ANTENTOP- 01- 2013 # 017

Helical Antenna for the 20- meter Band

Igor Grigorov, va3znw

The 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.

Let's see on the schematic of the Antenna (Figure 1A). Antenna consists of from a vertical radiator V1, helical inductor L1, antistatic resistor R1, antenna grounding and Antenna Tuning Unit. Figure 1B allows us understand the design of the antenna.

Vertical 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 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 (item 5). 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 at: www.antentop.org/wires_017.html)

Helical Antenna for the 20- meter Band

Now let 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.

Then 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.

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