dual j pole antenna system

Discussion in 'Survival Communications' started by slimgray, May 12, 2016.


  1. slimgray

    slimgray Monkey

    Has anyone tried a dual j pole antenna system? What effect if any would this have?( I'm talking about actually hooking up two j pole antennas to the same transceiver)??
     
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  2. AD1

    AD1 Monkey+++

    Slim

    Any reason why you want to try this?
     
  3. slimgray

    slimgray Monkey

    I'm just curious seems to me two is better than one.
     
  4. stg58

    stg58 Monkey+++ Site Supporter+ Founding Member

    Hams are (were) always trying new "stuff" it is more old school home brew ham rather than new buy it out of the box.
    I have a single 2 Meter copper J-pole that works great at a remote site.

    Are you trying to do 2 meters and 70cm?

    Try it out make sure you keep your vswr is OK and let us know.
     
    Last edited: May 12, 2016
  5. BTPost

    BTPost Old Fart Snow Monkey Moderator

    I have built a few for my Dual Band Radios, but went to using a Rugged Marine type VHF Antenna designed for 146-148 MHz and just let the UHF side deal with the Third Harnonic issues... I need VHF a whole lot more than UHF, where I live. (Alaskan Bush ) I also need an antenna that will stand up to 130 Knot Winds, when covered in Ice... That is hard to come by in my world...
     
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  6. slimgray

    slimgray Monkey

    Just thinking about 2M. Not trying to do dual band. Just curious to find out what might happen to run dual antennas. Like on a big truck dual antenna system it radiates more front to back rather than a circular pattern. Is this what would happen to the 2M dual antenna system ??
     
    Last edited by a moderator: May 12, 2016
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  7. stg58

    stg58 Monkey+++ Site Supporter+ Founding Member


    One on 2 should be good :) but thanks for asking lots of good RF knowledge here.
     
  8. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    I'm not so sure that two is better than one in this case. That said, one thing is for sure, two antennas will present a dramatically different reactance to the transmitter. I'd be inclined to use an antenna analyzer and work out the kinks with a balancing network of some sort before loading the finals with an unknown.

    Might be an interesting experiment to see if setting them up "just right" would make them behave directionally.
     
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  9. slimgray

    slimgray Monkey

     
  10. slimgray

    slimgray Monkey

    I was thinking if it could work. That it would produce an extremely wide oblong shaped radiation pattern. Sort of semi directional. But I really don't know enough about antenna systems. That is why I'm posting all of my questions here. Before I ruin a good radio. Lol. Thanks for all the info.
     
  11. hitchcock4

    hitchcock4 Monkey

    I have been a ham for less than a year so I could be wrong here. I think that it could help but it could also hurt -- it will really depend on the space between them as well as other objects in the area. If one antenna received (sent) the same signal 1/20 of a second later due to bouncing of objects, then there is interference. I would personally stick to 1 J pole and try 10+ different locations to see which location is the best in your AO.
     
  12. azrancher

    azrancher Monkey +++

    You would need a splitter/combiner, and lead lines to both antennas need to be equal, and instead of an omnidirectional pattern you would have a lobe perpendicular to the center line of the two j poles.

    BT, correct me if I'm wrong, I'm just a wanna be Ham, and am only using my EE cap on right now.

    Rancher
     
  13. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    You're doing fine. The radiation pattern will depend on the distance (in wavelengths) between the poles. At about 1/4 wavelength separation, the pattern is quite circular. Go closer or further, you can get patterns in line with the axis, or perpendicular to it. I forget which, and too lazy to look it up this morning.
     
  14. AD1

    AD1 Monkey+++

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  15. Idahoser

    Idahoser Monkey+++ Founding Member

    you also have phasing to consider. I believe the truck antennas are "cophased", fed in parallel with identical lengths of feedline from the transmitter. You could also feed them at different phases by feeding them in series with a specific length of feedline between them (a half wavelength feedline would put them at 180°). I would bet they'll end up behaving much like verticals. There is a whole field of study involving this, what you want to do is learn how to model antennas on the computer. There is a free program that comes with the ARRL books, I think it's called EZNEC, but I'm not certain the free version is able to do multiple antennas. I'm sure some version of modeling software is available on the internet.
     
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  16. Yes, you can connect two vertical antennas together, in the same way two beams can be "stacked" to provide added gain. Combining vertical antennas, though, is almost always done in AM broadcasting, to place a "notch" in the signal so that a broadcast station doesn't send too much signal toward another station which has been on the frequency longer and has precedence in signal coverage.

    For ham radio, it's mostly for fixed stations that want to "null out" a source of interference, e.g., so as to reduce unwanted signal input to a repeater from mobiles on the fringe of another repeater's area. The only time I've seen it done myself was by hams was for a repeater in Boston, where the guys who designed it wanted to have a "teardrop" shaped pattern to avoid sending power out over the ocean and increase their coverage going westward from the coast.

    The ARRL Antenna book will have a variety of patterns you can look at, but remember that phased arrays are most useful for putting a notch in the array pattern, i.e., they can introduce loss in one fairly sharp null, while favoring a broad pattern a little more.

    HTH.

    William Warren

    (Edit to make more clear)
     
    Last edited: May 14, 2016
  17. Good point: the usual "phasing harness" for stacked beams is two 1/4 wavelength sections[1] of 75 Ohm coax (such as RG-59), connected to each antenna and wired in parallel at the feedpoint. The 1/4 wave sections transform the 50 Ohm impedance of each antenna to 100 Ohms at the feedpoint, and since they're connected in parallel, they present a 50 Ohm load.

    The pattern of your array will be determined by the distance between the two J-poles, and the compass bearing of an imaginary line run between them.

    Good Luck!

    William Warren

    1. Don't forget that the physical length differs from the electrical length for any piece of coaxial cable. The cable's "velocity factor" will determine the needed length, so you'll need to do some research: the good news is that the resulting "matching section" will always be shorter than the physical measurement would indicate. For example, two meters is 78.74 inches, and 1/4 of that is ~19.7 inches, but the actual length of the matching section will be 17.1 inches, assuming coax with a velocity factor of .87 (do not use that number in your calculations: look up the velocity factor that's specific to your manufacturer!).
     
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  18. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Also note: As long as the Phasing Harness is of equal length, AND a multiple of that 1/4 WaveLength, including the Velocity Factor Correction, it will work correctly.... With the antennas, multiples of 1/2 wavelength apart....
     
  19. Idahoser

    Idahoser Monkey+++ Founding Member

    see? Piece of cake. :D

    I always laugh when I see pickups with pairs of antennas for their CB radios. They think it's cool but the inspiration doesn't translate well. A big rig with antennas on the mirrors has a scientific basis, trying to copy the style on a pickup is just vanity.

    There's a reason they call ham radio a "hobby", there's a LOT to it.
     
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