"Cecil Moore" wrote
Reg Edwards wrote:
A terminated rhombic is only 50 percent (or even less) efficient.

But it's 50% in the bad direction, not the good direction. :-)
===========================================

In the most simplistic of terms, a rhombic consists of four
1/2-wavelength wires plus a lossy resistor which gets hot.

If the four 1/2-wavelength wires are rearranged to form a dipole, plus
a reflector, plus two directors, we have only one good direction in
which 100% of the power is radiated. Nothing gets hot.

Common sense prevails. No need to refer to Eznec. Even a drunken
old-wife would know which arrangement to choose, if only because it
saves the cost and fitting of a high power, non-reactive resistor.
;o)

But no doubt US Army Field Manuals still call upon rhombics.
----
Reg.

In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:

In the most simplistic of terms, a rhombic consists of four
1/2-wavelength wires plus a lossy resistor which gets hot.

And, in this case, I think the "most simplistic terms" are a
misapplication of the way the term is usually used.

My recollection is that in practice, rhombics of the sort being
referred to have arms which are several wavelengths long. The total
double-arm-length of a wire rhombic is often 10 wavelengths or more.

In this sort of rhombic, the great majority of the transmitted power
is radiated before it reaches the termination resistor. There is little
power left to dissipate in the resistor. If the rhombic were
unterminated, and the forward-travelling wave were reflected at the
end of the rhombic, most of this reflected power would be radiated
before it reached the transmitter and were re-reflected.

Dissipating the remaining (small) amount of forward wave at the end of
the rhombic helps maintain a very high front-to-back ratio. This can
be advantageous both when transmitting (no back-spill) and when
receiving. I believe that rhombics were popular among U.S. government
radio sites for use at coastal sites, for precisely this reason - they
were very good at rejecting QRM from landside transmitters, and didn't
blast landside receivers with high power.

Long, terminated rhombics have another advantage - they maintain a
consistent directionality and feedpoint impedance over a wide range of
frequencies... rather wider than you can do with a resonant
standing-wave antenna such as a reflector/DE/directors beam.

Common sense prevails. No need to refer to Eznec. Even a drunken
old-wife would know which arrangement to choose, if only because it
saves the cost and fitting of a high power, non-reactive resistor.

If you're insisting that a "rhombic" may have arms of no longer than
1/2 wavelength, I'd agree.

Since that's not the only way to design 'em, though (and is not how
some of the better-known ones were designed), I think that your
conclusion is overbroad.

It's all a matter of serving your needs, whatever they may be. If
you're limited on space, and/or want a steerable beam, then a Yagi or
similar is probably the best choice. I certainly wouldn't try to put
up an effective HF rhombic on my roof!

If you've got oodles of space, want to listen (or transmit) only in a
single direction, need a lot of front-to-back isolation, and want a
broad bandwidth and consistent radiation patterns and a considerable
amount of gain, then a long-armed terminated rhombic may be a better
choice than the alternatives (e.e. a honking-big LPDA).

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

What??? Four 1/2 wavelength wires!! You gotta be kidding!!!

The Rhombic at AGA5HI in the early 70s was close to 6 wavelengths per side.

There are/were two different rhombic designs. [Roy may want to comment
on this.] A resonant rhombic without a termination resistor is
bidirectional. A non resonant rhombic, a traveling wave design that uses
a terminating resistor is unidirectional.

Gain increases as the length of each side increases. Directivity [gain]
is also dependent on the included angle which contributes to radiated
beam width.

AK

Reg Edwards wrote:
"Cecil Moore" wrote

Reg Edwards wrote:

A terminated rhombic is only 50 percent (or even less) efficient.

But it's 50% in the bad direction, not the good direction. :-)

===========================================

In the most simplistic of terms, a rhombic consists of four
1/2-wavelength wires plus a lossy resistor which gets hot.

If the four 1/2-wavelength wires are rearranged to form a dipole, plus
a reflector, plus two directors, we have only one good direction in
which 100% of the power is radiated. Nothing gets hot.

Common sense prevails. No need to refer to Eznec. Even a drunken
old-wife would know which arrangement to choose, if only because it
saves the cost and fitting of a high power, non-reactive resistor.
;o)

But no doubt US Army Field Manuals still call upon rhombics.
----
Reg.

Amos Keag wrote:
A resonant rhombic without a termination resistor is
bidirectional.

Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."

Why would that be of any advantage? Do you believe it is easier to solve
for the exact radiated fields by partitioning the standing wave into
two components?

The real challenge in antenna theory is determining the exact current
distribution. Further subdividing or superposing the current is a
trivial exercise in comparison, and such manipulation may or may not be
useful.

In other words, what Balanis says may be true, but so what?

