Download The Aerial explained without mathematics

The Aerial explained without mathematics
by Andy Choraffa, D.phys. G3PKW
Intro :
Have you ever thought how an aerial actually radiates or couples its
energy into the atmosphere ?
Here I have tried to explain in words of one syllable the way that an
aerial actually achieves this.
With all aerials there is nothing to see, no moving parts, so they are
normally described in mathematical terms. This is always a stumbling
block for many radio amateurs.
As a physicist I have lived and breathed Maxwell and Poynting Vector
equations for fifty years, I have tried here in simple words to explain
what is happening to a dipole, being a centre fed resonant two polarity
half wave aerial. I have explained further, how any aerial which is not
resonant, can be corrected to radiate efficiently.
Modus Operandi :
Consider a starting moment in time. The opened centre fed point rises
positive on one terminal and negative on the other, a normal balanced
drive situation.
This RF voltage drives a
merely re-plenishing the
radiation. In effect the
load, assuming other non
current which is in the same phase, as it is
radiated energy that has been lost due to
feeder sees the Radiation Resistance as the
radiating losses are negligible.
The circulating current within the aerial element comes from the energy
stored in its own L & C components and is much greater than the driving
feeder current. It has been built up over time as a standing wave. It
is this standing wave which couples our signal to the atmosphere or
aether as it is known.
The huge current comes
about from the stored
electric charge (in C)
and the magnetic field
energy (in L) components
of the radiating element
itself.
This current and voltage, in quadrature (90 deg.apart) is what is shown
in the classic dipole diagram.
Many amateurs get confused by this, and do not realise that the
intrinsic stored energy is different than the real matched energy being
transferred by the feeder.
The feeder cannot see these intrinsic reactive L & C components,
because at resonance, they are equal and opposite so therefore they
cancel, which is why the aerial is resonant in the first place. The
energy is being tossed back and forth between the L & C components of
the aerial element itself, in perfect time or harmony.
L & C are ALWAYS present, even though, at resonance, we don't see them
at the feed point, but they create these huge internal circulating
currents.
The transmitter drive is just replacing the energy which is being lost
(radiated) to the atmosphere. This apparent 'loss' of energy is what we
refer to as the Radiation Resistance. All resonant systems demonstrate
this attribute. In fact there is an illuminating mechanical analogy in
The Tachoma Narrows bridge, USA 1940. The gentle driving wind energy
acting on the mechanical resonance, slowly stored up energy, which
eventually destroyed the bridge, as it had no way to dissipate the
energy away. A breath of wind destroyed a solid steel structure with
stored energy.
So in our simple half wave dipole case, everything occurs at the
correct time (phase) so the energy builds up and then radiates
efficiently.
Restoranté :
That is fine for an aerial which is resonant by design. Let us now look
at the case for a doublette aerial which is non resonant or perhaps an
end fed wire which may not be resonant on any particular band.
We need to employ a phase correcting network, often mistakenly called
an Aerial Tuning Unit. It is not actually tuning the aerial, per sae.
It is merely providing two basic tasks as follows.
The correct name is an Aerial Matching Unit or 'Transmatch' as it
alters the transmitters RF driving phase to make it occur at the
correct moment in time to enable the radiating element to build up
energy in a standing wave. It does this by introducing an opposing
phase into the system known as a conjugate match. This corrects the
phase error due to the aerial not being resonant where we would wish it
to be. Remember from basics that time and phase are synonymous.
If, for example the aerial has more inductance than required for
resonance, this would cause any current to lag in time. Our AMU
introduces a surplus of capacitance into the whole resonant system to
make the current advance back to where it needs to be.
Conversely if the aerial has too much capacitance or not enough
inductance, it then adds extra inductance to correct this situation.
Remember from basics that current leads in a capacitor and lags in an
inductor.
This correction of timing is crucial because any deviation from the
correct amount of L or C anywhere in the system means that reactance
will be visible to the source. Any reactance present will only hinder
our current which we are trying to make flow in the real radiating part
of the aerial. Surplus reactance must be eliminated to get the aerial
to create a standing wave and radiate properly.
As
to
be
it
well as conjugate matching the AMU must also provide a second task,
transform the ratio of the real radiating resistance. As it may well
different from the 50 ohm which the rig likes to see, in order that
can transfer all of its generated power.
I refer the reader to my previous article which explained how a Pi
network actually achieves this transformation together with the
required phase correction.
In many practical cases, just using a single inductor or a single
capacitor could be sufficient to correct the phase (time) error, as the
real radiating part of the aerial element could be very close to the 50
ohms which the rig likes. Resulting in an SWR close to unity enabling
the full power to be transferred.
Et Al :
The Magnetic Loop aerial has become popular recently and is a
physically small aerial. Such a small system has a much smaller
Radiating resistance because it only occupies a very small volume of
space to couple its energy into the surrounding atmosphere. Therefore a
typical radiating resistance may be only a tenth of an ohm.
As the Radiating resistance is tiny it means that massive internal
circulating currents will flow to build up the standing wave on the
radiating element (even at low power).
In a typical Mag.Loop with 100 watts it can have 30 to 40 Amps of
internal circulating current when resonant. This in turn causes the
capacitor to generate a very high voltage across its plates.
Therefore the resonating part of the aerial due to its own L & C,
becomes much more critical because only tiny changes will cause non
resonance, hugely hindering this large current flow. This makes
resonance critical and this aerial type has a narrower bandwidth as a
consequence. Losses need more care to be kept small then these aerials
can have good efficiency.
Finale :
To answer a frequently asked question - the Radiation resistance of a
folded dipole is 73 ohms (free space value) although the feed impedance
is nearly 300 ohms. This occurs because the folding of each side of the
aerial gives a transformer effect due to the two conductors each
carrying half the current with twice voltage (same power) which results
in transforming the Radiation resistance to a real load of four times
its actual value.