Building Franklin JPole

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againzz

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Hi!

I want to build a JPole antenna and i would like to listen any advice.
Could you help me?

Best regards and thanks you,
 

majoco

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I've never understood what the bit sticking out of the bottom is for. The original Jpole was devised in the early 1900's to be hauled behind an airship and therefore had no mounting pole - this was one of the criteria to be overcome and the Jpole did it. Now it seems to have sprouted a mounting which is unnecessary.

see....
https://en.wikipedia.org/wiki/J-pole_antenna

and F.G. Rayer's original article for the "Slim Jim" - no conductive mounting pieces. My slim jim is constructed around a 32mm PVC pipe with 3mm ali rod - works a treat.
 

againzz

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The center frequency of antenna is around 700 MHz. But, the problem is that I need to have about 80 MHz of bandwidth and very directivity HPBW around 9 degrees and I don’t know as get it.

Thanks,
 

jonwienke

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Good luck with that. A J-pole antenna does not have a 9-degree emission pattern. To get that narrow of an emission pattern and wide of bandwidth, you'll probably require something like a 10-element log-periodic dipole array antenna with an expansion factor around 1.01.

Log Periodic Dipole Array Antenna
 

againzz

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Can i have a omnidirectional horizontal pattern with log periodic dipole Array antenna?
 

bharvey2

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Log periodic antennas are directional Remember the TV antennas that used to sit on everyone's roof in the past? those were log periodics.
 

ko6jw_2

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The bit at the bottom

I've never understood what the bit sticking out of the bottom is for. The original Jpole was devised in the early 1900's to be hauled behind an airship and therefore had no mounting pole - this was one of the criteria to be overcome and the Jpole did it. Now it seems to have sprouted a mounting which is unnecessary.

see....
https://en.wikipedia.org/wiki/J-pole_antenna

and F.G. Rayer's original article for the "Slim Jim" - no conductive mounting pieces. My slim jim is constructed around a 32mm PVC pipe with 3mm ali rod - works a treat.
Are you referring to the 1/4 wave matching stub? This is to allow a low impedance feed point since an end fed 1/2 wave antenna presents a very high impedance (theoretically infinite). The zeppelin antennas accomplish this with a 1/4 section of feed line. The theory is the same just the scale is different.

With regard to the actual question:

Making a j-pole for 700 Mhz would be impractical. The combination of the desired bandwidth and gain in an omnidirectional antenna would be difficult to achieve. High gain omnidirectional antennas will have a narrow bandwidth. High gain broadband antennas will be directional. If you find a solution to this problem let us know.
 

jonwienke

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Can i have a omnidirectional horizontal pattern with log periodic dipole Array antenna?
No. No antenna design will give you a 9-degree vertical radiation pattern, omni horizontal radiation pattern, and 80MHz of bandwidth withreasonably low SWR.

Pick any two.
 

againzz

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I have gotten an omnidirectional horizontal pattern and narrow band in vertical pattern (10 degrees). Until now, I only have done design with 4nec2.
Antenna have a bandwith of 90 MHz and the SWR is 1.5: 1 in this BW. The antenna have 3 verticals dipoles 1/2 wave and 2 horitzontal dipoles as elbows beetwen them of 1/2 too.
This afternoon if you want i would be able to upload a image with the geometry of antenna.

Thanks for all and regards,elbow
 

jonwienke

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Which is an antenna array you haven't actually built in the real world.
 

prcguy

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Those specs are not a big deal to design or build. When you get into the 10dBd gain range of a vertical omni the 3dB vertical pattern is in the 7deg range and at 700MHz it should not be difficult to achieve 80Mhz of BW.

A typical 10dBd gain antenna could be made from a 16 element colinear array made of 1/2 wavelength chunks of coax or an 8 element exposed dipole array. With about 30sec searching I found this one at 900MHz range with 6deg vertical beam width and 95MHz of BW. There may be an off the shelf antenna that meets the OPs specs. https://d2oc0ihd6a5bt.cloudfront.net/wp-content/uploads/sites/1819/2016/07/TWDS-7013.pdf
prcguy


No. No antenna design will give you a 9-degree vertical radiation pattern, omni horizontal radiation pattern, and 80MHz of bandwidth withreasonably low SWR.

Pick any two.
 
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