Antenna

An Antenna is to a two-way radio what an aerial is to a radio receiver (Hi-Fi, T.V. set etc). However, an antenna is a much more specialised piece of equipment. With an aerial almost anything metal, including a coat hanger, will do. If an antenna is not configured correctly, your radio won't transmit correctly, and could even be damaged beyond reasonable repair.

Having said that, The antenna is still just a piece of metal. This piece of metal is used to transmit the radio signal generated by the radio set. This is done, by the radio signal being emitted along the full length of the metal surface of the antenna. This is where the important bit comes in. The length of the metal bit of the antenna must match the wavelength of the radio wave being transmitted, or a fraction of it. What this means is that for our frequencies (86 Mhz), a quarter wave (i.e. 1/4 of the wavelength of the signal) antenna should be 832 mm.

The piece of metal actually needs to be a little bit longer that 832 mm as you need enough to insert into the antenna base. When SWRing the antenna, you need to be able to slide the antenna in and out of the base until the SWR is right. (Also, it's easier to take a little bit more off that it is to put some back on)

At the base of the antenna is the antenna base (No! Really?). The antenna base is used to fasten the antenna to wherever you want it putting - for a mobile radio, this is usually your vehicle. There are two points to consider when choosing an antenna base.

Firstly, the base must be compatible with the antenna. This is not usually a problem, because the antenna attachments (the little collars that the antenna whip slots into) usually have a standard sized screw thread on them. If they do not, then they may come with an adaptor. Sometimes a base is made specifically for one type of antenna and will not work with anything else. Make sure when choosing your base that it is suitable for your antenna.

Secondly, when choosing your base, make sure you know how - and where - you wish to place your antenna on your vehicle. There are a few commonly available options, and the one you choose will probably depend on how permanent you want the antenna to be, and whether you're happy taking a drill to your beloved automobile. Right over on the permanent side, is the traditional screw-in wing or roof mounted base. This involves drilling a whole into a panel of your car, through which you push the base. The base is then secured in place with a large nut screwed onto it from the under-side of the panel.

Swinging right over to the other end of the permanency scale is the 'mag-mount'. This consists if a large magnet fastened to an outer case. The outer case then has the screw socket for the antenna. The benefit of this is that you can install and remove the antenna in seconds onto any vehicle - as long as it's got a suitable metal panel. I would however, recommend placing a small piece of cloth between the vehicle's panel and the mag-mount just to help prevent scratches. The cloth should not be too thick as this could reduce the magnetic effect, and the antenna could come off while the vehicle is moving.

Smack bang in the middle we have boot-mount and gutter-mount bases. These are basically clamp style bases that are attached, usually by two screws to the edge of the boot lit in the case of the boot mount, or the roof guttering in the case of the gutter-mount. If like me you have a new shape Ford Escort, which is a hatch-back, and has no roof gutters, this makes life a little difficult. However, I have successfully attached a boot-mount base to the bonnet though.

The antenna base is connected to the radio set by a piece of co-axial cable. This is very similar to normal (T.V.) co-ax, but needs to be of a better quality. The cable needs to be 50 Ohms impedance. You don't need to know what this means, but you need to make sure that's what you ask for when you're buying it. Usually, these day when you buy an antenna base, it comes with a length of the correct co-axial cable already attached so you won't have to worry about it.

The only other thing to remember about antennas and the co-ax cable is the plug on the end of the cable. Older sets (mostly AM) use a screw on PL259 plug just like many C.B.'s, while newer sets use bayonet style push and twist BNC plug.