2m vertical antenna build

Following are directions to construct a 2 meter (144-148 MHz)1/4-wave groundplane antenna. Not only is this a very useful and effective antenna, but it is also quite instructive. Concepts taught include resonant frequency, input impedance, soldering, hand assembly, antenna analyzer use. Credit to Ken KG6T for taking careful notes, photos, and providing measurement details for this build process. Please see the pictures below the text.

Material

1. Coax connector, SO-239 (1 ea)

2. Attachment hardware, 4-40: machine screws (3/8 in), flat washers, nuts (all 4 ea)

3. Solid wire, insulated, #14 VERTICAL ELEMENT initially cut to 21 inches (1 ea)

4. Solid wire, insulated, #14 GROUNDPLANE ELEMENTS initially cut to 22 inches (4 ea)

5. Solder

Tools

1. Tooling block - to hold SO-239 and wire (see picture below)

2. Soldering gun

3. Safety glasses

4. Antenna analyzer

5. RG-8/x coax assembly -- 10 feet long should be sufficient to connect antenna analyzer to antenna

6. Wrench -- to tighten 4-40 nut

7. Screwdriver

SOLDER VERTICAL ELEMENT TO SO-239

1. While the tooling block is not necessary, it's very convenient and a nice way to demonstrate how simple tooling can greatly improve the ease of soldering a joint. Please reference picture below of a example tooling block.

2. Insert SO-239 into tooling block.

3. Strip ~1/4 inch of insulation off vertical element wire

4. Insert wire through tooling block hole and insert into SO-239 connector.

5. Solder.

install GROUND PLANE ELEMENTS

1. Strip ~3/4 inch of insulation off one groundplane element wire.

2. Bend the wire into a loop, clockwise to the end of the wire, as viewed from above the antenna.

3. Install the wire onto the SO-239 using 4-40 hardware. Note: if the wire's loop is installed in the proper orientation, tightening the screw will tend to close the loop. Please see picture below.

TUNE ANTENNA

1. Attach coax sufficiently long so no one is close enough to the antenna to affect its performance. The K7UAZ club has a sprinkler pipe in the ceiling to which we attach lightweight cord. The bottom of the antenna is at about 7 ft above the floor, and the coax is about 10 ft long.

2. Bend the ground plane elements down at a 45-degree angle to horizontal.

3. Using an antenna analyzer, measure the SWR from 120 to 150 MHz. This range encompasses the 2m band with a lot of extra room below it. This will allow you to observe the resonant frequency so that you can slowly move it into the 2m band by reducing the length of the vertical element.

4. Trim ONLY the center element (unless as noted below and you want to experiment). Make measurements after shortening the vertical element. Keep trimming until the resonant frequency is at your frequency of choice. We use 0.5 inch iterations, gradually approaching the resonant frequency. Here is one set of measurements from a recent build:

Trim # Resonant Freq (MHz)

0 122

1 126

2 128

3 132

4 135

5 135 -- trimmed the groundplane elements by 0.5 inch, but didn't affect the resonance

6 137

7 145

EXPERIMENT WITH INPUT IMPEDANCE

1. Now it's instructive (and fun!) experiment a bit with your freshly-tuned antenna. Bend the ground plane elements back up to a horizontal plane and measure the performance again using the antenna analyzer. Did the SWR change? Did the resonant frequency change? This is a good point for group discussion.