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Tx line vs lumped parameter
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From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Saturday, June 06, 1998 11:01 AM
To: Tesla List
Subject: Re: Tx line vs lumped parameter
>
>
> I agree with your transmission line comments to a point, but we need to
> remember antennas such as helical antenna, and also UHF resonator
> structures. These are both cases where the e-m wavelength is much longer
> than the biggest dimension of the helix in question.
For a helical antenna, the circumference of the helix is typically about a
wavelength, providing you want it to work in the "end fire" mode. (Ref:
Kraus, Antennas, etc.) If the wavelength is >> than the physical
dimensions, it is a "short dipole or monopole" and essentially an isotropic
radiator.
>
> These comments are in support of the ideas originally suggested by Terry
> Fritz and Dave Sharpe in last weeks' long post. If a magnetic field is
> coupling into lots of the secondary instead of only into the bottom turn
> (let's say), then the secondary won't look particularly "distributed" to
the
> primary.
Is this not why the diameter of a flat primary should be comparable to the
height of the secondary?
By the way, the primary acts not only as a source of magnetic flux, but
also as the "other" plate of the capacitor formed by the secondary and
ground. Has anyone ever tried putting a grounded plane (say a sheet of
copper) between the secondary and the primary? (You would have to cut a
slit in the copper to avoid circulating currents induced by the primary).
The magnetic flux goes right through the copper.
If you look at a delay line coax, they wind the center conductor in a
solenoid, to increase the L per unit length, which not only increases the Z
(to around 1000 ohms), but also decreases the propagation speed.