uhf antenna length

Hello obee,

Try about 150mm for a 1/4 wave, you can buy the so cheap now around $5 to $10.

In a wire antenna a 1/4 wave is a good size, if you go a 1/2 wave about 600mm you may have some problems radiating the power.

Regards Richard

The actual length will vary with each installation and the only way to check is with a swr meter, dip meter or antenna analyzer, if you cant lay your hands on the equipment then the theoretical length will be ok.

A dip meter is used for antenna radiating patterns only and will not show the antenna swr.

Regards Richard

Gday Obee
My wire antenna is 1250mm from nob to nut......

An antenna length of 140-150 mm is a good length - but this has to be measured from the feed point, not the top of the insulator. If you have the HF/VHF type base (the ones with the solder lug visable on the underneath side) then the length is measured from the sheet metal that the base is mounted through. If you have the UHF type base (the one where the co-axial cable disapears up into a hole in the bottom) or the above ground type, then the feed point is up in the insulator - at the top of the braid of the co-ax cable. The lengths I have quoted are less than the theoretical 1/4 wave, the thicker the antenna material the shorter its length will be compared to the free space calculated length.

A half wave stick would be 280-300 mm long, not 600 mm as stated above. The impedance (RF resistance) of an antenna varies along its length. It is extremely high at the open end. It reduces as you progress down its length. It reduces to around 35 ohms at the quarter wave feed point. If you lengthen the antenna the impedance again increases until at the half wave length the feed point impedance will be nearly as high as at the tip. Increase the length to 3/4 wave and the feed point impedance will again be low. This keeps going ad-infinitum.

The reason we should use the UHF bases for our UHF antennas is that the above ground feed point transforms the 35 ohm impedance to the 50 ohms necessary to match the impedance of the cable. They also withstand the weather better in exposed conditions like when on a bracket on a bull bar.

For those who want to know a little more – read on.

The problem with using a simple wire antenna longer than a quarter wave is the radiation lobe splits into two lobes and the longer the antenna the more the acute the angle is to the direction of the wire. We don't want our radiation going up in the air do we??? You will note that the longer antennas that have more gain than a 1/4 wave one, have decoupling coils mounted about every 1/2 wave length along them. These are some times known as decoupling coils and are there to phase the radiation of each 1/2 wave length so that their combined effect adds in the horizontal direction that we desire our signal to go. The outer elements are electrically ½ wave length long and the one at the feed point end is shorter (somewhere near ¼ wave) to transform the high impedance of the ½ wave elements at the top to the 50 ohm cable.

PeterD