[Antennas] HOW GAIN WORKS

[Christopher Boone]

I don't think it was Merv who asked about the omni gain of his 
vertical...but Merv did make an incorrect statement...you CAN have gain 
and omni directional at the same time. Using his own example, you can 
easily look at GAIN of a stacked element omni antenna (be it VERTICAL or 
HORIZONTAL) by using two balloons! Put one atop the other and then 
squeeze them together and watch the sides buldge out.....if you look at 
the balloons as the RF emitted from the antenna elements, you can see 
how squeezing it causes the signal to buldge in certain directions...but 
loses it on others (where you are squeezing it)....this is how a omni 
stacked antenna gets gain (be it stacked dipoles like a DB224 or DB420, 
a vertical colinear or stacked horizontal elements..IE dipoles....or 
stacked omni horiz anennas like a big wheel, etc).

A directional antenna does NOT work in this matter as there are 
reflectors and directors involved and thus, the flashlight example is 
much more obvious here.

As Mark said, an antenna cannot get something for nothing....when you 
GAIN in one direction, you LOSE in another....this is why vertical gain 
antennas do not radiate well directly below or above them. In broadcast, 
they use "Null Fill" so some signal is radiated close in below the 
horizon to allow listeners below the antenna site (especially when the 
xmtr is on a mountain and the city is close by!) to HEAR the station :) 
Otherwise, think of an umbrella....the antenna would be at the tip top 
of the umbrella and the antenna pattern looks like the umbrella 
itself...those close in and below the "umbrella" coverage get POOR 
reception because most of the signal is looking OUT toward the horizon.
Here null fill comes in handy....electrical beam tilt (usually 2-5 
degress down) lowers the signal AT the horizon all around (this cuts 
down on interference at greater distances and puts more signal into 
receivers that are not out on the fringe)....but does NOT really help 
close in.

A lot of times I run a simple 1/4 on 2, 220 and UHF FM because it works 
better in the city for rptr svc...now if ALL I did was simplex or out on 
the road, a gain antenna would be better...


Chris
WB5ITT
FCC Commercial PG-9-5322

AA6DX wrote:

> Well, Merv, your colinear aerial is CERTAINLY directional.... that is how it
> gets its gain!   There is NO WAY AN ANTENNA CAN GET SOMETHING FOR NOTHING!
> However, unlike the flashlight example, it is more like the ceiling fixture
> in your home, with the white shade behind the bare bulb ... the elements of
> your vertical antenna creates "gain" by taking away from the upward/downward
> components, and "squishing" the signal towards the horizon, if you were
> communicating with a station up overhead on a mountain, VS a dipole, it
> would (most likely!) suffer a decrease in signal strength received, and ...
> conversely, you would not receive that transmitter as well as you would with
> a dipole, or 1/4 wave vertical, as examples.  That is the reason why some
> ham, and commercial, stations employ "tilt" ... to enhance the signal to the
> "down the hill" station(s)....
> BTW, "tilt" works for verticals, AND other types.. Yagis, Quads, etc....
> AND mobiles ... which is why, if you operate in a mountainous area from your
> vehicle, it is sometimes wiser to use a 1/4 wave rather than a 5/8 wave or
> moreso gain antenna  --- you go up and down the hills, and the "GAIN"
> actually becomes a detriment!
> And.. the 7dB gain may .. just may ... be questionable... I have both a
> (claimed) 6 dB and a 7 dB gain 2M vertical, darn if I can see a difference!
> Seriously enjoy these chats, it is great to share info ... been hammin' for
> 4� years now, still learning something most every day!
> 73, y'all, Mark  AA6DX
> 
> From: <w8okn@charter.net>
> To: "AA6DX" <aa6dx@pacbell.net>; "Merv Stump" <mstump@hvc.rr.com>; "'Demetre
> Koumanakos'" <demetre@fastmail.ca>; <antennas@mailman.qth.net>
> Sent: Thursday, April 22, 2004 5:21 PM
> Subject: Re: [Antennas] HOW GAIN WORKS
> 
> 
> 
>>Then why does my vertical have a gain of 7 db?  It is not directional!
> 
> It's a colinear.  W8OKN
> 
>>

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