FPR1981
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(article transcribed from CB Radio Magazine – August, 1970 – pages 12 & 13)
HOW TO MAKE AN OMNI INTO A BEAM!
The Development Story Behind Avanti’s Wild New Astro Beam
By JOHN D. MARTINO
VP, Avanti Research & Development
A three-element beam usually consists of three half-wave dipoles. One is used as the driven element into which the power is fed. The other two are parasitic elements which are placed in front and in back of this driven element and used as a director and reflector. Theoretically in an assembly of this sort, the elements can either be spaced for maximum gain or maximum rejection, but not for both at once.
Well, a few years ago, one of the engineers at AVANTI came across with a brilliant idea. He reasoned that if a three-element beam was made with an ASTRO-PLANE antenna as a driven element, rather than a simple half-wave unity gain dipole, better results might be expected. After all, if you start with more power you must be better off than if you started with less power. The ASTRO PLANE proved uniquely suited for this purpose – no radials to interfere with the other elements, 4.46 db gain, and the ability to radiated from the top made it a natural choice.
However, there are a few more objectives to be considered. One of these was balance. Not only does an antenna need to be balanced as far as weight is concerned, but also as far as wind loading. Again the ASTRO-PLANE worked out well.
Because its design allowed it to be fit over the support mast, it would allow optimum spacing for gain as well as perfect balance on both counts.
By using a matching transformer which consists of a ¼-wave 35-ohm harness, Avanti not only eliminated the need for any exposed matching devices, but allowed the antenna to handle as much power as can be put through your coax. Incidentally, for those with eagle eyes, this phasing harness, due to the slower speed of propagation in the coax, only needs to be six-feet long.
After the company got the antenna up and with a little bit of tinkering, an unexpected benefit was discovered. Remember how we told you earlier that a three element beam was designed for maximum gain or maximum rejection? Well the 3-element Astro Beam using the ASTRO-PLANE as the driver, not only had more gain than the best 3-element beam, but also over 40 db front-to-back rejection!
The engineers working on this antenna didn’t know why (although they didn’t want to admit it), and set out to find why this phenomenon was occurring. As things developed, it was discovered that the coupling between the ASTRO-PLANE and the parasitic elements caused this unexpected effect. Coupling in this case refers to the efficiency with which one antenna element can be used to induce power into another.
Research & development insisted that models had to be constructed and subjected to at least one year of testing before they could be put on the market. Many times an antenna manufacturer will design and test a C.B. antenna on a higher frequency because the antenna is smaller and more convenient to handle. This method has its flaws however, because sometimes the scaling down will have an effect on the testing and an antenna that worked well on about 150 MHz will suffer at 47 MHz. Wishing to avoid this pitfall, Avanti designed and tested the first ASTRO-BEAM (as they have named it) at 27 MHz.
As far as mechanical construction, the elements are made from aircraft-quality drawn seamless aluminum tubing, so is the boom, while the hubs are molded of high-strength weather –resistant Cycolac. These molded hubs feature a pin lock design, which aligns the element. This feature also keeps the elements from twisting once they are up in the air – a problem so often encountered with many beams. The weight of this antenna is only 14 pounds, so it can easily be rotated by an inexpensive TV rotor, without going to more costly models.
We feel this antenna is going to be a winner. The CB’er needs an antenna capable of shutting off the unwanted noise and signals, especially in crowded areas where skip is prevalent. How great it would be to talk quietly and clearly for a change without being hammered by the confusion.
SPECIFICATIONS:
Rejection – 40db signal drop front to back
Forward gain – 11 db over an isotropic source
Impedance – 50 ohms nominal
V.S.W.R. – 1.3:1 or less
Wind survival – 80 MPH
Boom length – 10.5 feet
Weight – 14 pounds
Mode – Vertical beam
Turning radius – 63 inches
Material – Aluminum and Cycolac
Price - $59.95
TO EXPLAIN SOME OF THE SPECIFICATION
A 40db+ rejection means that if a signal is coming at a certain strength and the ASTRO BEAM is turned around so that the back is toward the signal, the signal will drop 40 db or more (see Polar Plot Graph).
Forward gain of 11 db over an isotropic source is a means of rating the antenna’s ability to increase receive and transmit signals. The point here is that the ASTRO-BEAM has about 1db more than the best 3-element beam currently on the market. The gain has the equivalent of multiplying about 12.5 times your power.
Impedance of 50 ohm nominal simply means that the antenna is of an impedance that allows it to match RG/8U or RG/58U cable used for C.B.
V.S.W.R. of 1.3:1 means that this antenna when built and installed correctly will have a standing wave of 1.3:1 or less. Most ASTRO-BEAMS will probably tune to 1.1:1 or 1.2:1. Anything below 1.5:1 is considered acceptable in professional fields, and to go below 1.3:1 will result in diminishing returns.
