RAJNI ELECTRONICS SERVICES

2M J- Antenna

Most popular and easily made 144 - 146 Mhz vertical gain antenna.

From pages of my old note-book on electronics circuits and ideas……

2M J-Pole Antenna  is very popular among radio amateurs across the globe. The reasons for its immense popularity are:

1. Simple to make & tune
2. Gain over conventional λ/4 whip / Ground-plane Antenna
3. Does not require ground plane for operation
4. Has higher radiation resistance & hence better efficiency
5. Has lower angle of radiation compared to λ/4 whip

To understand its functioning refer to Fig.1.

(Click on Right side lower corner of figure to expand)

J-Antenna has an over all height of about ¾ λ. The upper 1/2 λ portion can be considered as 1/2 λ antenna; the lower 1/4 λ section can be viewed as 1/4 wave section of transmission line for matching the upper 1/2 λ antenna.

We know that impedances at each end of λ/2 transmission line are very high. We also know that a λ/4 transmission line shorted at one end presents very high impedance at its open end.  Combing the two, as shown in the figure, we have a ¾ λ antenna with low input impedance at bottom most portion (shorted end) and increasing input impedance as we travel upwards towards λ/4 point.

So at a point between shorted and open ends of lower λ/4 transmission line, we can find a point where the input impedance of antenna matches to our feed line impedance of coaxial cable, say 50 ohms!

Figures on left show the details of J – Antenna I am using at my QTH at Noida, India.

SWR across the band 144 to 146 MHz is less than 1.3; about 1.1 at 145 MHz. RG-8U coaxial cable has been preferred over RG-58U for feed-line to antenna even though it is heavy & cumbersome to use. The reason being RG-58/U gives about 7.8 db loss / 100 feet at 100MHz as compared to just 2.7db for RG-8/U. I would like to save every milli-watt of RF output power from my QRP handy of 2 to 5W at 2M.

Construction is simple. Hot melt glue has been used for fixing Bakelite sheets, pipes and aluminium rods.                  

References:

1. J-pole Antenna, Radio Electronics, Feb. 93
2. A Collinear for 144 MHz, Practical Wireless,  March 90
3. 2-element Extended Collinear Antenna for 144 MHz, Practical wireless, Nov. 90
4. An End-fed extended Double Zepp for 2M,  QST, June 82

Nice Links on J – Antenna:

• A 146 MHz J-Pole Antenna
• Simple j-pole antenna for 2 meter
• A J-Pole for 2-meters and 70-cm
• The Copper cactus antenna
• 6 Meter J-Pole Antenna
• J-Pole antenna design
• Emergency Antennas
• 2 Meter J-pole from 450-Ohm Ladder Line
• J Pole calculator
• T-Hunting Antennas -
• 50MHz J-Pole Antenna
• J-Pole theory
• Easy J-pole antenna
• Wireman-J-Poles for 2m-40m
• J-pole Design Prg
• 5/8 Collinear antenna

                               VU2PPP