Ian Jackson wrote:
In message ,
writes
Ian Jackson wrote:
In message ,
writes
The ideal radial length for ANY ground plane antenna is slightly longer
than 1/4 wavelength, no matter for what frequencey.
Why is this? I would have thought that a 1/4 wave would be best, as it
offers the lowest impedance.
First you have to define what "best" means.
Yebbut ........
You've just said "the ideal radial length for ANY ground plane antenna
is slightly longer than 1/4 wavelength, no matter for what frequency". I
assumed that "ideal" = "best".
.
All antennas are a trade off for impedance, bandwidth, gain and in most
cases physical ability to build the structure.
Changing the radial length will have a small effect on impdedance and resonant
point but changing the radial angle will have a bigger effect on impedance
and a very small effect on resonant point.
True - but what's the angle of the radials got to do with their length?
I would suggest downloading the demo version of EZNEC and modeling a GP to
see what small changes in various parameters do.
I had presumed you had already do this (or something similar) in order
to say that slightly longer than a 1/4 wavelength was ideal. However, I
have always assumed that the steeper the angle of the radials, the more
the groundplane becomes like a vertical halfwave dipole - and the lower
becomes the angle of radiation.
OK, let's look at some numbers and see what is actually happening.
First, design a GP for 28.3 Mhz, 1/2 inch 6061 aluminum tubing with all
elements the same length and look at the element length, impedance and gain
in free space.
Then change the radial droop to 30 degrees and 45 degrees, retune for 28.3
and look at the numbers again.
All lengths are free space wavelengths of the driven element.
droop impedance gain length SWR
0 deg 23.6 Ohms 1.34 dBi .247884 lambda 2.12
30 deg 41.6 Ohms 1.83 dBi .238687 lambda 1.2
45 deg 49.1 Ohms 2.2 dBi .234493 lambda 1.02
OK, now repeat with the radials 5% longer than the driven element.
droop impedance gain length SWR
0 deg 23.3 Ohms 1.29 dBi .245373 lambda 2.15
30 deg 41.3 Ohms 1.81 dBi .236106 lambda 1.18
45 deg 50.4 Ohms 2.19 dBi .232007 lambda 1.0011
From the above the best SWR occurs with radial 5% longer than the driven
element and the droop at 45 degrees.
This is also the point of maximum 50 Ohm bandwidth. I will leave it as an
execise for the reader to get the demo EZNEC and view the bandwidth graphs.
In all cases the elevation angle of maximum radiation is 0 degrees.
Now let's come down from free space and put the longer radial version on a
typical single story house roof mounted on a pole.
The house peaks around here are about 13 feet and 10 foot TV masts are cheap,
so let's mount the antenna at 23 feet, which is .662 lambda at 28.3 Mhz, and
see what happens.
Note than because we are now over real ground vertical lobes are formed.
Again I will leave it as an exercise for the reader to get the demo EZNEC
and view the graphs.
droop impedance max gain length SWR
0 deg 22.6 Ohms 2.48 dBi @ 35 deg .245373 lambda 2.12
30 deg 43.4 Ohms 2.24 dBi @ 40 deg .236269 lambda 1.15
45 deg 51.1 Ohms 1.94 dBi @ 45 deg .231667 lambda 1.022
It should be noted that there is a large second lobe:
0 deg 1.09 dBi @ 12.5 deg
30 deg 1.37 dBi @ 12.5 deg
45 deg 1.66 dBi @ 12.5 deg
So which antenna is "best" in the real world?
I would go for 5% longer radials drooping at 45 degress.
--
Jim Pennino