In my review of the comments posted, I think I'm detecting a discussion of several different antennas going under a general category of "J-Pole."
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As Majoco points out, the "J" design is essentially an end fed radiating form of a wave guide, first introduced in the era of the Zeppelins- a long trailing wire of a half wave length radiator end fed thru quarter wave line. Without getting int the theory of this design, it basically follows a folded antenna analogy for an end fed antenna; fed at the centre of symmetry (a half wave section.)
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For a "J" antenna, the radiating section length becomes critical. It is at ground potential in relation to the matching section, both of which are impedance tapped to provide a 300-600 Ohm match. But the long element does not form part of the support mast. My reservations with the "J" is a mechanical issue- the construction of a stable matching transform'r, the selection of the proper feed points, and the stable isolation of this assembly from the support structure. This last one, its mechanical stability, is the bear for me; the mast can't be part of the antenna, although there are designs I have seen that do use it as such. Invariably these antennas exhibit high degrees of coupling (unfavorably) to other metallic structures, and their radiation patterns are off the charts.
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Which brings me back to my opening- is this a discussion of several antennas all being lump'd under a "J-Pole?" A vertical radiator with a Gamma matching arm looks just like a "J" - but isn't. Personally, I love Gamma matching -with it I can get almost anything to work as an antenna (albeit with some times really Half A*s'd radiation patterns but terrific 1:1 SWR's...)-- but a Gamma match doesn't make it a "J."
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I will fall back to my original statement- the necessity of adherence to design... otherwise we are comparing unknowns and getting a wild variety of results.
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Ok, enuff of the Professor's discourse....
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Here's a story of one of my experiences with a "kind'a" Zepp antenna. Since I was talking about 'kind'a J's" I think this is will be okay.
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On a project in the Far Far North, we were studying auroral stimulation. One portion of the tests involved flying high altitude aircraft carrying, among other equipment, instruments measuring very very low radio frequencies (<10 khz's.) These planes trailed what can most poignantly be describ'd as a "very Very! long antenna wire, " .... so long in fact Notice to Airmen were filed to warn other aircraft to stay well away from us. This wire- this Cable, had to be of a considerable diameter bronze alloy to trail a long distance behind the aircraft- it was Sturdy!
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As luck would have it, on one mission the winch that retracted that cable before landing (but of course it had to be pulled in !) malfunctioned. It indicated to the crew that the entire length had been recover'd-- when in fact,,, well, you can fill in the blanks.
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As the plane swept'd over the airfield outer markers, that cable and its attached trailing cone, snached up VOR antennas, runway lights---- Ye god's.. what a bloody mess it made.
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Someone (anonymous ) had to do a lot of soft pedalling to patch that muck- up... I wonder'd at the time how many Zeppelin captains had to do the same .....
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....................................CF
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As Majoco points out, the "J" design is essentially an end fed radiating form of a wave guide, first introduced in the era of the Zeppelins- a long trailing wire of a half wave length radiator end fed thru quarter wave line. Without getting int the theory of this design, it basically follows a folded antenna analogy for an end fed antenna; fed at the centre of symmetry (a half wave section.)
.
For a "J" antenna, the radiating section length becomes critical. It is at ground potential in relation to the matching section, both of which are impedance tapped to provide a 300-600 Ohm match. But the long element does not form part of the support mast. My reservations with the "J" is a mechanical issue- the construction of a stable matching transform'r, the selection of the proper feed points, and the stable isolation of this assembly from the support structure. This last one, its mechanical stability, is the bear for me; the mast can't be part of the antenna, although there are designs I have seen that do use it as such. Invariably these antennas exhibit high degrees of coupling (unfavorably) to other metallic structures, and their radiation patterns are off the charts.
.
Which brings me back to my opening- is this a discussion of several antennas all being lump'd under a "J-Pole?" A vertical radiator with a Gamma matching arm looks just like a "J" - but isn't. Personally, I love Gamma matching -with it I can get almost anything to work as an antenna (albeit with some times really Half A*s'd radiation patterns but terrific 1:1 SWR's...)-- but a Gamma match doesn't make it a "J."
.
I will fall back to my original statement- the necessity of adherence to design... otherwise we are comparing unknowns and getting a wild variety of results.
.
___________________________________________________
.
Ok, enuff of the Professor's discourse....
.
Here's a story of one of my experiences with a "kind'a" Zepp antenna. Since I was talking about 'kind'a J's" I think this is will be okay.
.
On a project in the Far Far North, we were studying auroral stimulation. One portion of the tests involved flying high altitude aircraft carrying, among other equipment, instruments measuring very very low radio frequencies (<10 khz's.) These planes trailed what can most poignantly be describ'd as a "very Very! long antenna wire, " .... so long in fact Notice to Airmen were filed to warn other aircraft to stay well away from us. This wire- this Cable, had to be of a considerable diameter bronze alloy to trail a long distance behind the aircraft- it was Sturdy!
.
As luck would have it, on one mission the winch that retracted that cable before landing (but of course it had to be pulled in !) malfunctioned. It indicated to the crew that the entire length had been recover'd-- when in fact,,, well, you can fill in the blanks.
.
As the plane swept'd over the airfield outer markers, that cable and its attached trailing cone, snached up VOR antennas, runway lights---- Ye god's.. what a bloody mess it made.
.
Someone (anonymous
) had to do a lot of soft pedalling to patch that muck- up... I wonder'd at the time how many Zeppelin captains had to do the same ......
.
....................................CF
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