[TowerTalk] Perfect Field Day beam?
Wed, 5 May 1999 11:08:22 -0400
From: "Joe Subich, W8IK/4" <W8IK@ibm.net>
Date sent: Tue, 4 May 1999 20:05:44 -0400
I hope one more post clears this up. It would be nice if we kept
things in perspective.
My points were:
1.) The TA-33 measures with a fair amount of F/B and with good
2.) Power applied to the antenna has to go two places, out as
radiation or out as heat.
How do we know where it goes based on a groundwave
> > We seem to be going around in circles. If different components of the
> > radiated signal arrive at a distant point with equal and opposite
> > phases, the net signal at that point is zero, right?
> > I don't think it violates the conservation of energy to argue that some
> > antennas may be more prone to this sort of cancellation than others.
It indeed DOES violate the law. If we cancel radiation in any
direction, the power applied MUST radiate in some other direction
or by a change to some other energy.
You can't have wave interference without increasing FS in other
directions, unless you've turned that cancelled radiation to heat.
If I enclose an antenna in a lossless conductive box many
wavelengths in diameter, radiation leaving the box stops. Antenna
current increases, until every bit of applied power turns to heat. If
there is no loss resistance, feedpoint impedance goes to zero and
we can't apply power to the antenna.
That's even one useful way we can measure loss and radiation
resistance of an antenna! That's also why, when we have
directional antennas, they have very high voltages and currents
compared to the same non-directional antenna.
> Certainly, different antenna designs have different patterns and thus more
> or less gain. If as previously stated however, an antenna shows a F/B
> ratio of about 13 dB over a 90 degree sector it must have a forward gain
> of 3 to 4 dB over a similarly situated dipole. The only possible reason
> for a lower total radiated field from one antenna than another given
> identical levels of applied power at the feedpoint is a lower current in
> one antenna than the other. The lower current can be due to a higher
> radiation impedence (not the same as feedpoint impedence) or higher loss
Not quite. We can have a higher feedpoint impedance, but that has
nothing to do with gain or radiated power. It is the currents in ALL
the elements and all areas of the elements that contributes to
radiation, not just at the feedpoint.
1 ampere spread over ten feet radiates just as well as two amperes
spread with the same shape over five feet. The feedpoint doesn't
tell us squat, unless we compare the impedance change when the
antenna is put in a "box" that stops radiation.
> > Moreover, why isn't it plausible that such cancellation effects could be
> > more prevalent in the forward field than to the rear?
> Such cancellation effects would effect only the shape of the pattern (F/B,
> F/S, etc.) and not the total radiated field. Antenna "gain" is like
> squeezing a baloon ... what one moves from one direction pushes out in
> some other direction. It is the antenna designer's task to see than the
> "air" is is pushed in a useful direction.
That's a great analogy Joe! Maybe we don't view it that way
because the handbooks never tell us why antennas have gain.
The power has to go somewhere. It can't just vanish from the back
and sides without being radiated as either heat or useful EM
radiation. Cancellation only means it has been moved somewhere
73, Tom W8JI
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