HOW TO MAKE AN OMNI INTO A BEAM!
The Development Story Behind Avanti’s Wild New Astro Beam
By JOHN D. MARTINO
VP, Avanti Research & Development
A three-element beam usually consists of three half-wave dipoles. One is used as the driven element into which the power is fed. The other two are parasitic elements which are placed in front and in back of this driven element and used as a director and reflector. Theoretically in an assembly of this sort, the elements can either be spaced for maximum gain or maximum rejection, but not for both at once.
Well, a few years ago, one of the engineers at AVANTI came across with a brilliant idea. He reasoned that if a three-element beam was made with an ASTRO-PLANE antenna as a driven element, rather than a simple half-wave unity gain dipole, better results might be expected. After all, if you start with more power you must be better off than if you started with less power. The ASTRO PLANE proved uniquely suited for this purpose – no radials to interfere with the other elements, 4.46 db gain, and the ability to radiated from the top made it a natural choice.
However, there are a few more objectives to be considered. One of these was balance. Not only does an antenna need to be balanced as far as weight is concerned, but also as far as wind loading. Again the ASTRO-PLANE worked out well.
Because its design allowed it to be fit over the support mast, it would allow optimum spacing for gain as well as perfect balance on both counts.
By using a matching transformer which consists of a ¼-wave 35-ohm harness, Avanti not only eliminated the need for any exposed matching devices, but allowed the antenna to handle as much power as can be put through your coax. Incidentally, for those with eagle eyes, this phasing harness, due to the slower speed of propagation in the coax, only needs to be six-feet long.
After the company got the antenna up and with a little bit of tinkering, an unexpected benefit was discovered. Remember how we told you earlier that a three element beam was designed for maximum gain or maximum rejection? Well the 3-element Astro Beam using the ASTRO-PLANE as the driver, not only had more gain than the best 3-element beam, but also over 40 db front-to-back rejection!
The engineers working on this antenna didn’t know why (although they didn’t want to admit it), and set out to find why this phenomenon was occurring. As things developed, it was discovered that the coupling between the ASTRO-PLANE and the parasitic elements caused this unexpected effect. Coupling in this case refers to the efficiency with which one antenna element can be used to induce power into another.
Research & development insisted that models had to be constructed and subjected to at least one year of testing before they could be put on the market. Many times an antenna manufacturer will design and test a C.B. antenna on a higher frequency because the antenna is smaller and more convenient to handle. This method has its flaws however, because sometimes the scaling down will have an effect on the testing and an antenna that worked well on about 150 MHz will suffer at 47 MHz. Wishing to avoid this pitfall, Avanti designed and tested the first ASTRO-BEAM (as they have named it) at 27 MHz.
As far as mechanical construction, the elements are made from aircraft-quality drawn seamless aluminum tubing, so is the boom, while the hubs are molded of high-strength weather –resistant Cycolac. These molded hubs feature a pin lock design, which aligns the element. This feature also keeps the elements from twisting once they are up in the air – a problem so often encountered with many beams. The weight of this antenna is only 14 pounds, so it can easily be rotated by an inexpensive TV rotor, without going to more costly models.
We feel this antenna is going to be a winner. The CB’er needs an antenna capable of shutting off the unwanted noise and signals, especially in crowded areas where skip is prevalent. How great it would be to talk quietly and clearly for a change without being hammered by the confusion.
SPECIFICATIONS:
Rejection – 40db signal drop front to back
Forward gain – 11 db over an isotropic source
Impedance – 50 ohms nominal
V.S.W.R. – 1.3:1 or less
Wind survival – 80 MPH
Boom length – 10.5 feet
Weight – 14 pounds
Mode – Vertical beam
Turning radius – 63 inches
Material – Aluminum and Cycolac
Price - $59.95
TO EXPLAIN SOME OF THE SPECIFICATION
A 40db+ rejection means that if a signal is coming at a certain strength and the ASTRO BEAM is turned around so that the back is toward the signal, the signal will drop 40 db or more (see Polar Plot Graph).
Forward gain of 11 db over an isotropic source is a means of rating the antenna’s ability to increase receive and transmit signals. The point here is that the ASTRO-BEAM has about 1db more than the best 3-element beam currently on the market. The gain has the equivalent of multiplying about 12.5 times your power.
Impedance of 50 ohm nominal simply means that the antenna is of an impedance that allows it to match RG/8U or RG/58U cable used for C.B.
V.S.W.R. of 1.3:1 means that this antenna when built and installed correctly will have a standing wave of 1.3:1 or less. Most ASTRO-BEAMS will probably tune to 1.1:1 or 1.2:1. Anything below 1.5:1 is considered acceptable in professional fields, and to go below 1.3:1 will result in diminishing returns.
