Antennas
Antennas can be constructed for use on PMR446. Quite often 70CM Amateur antennas are reasonant enough to work on PMR446 with a low SWR.
This is because the 70CM band is very close in frequency to the PMR446 band, thus having a similar wavelength (67 CM for PMR446, 70CM for 433).However a purpose built antenna may potentially perform better than a 70cm antenna as well as being a fun project to learn more about antenna designs
PMR446 Wavelength and characteristics
- The wavelength of 446.09375 (Channel 8) is 67.2039 centimeters (Wavelength = Speed of Light / Frequency ) So 299792458/ 446093750 = 0.67203913527 Meters
- An SWR meter is necessary to ensure the antenna is reasonant.
- Use high quality coaxial cabling between the antenna and radio, RG-213 will suffice for UHF frequencies, RG-8 is unsuitable.
- Always cut antenna elements slightly larger than instructed, this is to allow more leeway when testing and trimming elements to maximum efficency.
- A multimeter can be used to check for continuity (contact) between two metal points on an antenna.
Quarter wave (1/4) Omnidirectional Antenna
This is a basic antenna design that can be constructed quite simply from spare parts, A quarter wave antenna is as the name suggests, tuned to the quarter length of the wavelength.
To construct a simple quarter-wave antenna, firstly measure five pieces of metal rod to 17 cm, four of these will become the ground plane elements and one will be the main antenna element.
The next step is to use a thin sheet of aliminium or similar metal sheeting to cut a small 5x5 cm square, this will act as the metal groundplane to attach the 4 ground elements to.
Cut a hole inside the metal plate and solder and attach the SO239 connector. Make sure the outer chassis of the connector is making contact with the metal plate to ensure contact to the elements we shall attach.
Use a hot glue gun to attach the the 4 elements making reference to the diagram above, make sure they are pointing downwards at a 45 degree angle.
Insert and solder the main antenna element into the direct pin of the SO239 connector, be sure that it does not make contact with the ground chassis, it is advisable to use hot glue to help prevent shorts.
Optional
Feed a RG213 coax cable to the connector, then take a PCV pipe and feed it around the cabling.