On Jul 25, 11:47 am, Bam Bam wrote:
I am making a simple 1/4 wave ground plane antenna and have two rather
silly question regarding its measurements.

1. I've attached an SO-239 connecter to a 5x5 inch aluminum plate.
When measuring the length of the radials, I measure from the center
conductor of the SO-239 to the end of the radial taking into account
the plate, correct?

2. If I make a loop in the vertical radiating element so that I may
hang the antenna, does the circumference of the loop count in the
measurement of the element or the diameter? (e.g., if I need 20 inches
for the element, do I measure from the bottom of the SO-239 to the top
of the loop or do I take a 20 inch wire and make a loop not caring
that it has shortened the element slightly?)

Thanks

Bam Bam

You need a grid dip meter or a good SWR bridge and get ready to tune
"a little"
here is why.... The PLATE adds capacitance and that will make the
'need' for your radial lengths to actually wind up shorter than you
would calculate. Also the loop at the top of the antenna will add a
little effective length because of the 'capacitance' of the loop (see
capacitive hats in any antenna book) so rule of thumb ... is go ahead
and use the formulas for 1/4 antenna wire, make the antenna that
length and then 'prune' to get a better match at the frequency you
plan to use. Also, you can change the angle of the radials in respect
to the plate to change the impedance of the antenna. If you lower the
radials (from straight out 90 deg) you will increase the antennas
impedance and vice versa. This is done 'last' after you get the wire
as best match after puning. Then experiment with the radials to
further match the antenna. Donot worry about getting a 1:1 match.
your time will be wasted with little improvement in performance I
found less than 2:1 is usually very good and most solid state finals
work with this well. Good Luck and have fun ... KD4Q Dean