Re: [TCML] Tesla Coil + Transmitting Antenna ?

[Electronic Battle <electronicbattle@xxxxxxxxxxxxxx>]

Hello

One point to note is that the helical antenna model we're talking about
 (when we ask if the TC secondary is like an antenna) is a normal mode
helical antenna (NMHA) as opposed to an axial mode helix. In the former
case the radiation pattern is broadside firing and in the latter case it
is an end fire (i.e. the NMHA looks a bit like a monopole -
omnidirectional pattern in the azimuth plane i.e. normal to the axis -
and the axial mode squirts all of its radiation out of the end i.e.
along the axis). John Kraus was one of the joint discovering antenna
researchers of the helical antenna and hence is a useful reference source.

However, the TC secondary is so electrically small, the radiation
resistance (that part of the antenna equivalent circuit which accounts
for power lost to the far field) is swamped by the loss resistance
(conductor loss, predominantly).

The NMHA is often modelled as a short dipole in between each
loop. The turn-to-turn spacing of the TC secondary sets the
dipole length and the loop circumference is the TC secondary
circumference. At the usual TC frequencies i.e. <1MHz, typically a few
hundred kHz where wavelengths are in the region of a kilometre, you can
see that the TC will look to be very very electrically small and a poor
radiator.

So - there is a strong near field (E and H) around the TC but not much
far field (the far field's the bit that radiates away as an
electromagnetic wave, E and H fields in-phase in time, but  phase
quadrature in orientation, capable of dissipating power in a distant
load such as a radio receiver, conductor, etc.).




On 07/06/12 05:20, ED wrote:
> This configuration is usually called a 'helical antenna' and is very
> inefficient if the length is short compared to a wavelength.  That's
> true for every TC I've ever seen described.  Look up 'helical antenna'
> on Google and you should be able to find design information.  I have the
> references in a couple of handbooks but that won't help you.
> 
> miles wrote:
> 
>> To comment on the post made earlier by Matt Siri
>>
>> I have always wondered if the secondary of a tesla coil has any
>> interesting antenna properties, as well as simply acting as the high
>> voltage end in an pair of mutually coupled resonant LC circuits. I
>> believe, in the transmitting dipole/quarter antenna case, the length
>> of the secondary tube, not the length of the wire wrapped around the
>> tube, would be the interesting variable.
>>
>> Miles
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>>
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