Homebrew a quarter-wave vertical antenna
September 24, 2009 by Admin
Filed under Radio Antennas
You can build a vertical quarter-wave antenna like this in your own back yard.
A quarter-wave antenna is the standard base-station antenna for many radio base stations, mobile transceivers and even for many VHF and UHF portable handheld radios.
On the MF and HF bands a quarter-wave antenna would be much too large to be portable. The MF = 300 KHz to 3 MHz needs antennas hundreds of feet long, while HF = 3 to 30 MHz has quarter-wave antennas that range from more than 70 feet in length for 3 MHz shortwave to about 8 feet in length for 10 meter band VHF.
The formula in feet for a quarter-wave antenna is 234/freq in MHz. Therefore, if we look at the Amateur Radio bands, our standard quarter-wave vertical antennas would be:
- 160m = 1.9 MHz = 129.5 feet
- 80m = 3.6 MHz = 68.3 feet
- 75m = 3.98 MHz = 61.8 feet
- 60m = MHz = 5.3 MHz = 44.15 feet
- 40m = 7.15 MHz = 34.45 feet
- 30m = 10.1 MHz = 24.4 feet
- 20m = 14.2 MHz = 17.3 feet
- 17m = 18.12 MHz = 13.58 feet
- 15m = 21.2 MHz = 11.6 feet
- 12m = 24.95 MHz = 9.86 feet
- 11m (CB) = 27.175 = 8.61 feet
- 10m = 29 MHz = 8.48 feet
- 6m = 52 MHz = 4.72 feet
- 2m = 146 MHz = 1.68 feet or (20 inches)
Note that the lower the frequency, the shorter the vertical antenna can be. It is easy to hand-hold a quarter-wave antenna for the 2m VHF ham band (144 to 148 MHz), and it is possible to have a full quarter-wave whip antenna on your mobile CB radio in a vehicle – that’s an 8.6 foot steel or fiberglass whip. But then the antennas start to get really tall! Ham and other HF radio operators get around this problem by using loading coils (inductors) in their antennas to make the transmitter see a 50-Ohm match, and many base-station antennas for frequencies lower than 14 MHz also use loading coils. The coils can be placed anywhere on the antenna, however having the coil at the top is most efficient. (It is also technically more dificult.) Helical loading (having evenly-spaced turns along the whole length of the antenna, is an excellent way of managing the loading and is very popular with CB antennas for 27 MHz.
This homebrew 1/4-wave vertical antenna needs to have three or more radial wires to make it work properly. At a pinch, one radial will work, but four or even more is best. The lengths of these radials (sometimes called counterpoise wires) should be each be 1/4 wave length also. Then you need to have a length of 50-ohm coaxial cable (such as RG-213 or RG-58U) between the antenna and your two-way radio. At the antenna end, you should solder or screw the center conductor to the base of the vertical antenna, and the braided coaxial shield wire should be soldered or connected to all of the radial wires. The radials can also be used to keep your antenna from falling over!
Be sure to weatherize the outdoor antenna connections with silicone rubber cement or other waterproofing agent. If you forget to do this, rain will get into the coaxial cable and corrode it on the inside. This will wildly affect your SWR readings and will harm your radio signal. The end of the cable that reaches your radio transceiver (CB or otherwise) will usually connct to a UHF plug (PL-259 or similar). Get the type that gets soldered on, unless you have access to a professional crimping tool. A poor connection here will forever be giving you problems, so get someone’s help if you need to. And check with an Ohm-meter for short-circuits or open circuits (broken wires) before you switch on the radio to transmit!
Now, in case you are interested, I can show you how to build a Homebrew Half-Wave Dipole Antenna. Just add two or more trees or posts as handy supports to hold it up!
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