Somehow, after being in the
RF business for four decades, I have to admit to not being familiar with the term
"acceptance angle" for antennas. That is after having read scores of articles on
antennas. Maybe I did and just don't remember - embarrassing. Acceptance angle is
mentioned and explained in this 1949 Radio & Television News magazine article during the description of rhombic antenna
characteristics versus dipoles and multi-element designs. Although the author focuses
on television installations, information provided on signal reflections, shadowing,
ghosting, multipath, etc., it is applicable to radio as well.
Rhombic Antennas for Television
By Woodrow Smith
Author, "The Antenna Manual"
For fringe and bad ghost areas the rhombic antenna will always outperform an
ordinary dipole array.
The rhombic antenna has for many years been a favorite for high-frequency sky-wave
transmission and reception. This is explained by its simplicity of construction
as compared to dipole arrays having comparable gain, by its broadband characteristics,
and by its sharp, unidirectional directivity pattern throughout its broad frequency
The same characteristics which recommend the rhombic array for high-frequency
sky-wave applications make it an ideal TV receiving antenna for use in fringe areas
or bad ghost areas when sufficient room for erection is available, particularly
when several stations lie in very nearly the same direction.
Even when the desired stations do not lie in approximately the same direction,
the rhombic array often will be found useful in providing a usable picture on one
channel at distances so great that an ordinary dipole array will not perform, or
in providing a useful picture on one channel in mountainous areas where ghosts are
so bad that a ghost-free picture cannot be obtained on even one channel with an
ordinary dipole array.
Unlike an ordinary dipole array using one or more parasitic elements, a rhombic
array exhibits a good front-to-back ratio over a wide frequency range and also has
a very narrow acceptance angle off the front side. This means good rejection of
ghost-producing reflected signals on all channels, both from the side and off the
The wide acceptance angle of conventional dipole arrays makes them highly vulnerable
to ghost-producing echo signals arriving obliquely from the front side. Contrary
to widespread belief, ghosts are not necessarily produced by echo signals arriving
from the back side of an antenna; they often arrive from the front.
Front side echo signals sometimes are not apparent as ghosts, because they may
have a comparatively short delay time. Under such conditions a separate image is
not discernible; the echo signal simply degrades the definition of the picture without
producing a separate image.
A high-frequency rhombic array may be designed for effective sky-wave transmission
or reception over a frequency range as great as 4 to 1. While the vertical angle
of maximum radiation or response of such a rhombic increases considerably with decreasing
frequency over such a wide frequency range, higher angles become effective as the
frequency is lowered. Therefore, the change in vertical directivity is not particularly
objectionable for sky-wave applications.
The situation is different, however, in the case of TV reception or other v.h.f.
ground wave applications. The only effective vertical angle is that of the angular
elevation of the horizon at a point where the arriving wave passes over it. There
is only one useful vertical angle, and this angle does not change with frequency.
The only useful gain is that which occurs at this angle.
As a result, the useful frequency range of a rhombic array designed for TV reception
or other v.h.f. applications does not exceed approximately 2 to 1, and preferably
the range should not exceed 1.6 to 1. This means that it is not possible to design
a rhombic array which will provide near-optimum performance on both the low and
high television bands. The ratio of 216 mc. to 54 mc. is 4 to 1, and any attempt
to cover this range with a single rhombic array will result in mediocre performance
over much of the range.
Where two-band coverage is required, a high-band rhombic can be strung inside
a low-band job from the same poles. The separation will be sufficient to avoid undesirable
interaction. For short runs separate feed lines and a suitable switch should be
employed. For long runs a d.p.d.t. relay can be placed at the antenna end and a
single line run from the relay to the set.
For an excellent DX FM receiving antenna, simply double the dimension s given
in Fig. 1 for the high TV band.
Fig. 1 - Rhombic antenna design characteristics.
Design data are given in Fig. 1 for four rhombics: (1) a long-leg rhombic
for use on the low band for maximum gain and directivity where space permits; (2)
a short-leg rhombic for use on the low band when space restrictions will not permit
a long-leg rhombic or where less horizontal directivity is desired due to a slight
spread in the station directions; (3) a long-leg rhombic for use on the high band
for maximum gain and directivity; and (4) a short-leg rhombic for use on the high
band when less directivity is desired or when it is desired to hang the array from
a single pole and two cross arms, or from two poles and a spreader. The latter array
is small enough to be mounted on an amateur beam antenna rotator.
