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I'm building my first J-Pole and the different projects and articles I read about differ slightly but one commonility is a J-Pole does not need to be grounded. Why is that?

I read that it is optional to ground the J-Pole antenna to deal against lightning strikes but does not add benefit to the quality of transmission.

Please feel free to correct my current knowledge or add what you do for your J-Pole as I am a newbie to building these. Thanks!

john_ham_radio
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    While it isn't directly grounded, your feed into your shack should be through a lightning arrester and then into a good ground... – David Hoelzer Apr 08 '23 at 23:35
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    counterquestion: *why* do you *think* an antenna needs to be grounded? I think that's the surprising assumption here! (Occam's Razor says that you should go with the model of the world that needs to make the least assumptions. Not assuming an antenna needs to be grounded is one assumption less…) – Marcus Müller Apr 09 '23 at 09:20
  • @MarcusMüller I suppose I could test the difference by grounding the antennae versus not. But I would say the grounding has to happen because (and this is going to sound weird since I don't really understand how electrons work) I reckon the potential for electrons is higher if the antenna is connected to a place for the electrons want to go. So in a sense the antenna is "pulling the signal" better if the electrons in the antenna are going somewhere. (This is coming from a guy who can calculate Ohm's Law, etc. but doesn't really understand the nature of electricity.) – john_ham_radio Apr 10 '23 at 04:18

2 Answers2

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There are 3 kinds of ground in radio:

  • Lightning ground
  • Electrical ground
  • Antenna ground

Electrical ground is irrelevant for antennas.

Antenna ground is a misnomer, it's really just the other half of the dipole when working with a quarter wave monopole.

J-pole antennas don't need to be "grounded" because they are a half wave end fed dipole instead of a quarter wave monopole. Anything below the short of the parallel matching section is technically not part of the antenna.

user10489
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The ground you are referring to is a slight misunderstanding. True, some antennas require a ground to function correctly but the reason for that is the ground is the other half of the antenna, such as a marconi or ground mounted vertical, the ground is the other half of the 1/4 wave vertical.

Horizontal dipoles don't require a 'ground' because both 1/4 wave sections are already there and there is no need for any more connections to make an antenna.

A J-pole is a end-fed vertical which is already 1/2 wavelength long (or longer) with a matching stub at the bottom to meet SWR requirements for the coax. No ground is required because it's 1/2 wave or longer to start with. You can ground a J-pole for lightning protection simply by mounting it to a metal grounded mast.

A basic law of almost all antennas is that they form a complete circuit, that is, a minimum of an electrical 1/2 wavelength to be efficient. A dipole meets this by using 2 quarter wave sections joined in the middle. Most car antennas require the metal body of the car to complete the other half of the antenna to complete a full 1/2 wave.

The loop antenna is an exception to this rule as its a complete circuit tuned to a particular frequency. There are others too. But most people use antennas based upon 1/2 wavelength total with 2 quarter wavelengths jointed. All yagi antennas are built this way. The J-pole is similar to the bazooka antenna but its length is still a 1/2 wave or 5/8th wave or even 3/4 wave and the U-shaped joint at the bottom is a 1/4 wave tuning stub and thus no outside ground is required.

user3486184
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T.Bren
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