73,
Gene
W4SZ

On Tue, 10 Jan 2006 15:25:35 GMT, Cecil Moore wrote:

Amos Keag wrote:
A resonant rhombic without a termination resistor is
bidirectional.

It may be bidirectional, but it still has directivity in the direction
away from the feed.


Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."

If the unterminated rhombic is long enough to be considered a -real-
rhombic, it is both a standing wave and a traveling wave antenna.

If you want to think of it in transmission line terms, it's a very
lossy line. The 100% reflection at the open end is 100% of what's
left after the effects of resistive and radiated loss are factored in.

The rhombic as the positive attribute of simplicity but that is about
the only positive. It takes a huge amount of space (in wavelengths)
and it's really a crappy antenna from the standpoint of minor (and
some not so minor) lobes. The Laport version is much better, but much
more complicated.

Gene Fuller wrote:
Cecil Moore wrote:
Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."

In other words, what Balanis says may be true, but so what?

The "so what" is the additional knowledge to be gained by
not choosing to ignore the underlying physics. When the
forward EM wave hits the end of the dipole, what happens?
Essentially the same thing that happens when a forward EM
wave hits the end of an open-circuit transmission line.
The H-field (current) goes to zero and the E-field (voltage)
doubles, i.e. the forward wave existing at that point is
completely reflected.

That explains why the feedpoint impedance of a 1/2WL dipole is
50-75 ohms instead of the physical characteristic impedance
of ~1200 ohms. The feedpoint impedance of a 1/2WL dipole is
a virtual impedance caused by destructive interference
between the forward and reflected voltages, Vfp = |Vfor|-|Vref|,
and constructive interference between the forward and reflected
currents, Ifp = |Ifor|+|Iref|, and Zfp = Vfp/Ifp

It is interesting to note the consistency of the arguments
here on r.r.a.a. Someone says, "'A' is true". Someone else
says, "No, 'A' is gobbledygook". After 'A' is proven to
be true, the argument shifts to, "OK, so what? Those grapes
are probably sour anyway." :-)
--
73, Cecil http://www.qsl.net/w5dxp

Cecil,

I don't understand the "sour grapes" reference. To the contrary, I
believe I have argued with you many times that there are multiple ways
to solve a problem.

Use standing waves or traveling waves as you choose for computational
convenience. The only thing that goes into the field-defining equations
is the current, not the waves.

Your original message implied that may be some special benefit to a
*standing-wave* antenna over a *non-standing-wave* antenna. Other than
all of the hand-waving, which seems to somehow be connected to your
intuitive thinking, there is no physical difference.

In other words, what Balanis says may be true, but so what?

73,
Gene
W4SZ


Cecil Moore wrote:
Gene Fuller wrote:

Cecil Moore wrote:

Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."


In other words, what Balanis says may be true, but so what?


The "so what" is the additional knowledge to be gained by
not choosing to ignore the underlying physics. When the
forward EM wave hits the end of the dipole, what happens?
Essentially the same thing that happens when a forward EM
wave hits the end of an open-circuit transmission line.
The H-field (current) goes to zero and the E-field (voltage)
doubles, i.e. the forward wave existing at that point is
completely reflected.

That explains why the feedpoint impedance of a 1/2WL dipole is
50-75 ohms instead of the physical characteristic impedance
of ~1200 ohms. The feedpoint impedance of a 1/2WL dipole is
a virtual impedance caused by destructive interference
between the forward and reflected voltages, Vfp = |Vfor|-|Vref|,
and constructive interference between the forward and reflected
currents, Ifp = |Ifor|+|Iref|, and Zfp = Vfp/Ifp

It is interesting to note the consistency of the arguments
here on r.r.a.a. Someone says, "'A' is true". Someone else
says, "No, 'A' is gobbledygook". After 'A' is proven to
be true, the argument shifts to, "OK, so what? Those grapes
are probably sour anyway." :-)

Gene Fuller wrote:
I don't understand the "sour grapes" reference. To the contrary, I
believe I have argued with you many times that there are multiple ways
to solve a problem.

Actually, you have gone from saying I was wrong to saying that I am
right but it doesn't matter. I guess that's an improvement. :-)

Your original message implied that may be some special benefit to a
*standing-wave* antenna over a *non-standing-wave* antenna.

Sorry, I never said anything about a standing-wave antenna having some
special benefit over a traveling-wave antenna. It was a problem with
the inferrence function, not with the implication function. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Unless a self-respecting radio amateur wishes to set up a
point-to-point communications link, he would not consider a rhombic
which wastes half the transmitter power in a resistor.

How popular are rhombics, such as you describe, amongst amateurs?

There are, of course, some rich, perhaps contest participating
amateurs, with time any money to burn. I have no objection to their
existence. They are very welcome to the fraternity. They demonstrate
just what can be achieved.
----
Reg.