Ron D'Eau Claire wrote:
> We simply do not have conductors that will handle RF with
> anything like the efficiency they will handle DC or low frequency
> AC. That's because all the RF current 'crowds' onto the very
> surface of a conductor.
When Skin Effect losses become an issue in RF conductors and
inductors, Litz wire is typically used. I see no reason why a
similar approach cannot be used (one of using multiple conductors
tied in parallel with each insulated from one another).
An "El Cheapo" approach might be to use multi-conductor cable
instead of a single, fat conductor to minimize Skin Effect losses.
Cookie, K5EWJ wrote:
> The capacitive reactance will require 397 microhenrys
> to cancel out which in turn will require a coil 4
> inches in diameter and about 24 inches long with 160
> turns (about 168 feet of wire). Then you would need
> an 8333/1 balun transformer with its associated wire
> resistance.
Capacitance hats can be used to reduce the amount of
inductive loading required to establish resonance.
I agree that the impedance transformations appear astronomical,
but if performed in manageable steps (instead of a single
transformation), efficiency might be improved, and the process
might not seem so formidable or lossy.
David Woolley wrote:
> For antennas in practical locations, at least one other thing
> will happen:
>
> d) The antenna will convert part of the AC power into (near field)
> electromagnetic energy which will induce currents in the ground,
> building structure, wiring, water and gas pipes, etc. Much of that
> energy will be converted to heat after it has lost, although some
> will be re-radiated (I believe, in extreme cases, if the current
> is induced in something large enough and resonant, the re-radiator
> can become the actual antenna and the antenna act as a feed device,
> but, normally, for low, and indoor antennas, this is where most of
> the energy turns to heat).
I fully agree that the surrounding environment is part of the antenna
SYSTEM, and should be taken into consideration regardless of whether
the antenna is half-wave or an electrically shortened version of a
half-wave antenna.
> This reflection abstraction causes a lot of confusion. It is
> possibly easier to see it as simply a bad match between the
> transmitter source impedance (which is usually rather different
> from the optimum load impedance) and the antenna impedance,
> causing most of the DC input to the transmitter to end up as
> heat in the output devices.
Provided measures to re-reflect the energy back to the antenna in
phase with the incident power aren't made. (And the device that
does this very nicely is the antenna tuner.)
Actually, I mentioned reflected power to dispell the untrue but
widely held belief that reflected power "cancels" forward power
on a transmission line. Forward and reflected power can peacefully
co-exist on a transmission line without interaction. If this were
not true, repeater systems using duplexed antenna systems would
be impossible.
> As already pointed out, skin effect means that this is not true.
> People experimenting with small magnetic loops have to use large
> copper pipes to keep ohmic losses manageable.
The problem (in my mind) is that the copper pipe is really only a
single conductor, rather than the more efficient approach taken by
Litz wire where multiple conductors, each a skin depth in diameter,
are operated in parallel.
> For example, the KAT2 has a 10:1 SWR matching specification, but
> matching the antenna discussed here, at infinite height above the
> ground, needs a 250:1 range, or more. They can also have power
> losses.
Agreed. The impedance transformation has to (in my opinion) be
step-by-step process -- the same as a modern receiver design.
(In a receiver, we don't try to get all our gain and selectivity
in a single stage. By the same token, we shouldn't try to make
a large impedance transformation in a single "stage", either.)
(Just my opinion...)
> > The penalties for using physically shortened antennas are:
> >
> > (a) Decreased operating bandwidth
>
> I'm not sure that is inevitably true. My reference for normal mode
> helices included them in the section on broadband antennas.
I believe the Normal Mode Helix, as is used in HI-VHF television
broadcasting, is a travelling wave antenna, rather than a
resonant standing wave antenna, such as a dipole.
The bandwidth decreases in shortened resonant antennas as a
result of the antenna's increased Q. The Q rises because the
RF energy must oscillate back and forth between feedpoint and
endpoint of the dipole many more times than it does in a
full-sized antenna before all the energy applied to the antenna
is finally radiated into space.
In travelling wave (non-resonant) antennas (rhombics, Beverages),
the RF energy travels down the length of the antenna just once.
Therefore, they must be very long in terms of wavelength to radiate
all the energy into space on the first shot.
Incidentally, I'm not advocating that everyone should cut their
antennas to 1/40th of their natural resonant length and compensate
for the shortened antenna using LC networks.
What I AM saying is that there are approaches that can be taken to
minimize losses and increase the performance of shortened antennas,
and these approaches are seldom used or discussed in Amateur Radio
circles.
Take a look at some of the antenna systems used at LF and VLF, and
apply those techniques to 80-meters. Even if your efficiency is
only 50%, that's only half an S-Unit loss -- hardly a cause for
concern.
73, de John, KD2BD
Visit John on the Web at:
http://kd2bd.ham.org/
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