in strong
black insulation (33-cent/m, Home Depot is supplier). The wire
has initial length 10- meter. Coils spread uniformly on the form.
A plastic water bottle (item
2) with cut bottom is inserted (through a hole in the
cap: the hole should be a little smaller the diameter of the whip)
on to the whip antenna. The bottle protected the water pipe from
water and dust leaking inside in. Antenna wire is fixed on the
tube (at several places) with help of a Vinyl Electrical Tape.†
I used to a special outdoor tape with temperature range
Ė 35 + 90 C. The tape was bought by me in Sayal Electronics for $2
for a reel.
The water pipe (item 3) was installed on to a surplus microphone
stand (item 4). The microphone stand (I bought it in
Sayal Electronics for $10) has heavy metal circle base with
a thick vertical pin. On to the pin the water pipe is inserted.
On to the water pipe is installed an RF socket (SO- 239) (item 6) and antistatic resistor
Copper strip (item
8) is installed on to the water pipe. The copper strip
is connected with iron microphone stand and with antenna grounding.
Antenna grounding includes 10 burred counterpoises.
The counterpoises surrounded the antenna similar to the figure
star. Each counterpoise (3- meter long) consists of two wires.
There is copper wire in strong insulation (similar to antenna
wire) and bared stainless steel wire in diameter 19- AWG (I bought
it in Home Depot). †(Antenna grounding system also is described
Helical Antenna for the 20- meter
letís me say some words about tuning and matching of the antenna.
When antenna was installed it should be tuned and matched for
the 20- meter Band. The matching is very easy. At first you need
to know the resonance frequency of the antenna and the antenna
input impedance. It may be measured with help of an antenna analyzer
or SWR- meter. The resonance frequency is measured across resistor
R1. I used to a MFJ- 259B to find the resonance frequency. In
my case I have got the resonance at 13900- kHz.
when near a 70- cm length of wire from the helical inductor was
removed the resonance frequency of the antenna became 14.100-
kHz. In my case the input impedance of the antenna on the resonance
frequency was 8- Ohm.