Half-Wave Dipole Coax
May 28, 2009 by Admin
Filed under Transmission Lines and Cabling
Most times, you will need a length of coaxial cable to run between your two-way radio set and its transmit antenna, such as a half wave dipole or a perhaps a vertical quarter wave antenna. These two types of antenna are easy to make or buy, and will match well to commonly available coax cable.
A quarter-wave ground-plane vertical antenna has a typical impedance of 30 to 50 Ohms. And a half-wave dipole wire antenna, hung horizontally and fed in the center has an impedence close to 70 Ohms.
Most times, all you need to do is connect the antenna to a length of standard 50 Ohm coax cable that is designed for use with a radio transmitter.
A common type is RG-213 for VHF or UHF frequencies or for long runs of more than 30 feet or so. The thinner and cheaper RG-58U cable is used for short runs in a car or truck, or moderate distances on the HF bands between 3 MHz and 30 MHz.
The thin RG-58 has a higher loss than the thicker and higher-quality RG-213 coax or the older, now-obsolete RG-8 coaxial cable. Make sure you buy good quality coax that was made for transmitters, and not cheap stuff from Tandy/Radio Shack, or coax for old style computer networks. It won’t have thick enough braid to shield the signal properly.
On the radio end of the cable, your transceiver will commonly have a PL-259 connector, and that fits an SO-239 plug. But other plug types are possible as well, such as ‘N’ connectors, the bayonet-twist BNC connectors and even the new tiny SMA connectors that are popular with modern handheld walkie talkie radios.
Do make sure you have connectors that match. Strip the insulation off the ends the cables and cut the inner conductor and the outer braid to the lengths you need to fit the plug or socket. You will need to solder the parts together, as per the manufacturer’s recommendations, or use a set of crimping pliers designed for that exact plug and cable. (They are expensive, specialist workshop tools.) Most hobbyists have to use a soldering iron or a soldering gun to get the job done.
If you happen to live in a country where getting 50 Ohm coax is very difficult, do not fret. You can feed a dipole antenna very nicely with TV coax, which is typically 75 Ohm at radio frequencies. This actually makes television coax cable a better match for the halfwave dipole with its nominal 70 Ohms impedence. But yes, there is a catch…
You will find that television cable is a different diameter (thickness) than the 50 Ohm 2-way radio coax cable types. TV cable is thicker than RG-58 and thinner than RG0213, so it doesn’t fit their plugs very well. It doesn’t mean you can’t make it work. You cartainly can. Just be prepared for quite a bit of extra fiddling around to get it all set up right.
Before you connect the coax for your antenna to the radio set and try to transmit, even at its lowest power setting, make doubly sure there is no short-circuit or open circuit between the radio aerial and the radio.
Check continuity with a cheap pocket Ohm meter from the plug center conductor to one side of the antenna, and from the outer part of the plug, down the braid / shield of the coax to the other side of the dipole. It should read less than 2 Ohms or so. A closed circuit. Then check the outer part of the connector plug with the inner tip. It should show an infinite (or very high) Ohms value… that’s an open circuit.
As long as you get these readings, all is well. If not, check your work carefully to find your error and correct it. then text and measure again. Finally, connect the two way radio and test with a VSWR meter or an antenna analyzer. Use minimum power, until you are certain your SWR is less than 2.0 to 1.
And if you are in doubt at any time, don’t be afraid to ask someone with more experience to give you a little help or advice. We all had to start somewhere.
