I think it really boils down to 'what is the definition of directivity'?
Witness Vertical antennsa used in the commerical world...they may have
10-15db of gain...as a so called OMNI directional antenna...but the gain
comes out of depressing the antenna pattern so that the majority of the
major lobe has a 'footprint' that makes the radiated energy 'usable'...and
of course the 'gain' is ...as measured against some 'standard' radiating
element...1/4 wave, dipole, etc..
Marty
W1MD
-----Original Message-----
From: Barry Kirkwood [mailto:bjk@ihug.co.nz]
Sent: Thursday, March 14, 2002 11:54 AM
To: Gary Schafer; W8JI@contesting.com
Cc: Steve Thompson; Amps reflector
Subject: Re: [Amps] Antenna vs. amp (was: al-1200 question)
True,
but the converse is not true,
You can have directivity independent of gain as witness loop and other
directional receiving antennas.
Or yagis with lossy elements...
73
end
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240
NEW ZEALAND
www.waiheke.co.nz/signal.htm
----- Original Message -----
From: Gary Schafer <garyschafer@attbi.com>
To: <W8JI@contesting.com>
Cc: Steve Thompson <g8gsq@qsl.net>; Amps reflector <amps@contesting.com>
Sent: Thursday, March 14, 2002 3:19 PM
Subject: Re: [Amps] Antenna vs. amp (was: al-1200 question)
>
>
> Tom Rauch wrote:
>
> > > If both antennas have the same gain they will have the same
> > > directional properties.
> >
> > No, that is not true.
> >
> > The antennas can and often do have vastly different directional
> > properties even with the same gain.
> > 73, Tom W8JI
> > W8JI@contesting.com
>
> I guess I should have qualified that a little better. I did say that one
> may have a sharper vertical pattern and the other a sharper horizontal
> pattern. What I meant by that over simplification was that for a given
> amount of gain in one direction, gain has to be reduced in another
> direction.
>
> By the "same directional properties" I meant that they will have the
> same total beam width exposure to the sky. (not necessarily in the same
> direction) This total beam width is the sum of both vertical and
> horizontal beams.
>
> One antenna may be a vertical collinear type with 360 degree horizontal
> pattern but it's vertical pattern will be very narrow. Another antenna
> may be a bi directional vertically stacked dipole array. It will have 2
> major horizontal lobes and a very narrow vertical pattern. A yagi of
> course has a 1 major horizontal lobe in one direction and a suppressed
> vertical pattern.
>
> If you add up all of the area of the sky that each antenna beam is
> exposed to due to each ones horizontal and vertical beam widths, I think
> you will find that given the same gain for each they will also have
> equal total exposure area.
>
> In other words you can not have gain without without having a
> directional antenna.
>
> 73
> Gary K4FMX
>
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>
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