jonny290
Member
Project time!
the open stub J-pole is built and tuned slightly differently from the standard J. It is directly fed between the two element bottoms (one side hot, one side ground) which are electrically insulated.
pros (open stub versus shorted stub)
1: single element can double as 'long' element for 440 and 'stub element' for 2m
2: easier to adjust, one less variable
cons:
1: not dc grounded (edit: this is mildly inaccurate: the longest element (and biggest static/lightning risk) is DC grounded, but the hot element is not.)
2: harder to physically build (insulator required)
my tuned dimensions:
-long element: 1.44 m, 3/4" copper pipe
-short element: .47 m, 1/2" copper pipe
-cross stub: 1/2" copper pipe, cut so element centers are 15.2 cm spacing
-440 element: 15.0 cm coathanger wire, 5.5 cm on center away from and parallel to .47 m element. just clamp it on with a hose clamp, aim it vertical.
-insulator: CPVC tee, 1/2". pl-259 epoxied to one leg.
Ffeed at end of cross stub(ground) and .47 m element (hot).
Cut pipes 1" short; shove pipe caps on the end, drill holes and solder brass nuts on. use 2" brass screws for fine adjustment.
1.3:1 or lower across entire 2m band, my reverse power meter does not move with 50w forward at 146.500. ballpark 1.02 to 1.05:1 swr.
1.5 or lower between 445 and 449 (everything is +offset here), 2.0 between 440 and 450.
If you don't want to buy a torch, and you still have a 100w soldering iron, you can physically adhere the chunks together with JB Weld, and then go back with the 100w gun and solder all the joints where they meet up. Saves ya 20 bucks and it's still really strong and connections are still solid. Give the JB Weld 24 hours to cure, 36 if it's cold weather.
Further notes: I don't think taking it past two bands would be doable; the whole Arrow J 'trick' is that the 1/4 wave stub doesn't care if it's fed with one polarity or the other. So at 2m, your hot terminal is the shorter or 'non radiating' side, but at 440 the hot terminal is the radiating side. You can only do this with 2 bands without talking about traps or other weird stuff.
Diameter of the elements is extremely important, as is spacing of the stubs
Once you get a J-pole adjusted for minimum reactance at your target frequency, you can adjust the feedpoint impedance down by shortening the stub and lengthening the radiator, and you can raise the impedance by lengthening the stub and shortening the radiator
always always always use a coax balun at the feed.
The 440 element does not affect the 2 meter side much at all (deleting the element and comparing it in mmana shows almost zero changes in behavior at 146.5). Detuning the long element has a minimal effect on 440 SWR. The .47 m element, as expected, has severe detuning effects on both bands. It's definitely the most important piece of metal in the antenna.
I'm cramming for my Extra tonight/this weekend so no pics just yet, but I may take it down and tune it a bit more; i'll take pics at that time.
the open stub J-pole is built and tuned slightly differently from the standard J. It is directly fed between the two element bottoms (one side hot, one side ground) which are electrically insulated.
pros (open stub versus shorted stub)
1: single element can double as 'long' element for 440 and 'stub element' for 2m
2: easier to adjust, one less variable
cons:
1: not dc grounded (edit: this is mildly inaccurate: the longest element (and biggest static/lightning risk) is DC grounded, but the hot element is not.)
2: harder to physically build (insulator required)
my tuned dimensions:
-long element: 1.44 m, 3/4" copper pipe
-short element: .47 m, 1/2" copper pipe
-cross stub: 1/2" copper pipe, cut so element centers are 15.2 cm spacing
-440 element: 15.0 cm coathanger wire, 5.5 cm on center away from and parallel to .47 m element. just clamp it on with a hose clamp, aim it vertical.
-insulator: CPVC tee, 1/2". pl-259 epoxied to one leg.
Ffeed at end of cross stub(ground) and .47 m element (hot).
Cut pipes 1" short; shove pipe caps on the end, drill holes and solder brass nuts on. use 2" brass screws for fine adjustment.
1.3:1 or lower across entire 2m band, my reverse power meter does not move with 50w forward at 146.500. ballpark 1.02 to 1.05:1 swr.
1.5 or lower between 445 and 449 (everything is +offset here), 2.0 between 440 and 450.
If you don't want to buy a torch, and you still have a 100w soldering iron, you can physically adhere the chunks together with JB Weld, and then go back with the 100w gun and solder all the joints where they meet up. Saves ya 20 bucks and it's still really strong and connections are still solid. Give the JB Weld 24 hours to cure, 36 if it's cold weather.
Further notes: I don't think taking it past two bands would be doable; the whole Arrow J 'trick' is that the 1/4 wave stub doesn't care if it's fed with one polarity or the other. So at 2m, your hot terminal is the shorter or 'non radiating' side, but at 440 the hot terminal is the radiating side. You can only do this with 2 bands without talking about traps or other weird stuff.
Diameter of the elements is extremely important, as is spacing of the stubs
Once you get a J-pole adjusted for minimum reactance at your target frequency, you can adjust the feedpoint impedance down by shortening the stub and lengthening the radiator, and you can raise the impedance by lengthening the stub and shortening the radiator
always always always use a coax balun at the feed.
The 440 element does not affect the 2 meter side much at all (deleting the element and comparing it in mmana shows almost zero changes in behavior at 146.5). Detuning the long element has a minimal effect on 440 SWR. The .47 m element, as expected, has severe detuning effects on both bands. It's definitely the most important piece of metal in the antenna.
I'm cramming for my Extra tonight/this weekend so no pics just yet, but I may take it down and tune it a bit more; i'll take pics at that time.
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