On the Nature of Interacting Antennas

As some of my friends know, I enjoy building antennas in my free time (whenever that occurs) for my Amateur Radio hobby (which keeps me from being bored during my free time).

A while back I built a 2m Copper “Super-J-Pole” antenna which I attached to my chimney and use on a daily basis. Then a friend of mine (AD6IL) was kind enough to supply me with a 2m square loop for horizontal work which I attached at the bottom of the J-pole (for lack of a better place at the time). AD6IL and I were debating how much of a problem it would be to have them so close together and I figured I’d give it a try and see how much they interacted. If nothing else I’d learn from it. After putting them both up they both performed adequately though I was pretty sure I could detect a performance difference in the J-pole after adding the square loop to its base.

Recently another friend (K9RTY) loaned me his antenna analyser and I measured the performance of both antennas as they were mounted together and then moved the square loop to the opposite corner of the chimney and remeasured them both again. The following 4 graphs show the results of these tests.

For those without much background in antenna theory the goal here is to achieve a SWR of 1 (impossible), which is shown in red and is reflected in the left-hand Y-axis. The reactance should ideally be 0 and the impedance and resistance should be 50 ohms (in this case at least) and are all reflected numerically in the right-hand Y-axis. No antenna is perfect, however, but the goal is to shoot for getting as close as possible to ideal near the frequency where you want to use the antenna. In my case, the desired center frequency of the J-pole is at 147.000MHz and the desired center frequency of the square loop is at 144.200MHz.

2m Super-J Pole Graphs

These two graphs show the measurements taken on the Super J-Pole before and after the separation.

2m Square Loop Graphs

These two graphs show the measurements taken on the 2m Square Loop before and after the separation.

Conclusions

It is, of course, a no-duh that antennas affect each other when placed in proximity to each other. But these graphs show this interaction clearly in a real-world scenario.

For the J-Pole, the SWR (the poor-mans antenna measurement figure) has definitely dropped to a better range in general. The reactance has also improved, which means the antenna is closer to resonance which is the ultimate goal of any antenna (especially for transmitting). The square loop shows similar (even slighter bigger) improvements.

On the Air

Hooking them up to a radio and talking with friends (WB6ISO at ~20mi away and K6ERF at ~40mi away) I was told that my signals were definitely improved by an S-unit or two since the previous conversations. Of course, on-air measurements rely on constantly changing propegation conditions and thus require many more data points for a valid comparison but in this case they’re backed up by graphs that likely indicate the small sampling of on-the-air measurements were likely accurate.

1 Comment

  1. Frank Reshke, N6SNO Said,

    May 18, 2010 @ 9:14 am

    Great article Wes! Very informative. I’m on the roof as I type changing my j-pole!
    73, