BUILD THE J BEAM FOR 2 METERS AND
"Recently, while going thru some of my older Ham magazines,
I ran into this article by Ed, N3SDO
It struck me that some of you out there may not have seen
this fantastic project or maybe you do not get CQ VHF magazine,
which is back now better than ever!
LET'S GET STARTED
Now you've probably heard of a beam antenna, and maybe you've heard of a J-Pole.
But what is a J Beam?
It's a vertical directional antenna made of 1/2 inch copper pipe and wood or PVC. It uses a standard J-pole antenna as the driven element and center support, with two parasitic elements-----a reflector and a director, to provide directivity and gain. See J Beam pattern below. It can be built for around $15.00 (1998 prices) and you can use your old Jpole as a basis for the JBeam. You'll probably need to shorten the main 1/2 wave element by 1 to 2 inches, as the reflector and director tend to couple and lower the resonance of your original jpole toward the lower part of the band. See formulas for cutting to your desired frequency of operation or band . A small amount of trimming on the tip of the main active 1/2 wave element may be needed to get the SWR as low as possible.
You shouldn't have to adjust the 1/4 wave matching stub, as this is not strongly affected by the parasitic elements.
Design Details and Drawbacks
The reflector and director element spacings are equal at 16 inches. Slightly higher gain with reflector spacing at 18 inches from driven element and director spacing of 14 inches can be had but the antenna may tend to lean with unequal spacings if strong mast mounting is not used.
The bandwidth is not as wide as that of the standard jpole by itself. The SWR tends to rise faster toward the ends of the band, but you get an estimated 7dB gain and 20 dB F/B (front to back ratio).
High wind loading could potentiallly break off the jBeam at the bottom joint since the entire structure is supported by that point. You may want to reinforce it.
The lengths used in these plans are for the upper part of 2 meters and you can use the formulas to design it for your frequency.
1/2 inch copper pipe is used for all elements and standard soldering made with a small blowtorch. A soldering iron is just not hot enough!
Longest element = 58 1/2 inches (add about 2 feet for mast mounting) matching 1/4 wave stub = 20 1/4 inches
Position matching stub toward director for a bit of added gain reflector- 40 inches
director = 32 inches
Stub and main element spacing = 3 inches
Coax feed point = 3 to 5 inches from bottom
(use hose clamps for feed point coax attachment connections for easy repair/adjustment)
Center boom is 1/2 inch by 1-inch wood or PVC and about 36 inches long
Measurements should be made as close as
possible but are not extremly critical!
"One fellow built one with five
elements-----just cutting the extra directors shorter by eye-----and was
able to work a repeater 35 miles away, with a 5-watt HT holding the
antenna, from a valley that's hard to get out of with 50 watts and an
omnidirectional antenna at 30 feet!"........Ed Bathgate
J Pole Long element (in Inches) = 8568 / F mhz
J pole Stub (in inches) = 2952 / F in mhz
Reflector (in inches) = 5880 / F in mhz
Director (in inches) = 4704 / F in mhz
Spacing (in inches) = 2352 / F in mhz
Some examples for other bands:
REF : 26.52"
"A PICTURE IS WORTH A THOUSAND WORDS"
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