Rabu, 29 Februari 2012

G3TXQ Hex Beam For 5-Bands Built By WY3A

The G3TXQ Hex beam performs beyond my expectations.

In early 2008, I constructed a 5-band G3TXQ Broadband Hex Beam carefully following the K4KIO building instructions.  I wanted something lightweight to cover 5 bands, something that would handle power, something less expensive and less complicated than the 2 element Stepp-IR.  The Hexbeam intrigued me enough to subscribe to the HexBeam Group where I learned that the design has many satisfied buyers and homebrew builders.

The wires in the classic HexBeam attach to a light-weight fiberglass "upside-down umbrella" frame (see photo to the left) with the following geometry for each band:
 Classic HexBeam Geometry
In multiband versions, 20M elements sit at the top of the structure with 17M, 15M, 12M and 10M stacked below.  Fed with single run of coax, you end up with separate 2 Element Yagis on each band with useful gain and excellent F/B. 
However, the classic HexBeam is a bit narrow, displaying SWR > 2 across major portions of most bands, when tuned for best directivity.

The G3TXQ Broadband Innovation
In the autumn of 2007 G3TXQ (Steve Hunt) conducted reflector experiments in an attempt to broad-band the hexbeam.  An exciting hybrid scheme emerged which modeled and tested very well.  The driven elements were kept in the classic "W" or "M" shape.  However, the reflector was run around the fiberglass support structure to form a large "C" or "U".  This keeps the reflector as far away from the center post as possible.  It also makes the antenna about 15% bigger than the classic HexBeam. 
G3TXQ Hex Beam Geometry

When incorporated into a 5-Band array, this geometry produces F/B > 10dB and SWR < 2:1 across all of the 20m, 17m, 15m and 12m bands, and approximately 1 MHz of 10m.   That is a fairly dramatic improvement over the classic Hexbeam.  This design caught my attention, but I wasn't sure I knew how to actually build the antenna.

Working closely with G3TXQ, K4KIO (Leo Shoemaker) made his own G3TXQ Hex and provided detailed building instructions on his site to help those interested in home brewing the new design affordably.  (Update: QST published this design in March 2009 and K4KIO now offers a commercial Broadband Hex Beam kit.)
Making and Assembling the G3TXQ Hex Beam
With K4KIO's fully illustrated pages, I had a virtual Complete Idiot's Guide to building the antenna, and no more excuses for not getting started.  I began buying and ordering stuff using Leo's Parts-needed list, which even tells you which vendors carry the products.  Fiberglass tubes for the spreaders and center post were ordered from Max-Gain Systems.  I fabricated my own base plate per Leo's plans, which was a little intimidating for a guy who is more comfortable with a keyboard or a soldering iron than a reciprocating saw.  But in the end, it was no problem, even for me.  The majority of the work was done at a leisurely pace, indoors, during the winter months. 
Assembly was not difficult when the warm weather arrived (see construction & assembly photos below).  Once assembled, no tuning was required as the SWR looked good on all bands with the base at only 4 feet.  The antenna is a near perfect match to 50 ohm coax on all bands. I now have the base at 50 feet on an AB-577 surplus military mast (see comments on the mast below).  This height is a good compromise for signals arriving at various angles (See Hex Beam Height arrival angle studies by G3TXQ).
Performance has been simply outstanding.  I know what to expect from a directive array as I have owned and used 3 element trapped triband yagi antennas before. To say that this beam performs beyond my expectations, would be an understatement. 
Construction Photos

The K4KIO design has undergone revisions since I built this antenna. 
K4KIO now recommends two floor flanges instead of one -- a very important improvement.
You can also buy a
commercial K4KIO Hexbeam
which features a coaxial aluminum center post
requiring no wiring harness at all
(very cool).
Homemade Baseplate with U-Bolts and Floor Flange
Painting the Spreaders and Connector Blocks
I like the light blue gray paint color I chose here. 
The antenna is less visible against the sky on most days
Base, Spreaders, Wires, Spacers Coax Feeders
20 Meter Wires Installed
Wires for Other Bands Installed
Base and Center Post Detail
Dacron / Kevlar Spacers Between Driven Elements and Reflectors
G3TXQ Broadband Hex and Rotor On AB-577 Launcher
I used an old ALLIANCE  U-100 Rotor which will handle 5 sq. ft of wind load
Homebrew G3TXQ Broadband Hex Beam (Base at 50 feet)

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