The long-leg rhombics are four wavelengths on a side at their "design center"
frequency, have a gain of approximately 10 db.* over a matched half-wave dipole
(varying slightly over the band), have a useful beam width or acceptance angle of
approximately 8 degrees (varying slightly over the band), and exhibit excellent
ghost rejection (azimuthal discrimination) throughout their frequency range.
The short-leg rhombics are two wavelengths on a side at their "design center"
frequency, have a gain of approximately 7 db.* over a matched half-wave dipole (varying
slightly over the band), have a useful beam width or acceptance angle of approximately
13 degrees (varying slightly over the band), and exhibit good ghost rejection (azimuthal
discrimination) throughout their frequency range.
The low-band arrays employ 68 mc. as a design center, and the high band arrays
employ 194 mc. as a design center.
*When comparing gain figures, keep in mind that the high gains claimed by some
antenna manufacturers in their advertising are highly optimistic." A very elaborate
dipole array is required for a gain of more than 10 db. over a single matched dipole,
and the gain of such an array falls off at a comparatively rapid rate for departures
from the design frequency.
Antenna Patterns and Ghost Rejection
As a measure of vulnerability to ghosts, the common figure of merit for ordinary
dipole arrays having a wide acceptance angle is the "front-to-back ratio" on each
of the channels under consideration. For an array having an acceptance angle of
only a few degrees, however, we are interested in the relative response in all directions
outside the main lobe, regardless of whether it is off the back or off the front.
For this reason, "azimuthal discrimination" is a more appropriate term than "front-to-back
ratio" when referring to a rhombic array.
Like almost all large, high-gain arrays, a rhombic array exhibits various "minor
lobes," and it is the ratio of the amplitude of the main lobe to that of the various
minor lobes that determines vulnerability to ghosts. (See Fig. 4.) Generally
speaking, as the legs of a rhombic are increased in length and the included angles
are maintained at optimum values, the minor lobes become more numerous, sharper,
and lower in amplitude (compared to the amplitude of the main lobe).
Strictly speaking the response of a large rhombic to a signal whose modulation
envelope changes very rapidly with time, such as a television video signal, is not
quite the same as for a steady carrier or a signal containing only low modulating
frequencies. The effect is insignificant for a wave arriving "head on," because
a wave propagated from the fore end to the aft end of the rhombic via the wires
travels not more than about one wavelength farther than the direct distance between
these two points, and even on Channel 2 this difference is only about 18 feet.
For television signals arriving obliquely or from the back the effect is no longer
insignificant, particularly in the case of a long-leg rhombic cut for the low band.
However, the effective azimuthal discrimination will compare closely to that which
obtains under unmodulated conditions and, for practical purposes, may be considered
to be the same as for an unmodulated signal. It should be pointed out, however,
that if an attempt is made with a low-band, long-leg rhombic to receive a nearby
TV station off the back side, the picture quality will be poor even though the received
signal (due to the transmitter proximity) is of good strength.
Fig. 2 - Rhombic antenna construction details.
If a rhombic array is placed well above surrounding objects it is not necessary
to "probe" the available area for an optimum location. This is explained by the
sharp directivity pattern (making the array rather insensitive to phasing reflections
from nearby surrounding objects) and by the fact that the rhombic is spread over
a considerable area as measured in terms of wavelength.
While the exact location of the array is not critical, so long as it is "in the
clear," the direction in which the array is to be pointed must be determined very
precisely, particularly for the long-leg jobs. It is for this reason that dimensions
for still longer legs are not given in Fig. 1. While even greater gains may
be obtained, the beam width becomes embarrassingly narrow, making the array difficult
to orient. However, if you are sure you can get the array "dead on" and want to
pick up another 2 db., here are the dimensions for legs six wavelengths long at
the "design center" frequencies: Low band - L, 86' 5"; S, 64' 7"; D, 160' 2". High
band - L, 30' 7"; S, 22' 11"; D, 56' 8".
Flat Terrain Installations
If the antenna is to be located in open, flat, or rolling country, and the angular
elevation of the horizon is practically zero, then the orientation is fairly simple,
and the height of the array is not especially critical (though as much height as
practicable is desirable). The array should be pointed in the exact direction of
the transmitter. This can be determined by means of a suitable map and an accurate
compass. (Obtain the magnetic declination for your area from a surveyor, if this
is not already known.)
If the transmitter is on a peak which can be located with a pair of glasses:
or a telescope, the problem of orientation is still simpler. The orientation of
the short-leg rhombics of Fig. 1 should not be off more than about three degrees
and that of the long-leg rhombics of Fig. 1 more than about two degrees if
maximum performance is to be obtained.
