John,
Gain is derived from the simplest of antennas...the dipole
or if you want to take the professional route, an imaginary
antenna called the isotropic antenna which radiates likes a
soccer ball....equal in all directions...
Since such an antenna does NOT exist, we use the simplest we can
actually model...a dipole...of course a dipole radiates likes
a figure eight when viewed from the top (with the dipole horiz)..
A VERTICAL dipole radiates like a doughnut....to get gain, you merely squeeze
the doughnut down to put MORE at the horizon...
With a HORIZ antenna, its similar but you have to think in the opposite
plane....take a doughnut and turn it on a side...now you have the pattern of a
HORIZ dipole....note how it radiates
360 AROUND the dipole... Like a O...thats if you look at it on
the end of the dipole...from the side it looks more like a figure 8 but really
its the doughnut cut in half...(two circles..
one on top and on bottom of the horiz plane of the dipole)..
NOW if you SQUEEZED THOSE Os down, you put more signal at the
horizon (which is the goal!)....SO to stack another dipole at the right spacing
would do that....if the dipole was bent into a
loop or halo, the same thing would happen...the pattern off the
halo or other omni horiz antenna looks like a DOUGHNUT laying on the table
looking from the side...put another doughnut above it
and then imagine waves coming from both adding and subtracting to
cause more signal at the horizon...(hope this illustrates it easily)..the same
thing can be seen if you squeezed the doughnut down as we did above...more
doughnut goes to the outside and less above and below are there....this is the
same principle of a
horiz omni antenna getting gain (the theory IS the same as vertical gain
antennas...the radiation planes are merely swapped)
TV and radio FM stations have been using this technique for decades for high
gain horiz OMNI antennas....problem is most horiz antennas do not lend
themselves to easy mounting where the
pole doesnt get in the way of the pattern and the "omni" is now skewed
slightly....(the Batwing and End Fire Helix antennas are two of the few
antennas that doesnt suffer from this)...
Yagis have gain because of focusing the beam of a single dipole (the driven
element) into a certain direction...
Omni antennas get gain because of the squeezing of the radiation
below and above the horizon toward the horizon to increase signal strength
there...but with a penalty...signals close in suffer from signal LOSS because
of overshoot (think of an umbrella in this case and how it works...with the
umbrella being
the actual radiation....directly below the center of the pattern, there is
little signal compared to out at the horizon)..
Some omni horiz antennas get around this by using a feature called downtilt
where some of the signal is intentionally skewed DOWN to fill in the weak areas
close in...but the gain at the horizon will also suffer slightly because of
this...but its all a compromise (you dont get something for nothing! :)
Hope that explains it in less than 500 words!! Antenna theory is
all math and can get pretty complex...but its also fun to work with...and
experiment!
Chris
WB5ITT
BTW a stacked loop of say 4 stacked elements will do as much gain as a 4-5
element yagi....a YAGI DOES have the advantadge of not just having GAIN but
also NULLING out or rejecting unwanted signals from other
directions...something an OMNI antenna cannot do....and that can cause rcv
problems, etc....but for those wanting to work local SSB/CW/AM work on VHF/UHF,
the HORIZONTAL omni antenna is the best way to not miss any action..for DX
work, a YAGI is better....(best to have both at home so you can monitor on the
omni then switch to the YAGI to get through when conditions are not good).
-------Original Message-------
From: John Geiger <johngeig@yahoo.com>
Sent: 04/16/03 10:46 PM
To: vhfcontesting@contesting.com, vhf@w6yx.stanford.edu
Subject: Gain from an omnidirectional antenna
>
> I have an antenna question for the reflector. On the
local repeater here, the general consensus is "Get a
horizontal loop for VHF, they will outperform yagis".
(I know better, but hey, I just report what I hear).
So here is my question-I have heard gain figures
attributed to loops, or to stacked loops, but I don't
understand how. If an antenna is truely
omnidirectional, where does the gain come from?
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