[Antennas] small high gain 80m antennas (Correction)

[Tony Martin W4FOA]

Sorry.  Thor's new callsign is TF4M (not TF4A).
Tony
----- Original Message ----- 
From: "Tony Martin W4FOA" <w4foa at comcast.net>
To: <antennas at mailman.qth.net>; "David Kelley" <dkelley at bucknell.edu>
Sent: Tuesday, January 04, 2005 5:28 PM
Subject: Re: [Antennas] small high gain 80m antennas


> 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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