The theory of the crossed field operation (not
universally supported) is based on creating an electric (E) field in phase and
at right angles to a magnetic (H) field in suitable proportions. To do this two
E fields are generated with 90 degrees phase difference. The displacement
current from one of the two fields generates an H field in phase with the other
E field.
In the EH antenna with the L+L type matching network, a main
E field is developed differentially across the two dipole plates and a secondary
E field, at 90 degrees phase difference, is developed longitudinally between the
plates and the reference coax shield. In the EH antenna with the L+T matching
network, the main E field is developed between the top dipole plate and the
common connection of the coax shield and the bottom plate. A secondary E field
shifted by 90 degrees across the last coil in the matching network is developed
from the coil assembly to the reference common.
One characteristic of
the two types of EH antenna is that current runs down the outer shield of the
coaxial feeder. Where the coax is elevated, radiation occurs from this section
and can possibly be used to advantage. However where the coax continues past
building structure and close to earth, induction into these can amount to power
loss. Also with the coax led right into the radio shack, the current causes
undesirable fields within the shack itself.
To prevent the coax
shield current causing undesirable effects, a trap is placed in the coax line.
The trap may be placed close to the EH antenna input to ensure that the antenna
operates essentially in its crossed field or EH mode. If it is placed down
further, it should be placed well beyond the coax section which is directly
adjacent to buildings or earth. The nearer the trap is to the antenna, the less
the positioning of the coax will effect the antenna tuning. With the trap close
to the antenna, it is much more stable to tune. One thing observed in testing
the L+L type antenna is that the radiation tends to be concentrated more at a
lower angle if a short length of coax is used between the trap and the antenna
input. So you may choose to use a short length to the trap (say one metre)
rather than no coax at all.
There are critics who argue that the
cross field theory cannot work and that the EH antenna is no more than a simple
dipole with low radiation resistance and the main radiation is due to current in
the coax shield with resultant operation as a vertical antenna.
However in
practical operation:
The antenna does spring
to life when the 90 degrees phase shift is established between the two E fields
and the
phase shift seems to
be a necessary function to make the antenna really work.
Furthermore the antenna matches with matching network components
suited for a much higher radiation resistance
than a simple dipole and works without excessive loss in those
matching components. (**)
It also matches just
the same when the trap is fitted right at the antenna input connector and there
is no coax in circuit.
For these reasons, the theory of the critics seems
to fall down.
** Note: We are talking about series
resistance - In the case of the L+L network, matching is actually made to the
equivalent shunt radiation resistance which is lower than the equivalent shunt
loss resistance .
From my testing with the L+L type EH antenna, it works
quite well compared to a larger antenna. However I see no point in erecting an
EH antenna at height if you can achieve good efficiency with a wire or rod at
that same height. (Refer to my point (1) above).
But if you don’t have
the room for a larger or higher antenna which would be efficient, you might have
room to fit an EH dipole antenna to achieve much more efficient broadband
operation than a short dipole or monopole. There is one limitation; each
EH antenna is made for a single frequency band and you can’t use it on another
band.
For more detail, refer to
the following:
EH Antennas 20 and 40 metres
A New
Theory
Notes on the
L+T Network Antenna
Tests for the H
Field
Antenna without
Coax Shield Current
