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Radio Frequency Antenna Types

Amateur radio antenna construction

Imagine you sprained you ankle while bush walking and you can't make it back to the car by yourself. You are way out of cellphone range, of course, but you did remember to bring along a small VHF handheld Amateur Radio with you. You turn it on and select the frequency of a local Amateur Radio repeater, but the tiny antenna is too small to get into it. So, in true MacGyver-style, you strip the wire out of an AC adapter cable that you found in your back pack. You get three sticks and lash them together in the shape of a cross with your shoe laces. You cut five strands of wire to about half a meter long - about the length of your arm - and wrap them around the ends of the sticks. You unplug the handheld radio antenna and poke the wire ends into it. Trying the repeater again, you now get a good signal from it and can call for help. You can't believe how much better that makeshift antenna worked! An hour later your mates arrive to help you back to the car. "No problem. I had everything under control", you say to them, but actually you are thinking: "Maybe next time I'll bring along a friend".

Introduction

Radios need antennas to transmit and receive signals. Most antennas are just made of wire or metal rods, but their length, shape and orientation is critical to their performance. Knowing how to build antennas is a useful skill and it will certainly help you set up a great radio station of your own.

Preparation

You will need:

  1. VHF transceiver, preferably handheld.
  2. Multimeter (optional)
  3. SWR meter (optional)
  4. RG-58 coaxial cable terminated with a mating connector for your SWR meter or radio
  5. RG-58 coaxial patch cable (optional)
  6. Two hardwood batons, 90cm x 12mm x 12mm
  7. One hardwood baton 3m x 12mm x 12mm
  8. Predrilled holes in all batons
  9. Eight small brass wood screws

Activity

VHF Ground Plane Antenna

  1. Attach two 90cm metre batons 75cm from the to the top of the mast using brass wood screws.
  2. Cut the radiator and radial wires to length and bare the ends. Bare the coax wire ends too.
  3. Insert the bare wire ends into the screw holes and fix them with brass wood screws.
  4. The centre conductor connects to the vertical radiator. The braid connects to the four radials.
  5. Optionally test all the connections between the coaxial connector and the wire ends with a multimeter.
  6. Connect the VHF transceiver to the antenna via the optional patch cable and SWR meter.
  7. Prop the antenna up on the back of a chair.
  8. Caution: Do come within 30cm of the antenna or touch it when transmitting.
  9. Test the antenna: Does it work better than the small antenna on the handheld? Is the SWR low?

Homework

  1. Learn to identify different types of antennas: TV antennas, AM broadcast antennas, Cellphone antennas, Microwave dishes, CB antennas, mobile radio base station antennas, satellite dishes, WiFi antennas and, of course, Amateur Radio antennas.
  2. When you see a new type of antenna ask someone: "What is that antenna for?". You will often be surprised that no-one has actually ever noticed it.
  3. When travelling in the car play "spot the antenna" instead of "eye-spy"