Why do Rhombics work?
Long wire antennas develop gain differently than yagies.
Instead of reflecting and directing RF energy with near
resonant dipoles, long wires use the "traveling wave
effect". Consider a single long wire that is say three
wavelengths long on 14.200 Mhz. imagine a radio wave
approaching the far end of the wire at a 45 degree angle to
the wire.
The leading edge of the wave crosses the wire and induces
a voltage at it's far end that starts traveling toward the
feed point. As the rest of the radio wave sweeps across the
length of the wire it induces more voltages along the wire.
Some of these voltages add in phase, producing gain. Other
combinations cancel each other producing nulls. The amount of
gain or cancellation is a function of the length of the wire
(in terms of wavelength) and the angle at which the incoming
wave sweeps across the wire. The gain pattern of a long wire
can be visualized as a fat cone encircling the wire. For the
moment lets just concern ourselves with gain in the azimuth
plane. Imagine we are looking straight down at the wire and
the radiation cone, we would see a two dimensional cross
section of the cone that would look like this:
Angle "A" represents the angle from the wire
that produces maximum gain. As the wire is made longer, Angle
"A" gets smaller. Also gain increases linearly with
length. Basically, as the wire gets longer, the gain goes up
and the lobes become more focused and closer to the wire.
A V beam is simply two long wires feed 180 degrees out of
phase. Each wire has two lobes. If the apex angle of the V is
made two times angle "A", the lobes inside the V
add together yielding an extra 3 Db over a single wire. The
lobes outside the V cancel each other out so the resulting
pattern looks like this:
Note the somewhat smaller lobe 180 degrees from the main
lobe. This lobe is formed by the signal waves crossing the
wire starting at the feed point and sweeping across the wire
toward the end point. When the induced voltage hits the end
point, it is reflected, and goes back down the wire to the
feed point and then into the receiver. This makes the V beam
Bi-directional In real life there will also be some small
secondary lobes that appear at various angles off the main
lobes. The pattern looks similar to the illustration of a
rhombic's pattern presented up the page from here. If you
don't want the reflected lobes, you can ground the end points
of the V beam through 600 ohm non inductive resistors. This
will provide a path to ground for the signals that come in
from behind the feed point. Making the antenna essentially
unidirectional.
Since the V beam is a non resonant antenna, It really
isn't "cut" for any band but the relationship
between the length of the legs and the apex angle of the V
determine the band on which it will be most
"focused". Generally you get useful gain from 1/2
to 2 times the frequency at which it is "focused".
If you design for maximum focus on 14 MHz, it will work well
from 7 MHz to 28 MHz.
A Rhombic is simply two V beams joined to form a diamond.
The two extra legs add another 3 Db to the gain of the V beam
or 6 Db over a single long wire. This is similar to stacking
4 yagies to increase the gain over a single yagi by 6 Db.
One characteristic that is unique to the rhombic is the
ability to design the antenna to produce maximum gain at any
vertical wave angle. By changing the relationship between leg
length, tilt angle (how fat or skinny the diamond is) and the
height above ground you can design for very low angles for
radiation with respect to the earth. This is one reason that
Rhombics do so well with DX.
Like the V beam, if you want a unidirectional antenna,
simply connect the far ends of the antenna wires to a 600 ohm
non inductive resistor. This will provide very strong front
to back rejection. Some people have switching systems that
let them feed and terminate the antenna from either side.
For a through treatment of rhombic design methods
including charts to maximize gain and wave angle for a given
length please refer to the long wire section of the ARRL
antenna hand book
If you have any questions or comments, my Email address is
jbwieser@ktc.com.
If you would like to see some pictures of me and my Family click
here.
For a description of My
Rhombic Farm, click here.
Click here to see Ian Cummins excellent Rhombic web page.
http://www.mindspring.com/~cummings7/rhombic.html
