Re: Two Meter FM Antenna Question
- From: "RST Engineering" <jim@xxxxxxxxxxxxxxxxxx>
- Date: Tue, 22 Aug 2006 11:12:43 -0700
Let's not confuse "efficiency" with "gain". Efficiency asks, "Of the power
that is incident at the feed point of the antenna, how much of that power is
radiated into free space and how much is wasted as reflected or consumed in
losses (matching or the elements themselves)?"
Gain asks, "For a receiver far distant (in terms of wavelengths) from the
antenna, which antenna produces a higher signal strength?"
In the latter, which is what I presume you meant, we have to have a
reference of some sort ... we have a fictitious impossible antenna called
"isotropic", which says that all power is radiated from a point source that
is infinitely small and infinitely efficient ... that is, all the power
incident on the point is radiated equally in all spherical directions ... a
radiating molecular seed at the center of an orange the size of Yankee
stadium.
If you measure a perfect dipole with respect to this isotropic source, you
find a "gain" perpendicular to the dipole elements of 2.14 dB. Where did
this "gain" come from, since power can not be created by a passive antenna?
If you look at the radiation pattern of a dipole, this apparent increase in
power was caused by a deep hole in the pattern off the ends of the dipole.
THe dipole, in essence, squeezed the top and the bottom to let the sides
bulge out. Think of a donut dropped over the elements and sitting at the
feed point of the dipole.
For the ground plane, think of that same donut cut in half through the fat
part of the donut. Now since our "power" is really the volume of the donut,
if you cut it in half, you are going to have to start out with a fatter
donut if you are going to wind up with the same volume. Now drop that fat
donut over the radiating element and let it come to rest on the center of
the ground plane. The dipole radiated its energy so that half of it was
"up" and half of it was "down". If "down" into the ground isn't what you
wanted, then the ground plane, which radiates all of its energy "up" at some
angle to the horizon, has more gain. By a clever bending of the ground
plane wires down at some angle to the horizon, you can move that donut
pattern down to where it is nearly horizontal. Thus, for a person at some
far distance, a properly constructed ground plane will appear to have a
stronger signal relative to a dipole.
Jim
"N3" <N3isme@xxxxxxxxx> wrote in message
news:1156112798.027258.152330@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Which one of the the two is more efficient as a radiator & why?
1/2 wave vertical fed in the center with coax or one vertical 1/4 wave
with four 1/4 wave radials also fed in the center with coax?
.
- References:
- Two Meter FM Antenna Question
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- Two Meter FM Antenna Question
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