1/4 wave ground plane antenna.
This relatively easy project is ideal for SWLs and licensed.
John M0UKD has blessed us with a most useful hobby radio website via link below.
My favourite offering is his 1/4 wave ground plane antenna calculator. Enter the frequency you require and the element lengths are displayed along with construction details.
I've made one and it works fine for me on the GB3VH repeater. My element lengths are: Driven element A = 16.5 cm ground plane elements = 17.5 cm. Feeder RG58C/U.
The radiating element is a quarter wave and the radials are 12 per cent longer. There are usually four radials.
These antennas can easily be built for UHF or above by using a chassis mount N-Type (or SO-239) connector, some solid wire and solder. For VHF and below, as the elements get bigger, some more structured design is needed.
A quarter wave monopole mounted against a perfect ground will have an impedance of around 36 degrees but by bending the radials down at an angle of 45 degrees, we increase this to around 50 Ohm whilst at the same time lowering the radiation angle more towards the horizon. (42 degrees is the theoretical perfect angle.)
73 Bob G4PVB
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