Jpole vs. Coaxial Dipole EZNEC Shootout

In a recent email from the ARRL concerning ARES topics, a writer compares the coaxial vertical dipole antenna with a J-Pole for VHF work. He says…

“With no real models for comparison, it is difficult to compare patterns for each antenna. I would suspect that the J-Pole would not have an omni-directional pattern and would therefore produce a lobe or lobes giving gain in some directions”

Really?

Larry Cebik’s antenna model collection contains hundreds of antennas to examine with NEC or EZNEC and the J-Pole is no exception. However, I did not find a coaxial antenna in his models. So I examined the original patent for the Coaxial Dipole antenna and found it was originally composed of several elements running parallel to the coax to provide the bottom half of the dipole. Great. The models for the J-Pole, the coaxial dipole and a reference typical monopole with 45 degree radials are shown below…

All three examples are simulated with their feed points at 300 inches above ground. The J-Pole enjoys a height advantage for obvious reasons. This advantage become more pronounced at lower elevations since the J-Pole’s height delta is a larger percentage of height above ground the lower the overall height is.

Height increases gain for all three antennas as the ground reflection begins to focus the horizon bound energy. No real surprise there. It is the relative gain patterns we really care about here. Here it is for Azimuth at an elevation about 3.5 degrees…

Indeed, you can see the J-Pole’s lack of perfect symmetry results in a slight benefit in certain directions as correctly assumed by the ARES article author. However, the difference is not very significant at less than 3 dB. Also, there are no lobes of gain or loss, in azimuth, like the author theorizes. Lobes do exist for elevation for all three antennas.

Here is a comparison of elevation…

The J-Pole enjoys the most gain at around 6 dBi with the other two antennas pretty close to this value. Again, all three antenna’s gain is very dependent on the height above ground. Thus, the J-Pole always has a slight advantage. Otherwise, the J-Pole is just a half-wave radiator like the coaxial dipole with a slight height advantage.

What are we to make of all this? Well, don’t listen to anyone who tells you one antenna is dramatically superior in performance to another. In the end, they all, more of less, behave like a half-wave radiator and all are a little better than a 1/4 wave monopole over horizontal radials.

The Monopole is the one that bests matches to 50 ohm coax thanks to the angled radials. The coaxial dipole simulates to around 80 ohms impedance which is close to the 72 ohm half-wave value. The J-Pole feed is, indeed, a bit complicated, but matches 50 ohms very well.

So put performance issues aside and concentrate only on the mechanical issues when selecting your next VHF antenna.

Also, you now know better if someone tells you there are no real models for comparison. Simulation has its limits, but is an excellent first step. You can rest assured an antenna invented in the 1930s has been modeled at least once.