In open, flat, or rolling country, the higher the array the better (within the
range of practical pole heights) but there is not much profit in going above about
thirty feet for the high-band rhombics or more than about sixty feet for the low-band
rhombics unless it is necessary in order to get the array well in the clear with
regard to surrounding objects, particularly objects in front of the antenna. The
law of diminishing returns applies, and it is up to the individual how much pole
expense is justified. The higher the antenna the better, but the higher the antenna
the less difference another ten foot length makes, and the harder it becomes to
obtain another 10 feet.
When the receiving location is in comparatively flat country but is separated
from the transmitter by a range of mountains or high hills some distance away, in
such a manner that the angular elevation of the horizon in the transmitter direction
is more than about three degrees on the high band or more than about ten degrees
on the low band, then one must be careful not to get too much height. Under such
conditions the height at which maximum signal strength occurs (and above which it
falls off) will come within the range of practical pole heights, and high poles
may provide too much height.
If the angular elevation of the horizon exceeds the above limits, it is a good
idea to lower the array on the poles a few feet to see if the signal strength drops
off. If it increases instead, then the array should be run up and down the poles
to find the optimum height.
When the angular elevation of the horizon in the direction of the transmitter
exceeds approximately eight degrees for a long-leg rhombic or approximately twelve
degrees for a short-leg rhombic, it also is a good idea to try elongating the array
by increasing the dimension D of Fig. 1 a certain small percentage while checking
signal strength, dimension S being decreased accordingly to allow for the elongation.
(Elongation raises the elevation angle of the main lobe.) In installations where
the angular elevation of the horizon exceeds the aforementioned limits, the distance
between the fore and aft poles should be made about fifteen or twenty per-cent greater
than the distance D given in Fig. 1, to allow for experimental elongation of
When sufficient room to permit experimental elongation of the array is not available,
the array should be tipped upwards so that an extended line through the fore and
aft apices of the array would intersect the horizon. This requires that the front
pole be higher than the center poles and that the rear pole be lower than the center
poles (assuming level ground). The array must be kept in a flat plane, even though
it is tipped upwards.
Because of the sharp vertical directivity of the array, one of these two expedients
is required for maximum performance whenever the angular elevation of the horizon
exceeds the aforementioned limits. The dimensions given in Fig. 1 are for maximum
response at zero elevation with the array lying in a horizontal plane. Therefore,
for good response at an angle much above zero, the array must be either elongated
or tipped upwards. Elongation gives slightly better results and is the preferred
Fig. 3 - Three applications of a broadband impedance transformer
and line balancer (T) in conjunction with a rhombic antenna installation. The device
is used at the antenna end (A) to permit use of a coaxial line in locations where
the line must run through a region of high ambient noise. It is employed at the
set end (B) to match coaxial line to a receiver having only 300-ohm input. At (C)
it is used at the set end to match 300-ohm ribbon or an open-wire line to a receiver
having only 70- to 75-ohm input.
When the array is to be located in hilly country or down in a canyon, it is not
safe to orient the array in azimuth simply by aiming it at the transmitter. The
dominant signal may be taking a devious route. The safest procedure under these
circumstances is as follows.
Using four low, temporary poles and some willing assistants, determine from which
compass direction the main signal is arriving. During this operation keep the S
and D dimensions nailed down by tying strong string between opposite apices. After
the signal direction is determined, layout the location of the permanent poles,
allowing for experimental elongation of the D dimension if there is room. Then proceed
as before, checking to see if greater signal strength can be obtained by lowering
the antenna. If so, optimize the height and then try either elongating the array
in the D dimension or tipping the front of the array upwards, as previously described.
This may sound as though a lot of trouble were being taken, but it is necessary
in a hilly receiving location in order to insure maximum performance. By following
this procedure good pictures have been received in what were considered "impossible"
Occasionally when the terrain is very hilly and the spurious reflections very
bad, the discrimination of a rhombic is not sufficient to eliminate a ghost coming
in on a minor lobe. If this is the case, try varying the D dimension slightly either
way (at the expense of the S dimension). The numerous nulls can be steered over
a narrow arc in this manner, and usually one can be lined up on the troublesome
ghost without affecting the main signal.
The Terminating Resistor
For proper operation a rhombic array must be terminated in a substantially non-reactive
resistance of approximately 800 ohms. Satisfactory operation will be obtained simply
by connecting in series two 390 ohm metalized resistors of the insulated, hermetically
sealed type (such as an IRC type BTA), shown in Fig. 2. Two in series are preferable
to a single resistor having twice the resistance, for reasons which need not be
Care should be taken to make sure that the resistors used are not of the wirewound
type. In the low resistance range, 1 watt resistors are available in both metalized
and wirewound types, and the two cannot be told apart by inspection except by type
number. If in doubt, break one open to see.
