Ground a jpole or not?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KI5CXX, Feb 21, 2019.

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  1. KI5CXX

    KI5CXX Ham Member QRZ Page

    I am building a 2 meter jpole antenna, and I read pros and cons for connecting the antenna directly to a metal mast, or ground isolate it. I have a 21 foot pole I want to mount it on, thought about isolating the antenna with a piece of pvc. If I do, how long should I make the pvc, at least a wavelength, or more? What do the antenna experts say here?
     
    KX4O likes this.
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The idea of insulating a j-pole from its supporting mast is to help prevent the mast from being part of the antenna, since j-poles are not well decoupled.

    All you'd need is a few inches of PVC slipped over the metal supporting mast where the antenna attaches, to use as an insulator -- no "wavelengths" involved. But even then, the outer conductor of your coax is still part of the antenna (common mode), so it pays to install a choke balun just below the feedpoint to prevent that from happening. Just a few type 43 cores that fit snugly over the coax jacket and are zip-tied or heatshrinked (or something) to hold them there works for this.

    So, the advantages are: Possibly a better-performing antenna where neither the coax nor the mast will become an undesired part of the antenna.

    The downside is: Now, your antenna isn't grounded and is more susceptible to lightning. Personally, I like "grounded" antennas, and the easiest way to accomplish that with a j-pole is to use a very good conductive (metal) mast that is well grounded to earth at its base, and then ground the coax on the way into the house also, and create a single-point-ground by bonding that point to the electrical utility ground (at the service panel), so a lightning surge can't create a huge potential difference between the earth ground outside and the station ground inside (which is provided by your utility ground).
     
    KX4O likes this.
  3. KI5CXX

    KI5CXX Ham Member QRZ Page

    Thanks for the reply. Yes, for lightning safety I would feel much better with a good ground, and properly grounded coax, which I would do anyway. Think I will go with the grounded setup and experiment to see how swr's look, as i don't have an analyser. On the wavelength question, I wondered if it being closely coupled would affect the antenna's performance, as feedline lengths appear to. My thoughts being that isolating the ground would combat this.
    I like the idea of bldg. antennas vs just buying one. Being new to this hobby I thing homebrewing is a lot of fun. As a lifetime hvac/refrigeration guy, fab'ing copper pipe is simple. New tech studying for general.
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The j-pole is a vertical, and the mast below it normally has zero influence on "performance" at all. The only issue is decoupling the antenna from the conductive mast, to prevent the mast from actually becoming part of the radiating antenna system. SWR has nothing to do with it. SWR should be fine if you use a grounded mast.

    But as noted earlier, even with a non-conductive mast, the coax feedline is still part of the radiating antenna system because j-poles are poorly decoupled antennas. Now, if you add a set of horizontal radials to the system, 1/2-WL below the antenna feedpoint, that can provide reasonable decoupling and improve antenna performance -- this is exactly what Cushcraft discovered decades ago when they improved the "Ringo Ranger" to create a "Ringo Ranger-II" model that adds radials. The "Ringo" actually is a j-pole but instead of being formed like a "J," using a 1/4-WL of parallel line to match the antenna, they wrapped the matching network around the base of the antenna to create a tapped transformer which does the same thing but is adjustable and takes up less space. That's why the CC Ringo has been a popular antenna since the early 1970s.

    The Ringo is also made of thinwall aluminum alloy tubing, which is very strong but much lighter than copper so while providing exactly the same electrical performance as a common copper j-pole, it's about 1/4 the weight so it's easier to install and keep installed.:)

    The j-pole is a very old design that has been much improved by such techniques. Although it's 3/4-WL long, it's only a 1/2-WL radiator: The matching section occupies 1/4-WL in height without adding any gain or anything.
     
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  5. KX4O

    KX4O Ham Member QRZ Page

  6. KI5CXX

    KI5CXX Ham Member QRZ Page

    I had previously read the articles you posted which added to my questions about mast grounding affecting performance. I have already constructed a 2 meter vertical dipole, and have not mounted it on a mast yet, but it seems to perform well with my yaesu handheld. I most likely will build a jpole and play with it, and buy a commercial antenna and mount on a mast where I can properly ground for safety and peace of mind.
     
  7. W6KCS

    W6KCS Ham Member QRZ Page

    I've wondered why someone would prefer one of these over a simple ground plane with drooping radials. Is it because the angle of radiation is a little lower?
     
    NH7RO and K2XT like this.
  8. KX4O

    KX4O Ham Member QRZ Page

    Angle of radiation of the two appear to be more similar than different... a wash IMO. The J wins on DC to ground robustness and a more slim profile (although when fed with external feedline they are quite sad in appearance). The GP handles the feedline more elegantly although there is nothing stopping someone putting the feedline inside the J if made with pipe. To be honest though, there is just something cool about an antenna of one continuous piece of conductor, easily made with copper pipe, that operates as an antenna merely by proper shape. To understand the J is to understand a lot about antennas.

    All that said, the Diamond CP22e sits on my roof and is my go to antenna for volunteer work in the woods requiring 2m. If I need a relatively modest antenna that is also its own lightning rod (with mast decoupling stub of course), the J will get the nod. Aren't antennas a marvelous thing? Viva la choice!
     
    W6KCS likes this.
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Not really...most commercial/military ground planes are completely DC grounded by using a folded unipole radiator like this:
    [​IMG]
     
    NH7RO, WA7PRC and KX4O like this.
  10. KX4O

    KX4O Ham Member QRZ Page

    Ah yes of course... I did forget about this design. Thanks!
     

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