A certain inductance places at a correct place along the 80m HW will
make multy band EFHW. Here is the example
http://myantennas.com/wp/product/efhw-8010p/
73, Igor UA9CDC
12.02.2017 1:50, K9MA пишет:
On 2/11/2017 13:03, Brian Pietrzyk wrote:
Now comes the fantasy thinking... What about making it an end fed
multi band by either putting traps in the 80m end fed or better still
adding on a couple of fan EFHW wires for 40 and 20? Anyone tried
this? How would the efficiencies compare to their centre fed trap or
fan dipole counterparts respectively?
That won't work, because the shorter wires will present a much lower
impedance, and essentially short out the half wave wire. The whole
idea of the EFHW is that the high feedpoint impedance reduces ground
system current to practically zero.
A trap might work, but might not be compatible with keeping the
antenna very light. A QRP trap could be pretty small, I suppose, but
it still might be hard to support the wire with the extra weight on it
using a lightweight pole.
However, an EFHW will work on harmonics, though the pattern changes.
A 40 meter half wave works quite well on 20, but I'm not sure an 80
meter half wave would be all that great on 20, especially with a
single support. The free space patterns of long wire antennas are
shown in the ARRL Antenna Book. As frequency goes up, more lobes
appear in the pattern, and the main ones get closer to the axis of the
wire. For example, a full wave antenna has a cloverleaf pattern, the
lobes about 55 degrees from the wire axis, with nulls broadside. For
a 2 wavelength wire, there are 8 lobes, the main ones 35 degrees from
the wire axis. These patterns will, of course, be modified by the
effects of ground and the fact that the wire probably won't be
straight, but the free space patterns give some idea of how the
antenna will radiate. Modeling these antennas is very easy, and gives
much more accurate results.
Note that the pattern of an EFHW (full wave, etc.) is NOT the same as
that of a center fed dipole operated at even harmonics.
73,
Scott K9MA
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