> A question for the antenna experts.
>
> I'd like to know what happens when you go from a half-wavelength to a
> quarter-wavelength antenna, while trying to maintain the same output power
> from the receiver's POV.
>
> In other words, let's say that to compensate for shorter antenna length, I
> swap out my 1W transmitter with a 2W one. Would that even work? And if it
> would, what happens to the extra power -- does it get converted to heat
> inside the antenna, or does poor SWR cause the energy to be lost inside
> the
> output stage?

A bit late to the discussion, but I thought I'd make a couple comments.

To me, the geometry of an antenna has two major effects. These are the
driving point impedance and the directional characteristics. As another
poster noted, antenna are often rated in dBi or decibels above what an
isotropic antenna would do. The gain above isotropic is possible by giving
up some radiation in one direction and getting more radiation in another
direction. A half wave dipole has gain above an isotropic since it does
not radiate off the ends. As the length of an antenna varies, its
radiation pattern varies.

As mentioned above, a half wave dipole does not radiate off the ends. If
you drive the antenna at its center, it has an impedance of something like
70+j0 ohms. If you turn the antenna vertically and still drive it at the
center, it is now omnidirectional horizontally, but directional vertically
(no radiation straight up). You can replace the bottom half of the
vertical dipole with a ground plane. This is the common 1/4 wave ground
plane antenna that is used for broadcast applications. The driving point
is about 35+j0 ohms.

As the antenna gets shorter, the vertical radiation pattern changes and
the real part of the driving point impedance goes down while the reactive
part goes negative. You can compensate for the negative reactance by
adding an inductor in series, but the low driving point resistance becomes
a problem. As the driving point resistance decreases, other loss
resistance (ground losses, matching network losses) become significant.
You get more power lost (converted to heat) in these loss resistances.

Harold
WA6FDN
Former broadcast engineer




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