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"J-Beam" Antenna



									26 August 2002
	WA9AFM/5 de S.I.

At 21:23 8/21/02 -0500, you wrote:
>Does anyone recall the 'J Beam' antenna?  I believe it was manufactured in
>Britian.  Its' elements were bent to opposing 45 degree angles at the end.
>Are they still made?  Who made them originally?
>
>Tom Webb, WA9AFM/5
>
	***************************************************************
	Tom,

	Yes it was a most interesting antenna design. It had lots of gain and a
very wide bandwidth, covering 420-450 MHz. The bandwidth was particularly
attractive to ATV operators who have wideband channels at 420-426 MHz and
438-444 MHz (and others in between). Its official name was MULTIBEAM. There
were three models, the MBM28, MBM48 and MBM88. There was also a "68"
manufactured for a year or so.

	The design consisted of four (in a box formation) very close- spaced Yagis
fed with a common "skeleton slot" driven element and a common slot
reflector. There were also two dipoles on the boom; these were located
close to the driven slot. They were called phasing elements. Taking the
"28" as an example; the slot reflector, the slot driven element plus the
two dipoles counted as four elements. There were six "X" shaped directors
(from each equivalent Yagi); this counted as 24 more elements. Hence the
total was 24 + 4 =28 for the MBM28!

	Similarly the "48" had 11 "X" directors and the "88" had 21.

	All of the "X" directors were identical. If you wanted more gain, you just
added more "X" directors. There was virtually no change to the input
impedance.

	The "Skeleton Slot" antenna was conceived and developed by Bill Sykes,
G2HCG, in the late 1940s. He obtained a patent and the design became the
basis of his company, Jaybeam Limited. The original Skeleton Slot was a 2m
antenna. It was basically a vertical Loop Antenna that was fed, with a
balanced feed, on opposite sides; hence the polarisation was horizontal.
The loop was not circular, nor was it square. It was "stretched" vertically
into a rectangle. He then added a horizontal Director and a Reflector
element in the plane of the top and bottom sections; hence he had a pair of
3-element beams. But the driven elements were not "full length" dipoles.
They were just the centre section of equivalent dipoles (where the current
is maximum, which is what you want). Later 2 more Directors were added at
each level making it a pair of 5-element Yagis stacked one above the other
and having the common slot driven element. The D5-2M, as it was known, led
to the D8-2M by adding 3 more directors to each Yagi. This was a very
popular antenna in England and in Europe. The D8-2M was also imported and
sold for several years in America. 

	The MULTIBEAMS were a further development. The four (2 by 2) close-spaced
Yagis director assemblies were driven from the common, horizontal polarised
slot, Driven element. The common Reflector was also a slot element. A
unique mechanical design problem was to support the director elements with
a minimum of metal (less weight, less manufacturing cost and less
distribution cost). This was solved by mounting the directors on a single
boom, that was located in the middle of the structure, by using a molded
insulator on the boom at each director location. The inner ends of the
directors were bent towards the boom and held in place by the insulator.
The insulator kept the four director elements at each boom location
insulated from each other and from the boom.

	You ask "Are they still made?". Unfortunately NO. After Bill Sykes retired
from active management, a French Company purchased Jaybeam. They quickly
stopped production of all domestic TV antennas and a year or so later shut
down the Amateur antenna line. Jaybeam continued to make commercial HF, VHF
and UHF antennas and also a new line of VHF/UHF mobile whips.

	73s,   John  G3BVU/W1.
		
	Yours truly,

	C.J.Beanland, MSEE, C.Eng., MIEE, MRAeS.
	Sales Manager,
	Spectrum International,Inc.	spectrum@spectrum-intl.com
	P.O. Box 1084,			Tel: (978) 263-2145
	Concord, Mass. 01742.		Fax: (978) 263-7008

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