Saturday, Jan 27, 2007 at 10:42
Hi
John
I use a half wave dipole which I mount as an “inverted vee”. This is the antenna:
en.wikipedia.org/wiki/Dipole_antenna
“Inverted vee” simply means it uses only one high point for mounting ie. the centre of the antenna where the co-ax meets the arms, like this:
www.hamuniverse.com/invertedveelw.html
They are very simple to make; you need a length of co-ax, I suggest decent 50 ohm RG58 cable (NOT Jaycar – their RG58 is rubbish!) with a suitable plug for your radio, a piece of strong plastic about 125mm x 50mm (a chopping board from the $2
shop is ideal) and two lengths of any old (strong enough) wire for the arms. The arm length is calculated by the formula “length (in feet) = 468 / frequency in Mhz” so for 8.022MHz the arms would be 58’ 4” long in total or 29’ 2” each arm.
For 5.455MHz each arm would need to be 45’ 10” long so you could make a basic dipole for 8.022MHz at 29’ 2” and add an extra piece of wire 16’ 8” to each arm when you wanted to use 5.455MHz.
In use the higher you can get the antenna the better, I usually try for about 10m at the apex but they will work
well even when quite low, iirc I have used
mine at about 4m.
The ends of the arms carry a high RF voltage when the antenna is used for transmitting so it is _important_ to ensure they cannot be touched by anyone.
The most expensive part is the co-ax – shops which sell by the meter usually charge about 5 or 10 times their cost price – I can buy a 100m drum of quality RG58 for $60 so it may be better to do that?
Feel free to ask if that’s not clear – I can post pics. if required?
Mike Harding
mike_harding@fastmail.fm
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