The Feed Line
Only about 10 per-cent loss in signal voltage will result if a rhombic array
is fed directly into a 300 ohm line without benefit of a matching transformer. Therefore,
while it is possible to construct a matching arrangement which will result in a
precise match, the improvement hardly can be considered worth the trouble.
If the feed line must pass through a region of high ambient noise, and it is
desired to employ 75 ohm coax for lead-in, it can be done with the aid of one or
two of the broadband 300/75 ohm balanced-to-unbalanced transformers now on the market
(as manufactured by The Workshop Associates and the J. W. Miller Mfg. Co.) A transformer
is connected between the line and the array as shown at Fig. 3A. The device
should be made water-tight or protected from the weather. If the receiver does not
have provision for 75 ohm input, another transformer should be employed at the set
end of the line, as shown at Fig. 3B.
At Fig. 3C is shown how 300 ohm ribbon or an open wire line may be employed
with a set having only 75 ohm unbalanced input (such as certain receivers employing
the Du Mont "Inputuner").
Using Open Wire Line for Lower Losses
Where difficulty is experienced with 300 ohm ribbon close to the ocean becoming
very lossy after a few weeks, or where it is necessary to run 1000 feet or more
of line to put the antenna in the clear (as in some mountain locations), line losses
can be reduced greatly by substituting an ordinary open wire line constructed of
No. 14 or No. 12 copper wire spaced two inches. Suitable line spacers are available
at radio parts stores.
Fig. 4 - Horizontal directivity and gain pattern (voltage
across 300 ohms) of a typical long-leg rhombic array and of a typical stacked dipole
array using parasitic reflectors cut to the channel. The rhombic will maintain substantially
the same gain and discrimination over considerable frequency range. The dipole array
This type of line is particularly recommended for use on the low band but is
still useful on the high band. Somewhat closer spacing is desirable on the high
band, but suitable spreaders are not readily available and must be made up. It is
best to stagger the distance between spreaders a little, to make sure that all or
several do not fall upon exact multiples of an electrical half-wavelength on a desired
channel. (This will result in an undesirable effect called "periodicity.")
An open wire line constructed as above will have a surge impedance of from 450
to 500 ohms, or somewhat above the standard 300 ohm receiver input impedance. However,
this is nothing to worry about, because as the impedance of the line is raised,
and the match becomes worse at the set end, the match improves at the antenna end
of the line. As a result, it is possible to employ a 450-500 ohm line without running
into transmission line ghosts, loss of picture detail, or increased loss due to
Two examples of the improvement to be expected over 300 ohm ribbon used under
unfavorable conditions can be cited. Substituting an open line for 160 feet of ribbon
exposed to beach weather for four months resulted in an increase on two low band
channels of more than 10 db. After six months the performance of the open wire line
had not deteriorated enough to notice.
In another case an. open line was substituted for 1100 feet of 300 ohm ribbon
newly installed and not providing adequate signal strength. The location was not
near the ocean. The measured increase in signal at the receiver on Channel 5 was
approximately 12 db. (measured in dry weather), a very worthwhile gain. Wet weather
proved to have little effect upon the performance of the open line, which is more
than could be said for the ribbon line. It should be kept in mind, however, that
for a comparatively short run not near the ocean, ribbon or tubular 300 ohm line
is entirely satisfactory.
Where noise pickup by the line is not a serious problem, the same order of improvement
will be obtained with an open wire line on the high band when the installation is
near the ocean or the run is very long.
When expense must be kept down, plastic curlers of the type sold for use with
home permanent wave kits may be used for spacers. Get the clear ones rather than
Somewhat more care must be taken with an open wire line to avoid sharp bends
and to keep the line away from objects as much as possible. Nylon cord or fish line
may be used to support and position the line where necessary. The feed line in any
case should leave the antenna symmetrically for at least six feet on the high band
or at least fifteen feet on the low band before making a bend to the right or left.
Contrary to popular belief, little if any reduction in noise pickup will be realized
by twisting or transposing the line.
It is recommended that wood poles be used to support the array unless they are
spaced several feet from the apices. Guy wires should be well broken up where they
run within a few feet of the antenna, or else rope guys should be used. All joints
should be soldered.
Posted June 10, 2022
(updated from original post on