[Antennas] small high gain 80m antennas

[Tony Martin W4FOA]

David,
I guess "Thor's" (TF4A) antennas seen on QRZ.COM cannot be considered 
"small" by any stretch of the imagination.  What a system he has installed. 
You might remember him as 4W6MM, etc....
Check his webpage out....we can all dream about such a setup, hi.
73
Tony, W4FOA


----- Original Message ----- 
From: "David Kelley" <dkelley at bucknell.edu>
To: <antennas at mailman.qth.net>
Sent: Tuesday, January 04, 2005 3:38 PM
Subject: Re: [Antennas] small high gain 80m antennas


> Hi, Mark
>
> Unfortunately, there is no way to build an antenna (for any frequency)
> that both is electrically small and has high gain.  The main reason is
> that gain comes from being able to concentrate the radiated energy
> into a narrow range of angles.  Yagis have higher gain than dipoles
> because they force most of the radiated energy into a main lobe.
> Many people on this reflector in the past have used the analogy of a
> round balloon.  You can cause the balloon to bulge in one direction
> but only at the expense of shrinking it in another direction.
>
> Another inescapable law of physics is that an antenna has to be
> electrically large in order to concentrate radiation.  The larger the
> antenna, the more the radiation can be concentrated.  It's kind of
> difficult to visualize why this is unless you've studied antenna theory
> for a while, but it boils down to the fact that radiation is concentrated
> by taking advantage of constructive and destructive wave interference.
> That is, waves that travel in the same direction and that are in phase
> CONstructively interfere (add together), but waves that are out of
> phase DEstructively interfere (cancel out).  Waves that are neither
> totally in phase nor out of phase partially interfere.
>
> In the case of a large antenna, the radiation that comes from the
> different parts of the antenna interfere with each other (constructively,
> destructively, or somewhere in between).  However, a significant
> fraction of a wavelength between wave sources is necessary to
> achieve constructive interference in some directions and destructive
> interference in other directions.  Consequently, in order to achieve
> high gain, you need a large antenna.  Imagine using your two hands
> to create water waves in a pool.  If your hands are close together,
> the waves rippling away from your hands will either mostly cancel
> or mostly add in all directions.  However, if you separate your
> hands so that they are several feet apart, you can cause wave
> addition in some directions and wave cancellation in others.
>
> Interestingly, just about all antennas that are roughly a 1/4 to 1/2
> wavelength in size or smaller have essentially the same gain, if
> conductor and ground losses are neglected.  This is because
> almost all small antennas have a toroidal (or half-toroidal, in the
> case of a monopole) radiation pattern.  Ground reflection effects
> usually cause some lobing of the toroidal pattern, but they don't
> affect the gain much, if gain is defined as the peak of the main
> lobe.  When losses are accounted for, some small antennas are
> found to perform better (i.e., have higher gain) than others, even
> though their radiation patterns have essentially the same shape.
> The differences in gain are due to the differences in losses
> experienced by the various designs.  That's why short monopole
> antennas are often designed with top hats and large loading
> coils.  These devices have a negligible effect on the radiation
> pattern, but a profound effect on the efficiency.
>
> 73,
> Dave NB4J
>
>
>>Hi
>>     Are there any small, discreet, high gain 80 meter antennas?
>>                     God bless,
>>                     Mark WZ0K
>>God loves you!
>
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