Re: Antenna physical size & shape

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Gary Weaver wrote:

> The 8 element is physically larger than a 4 element and it is
> larger than a 2 element.   Reception and power output are both
> better with the physically larger antenna.

What's happening is that the extra elements are concentrating the
outgoing radiation in particular directions, so the power 'gain'
in some directions is increased at the expense of others.

> My next thought is, an amateur radio antenna like 80 meters, 40
> meters, 20 meters, 2 meters do not physically radiate visible
> discharge sparks like a Tesla coil.   WHY?

Well, they do sometimes - I used to get blue flames coming off
the driven element of a 2m beam.   But it's not really meant to
do that!  You want the antenna power to be radiated efficiently
as waves in the EM field, and not spent tearing up the air
molecules around the antenna.  So antennas are designed not to
give sparks (not easy in some applications, eg high power radars).

> If the amateur radio antenna were to be retuned would it radiate
> sparks that could be seen just like a tesla coil?

If mis-tuned, faulty, or like mine, driven beyond it's limits.
But you wouldn't get TC style streamers, just a corona.

> If so then would it still transmit a signal several 100 miles
> to a receiver?

It would continue to transmit, but not so well because some of
the power is diverted into the sparks.

> What if an amateur radio antenna were made of a toroid shape or
> sphere shape with smooth round surfaces that would not have
> premature discharges would it transmit better?

Sometimes high power antennas have features like spheres and
toroids to limit the E-field at the 'hot' ends of radiating
elements, etc.  They are trying to hold off the wasteful corona
discharge.  But if the antenna consisted solely of a TC-sized
toroid or sphere, it wouldn't transmit too well, except at VHF
where it do about as well as a dipole.   To get any sort of
reasonable radiating efficiency, you need the electrode to
be a decent fraction of a wavelength, say getting on for a
quarter wave, in size.  To get any sort of directivity, you
usually need to have more than one electrode (element), set
apart by another decent fraction of a wavelength.

--
Paul Nicholson
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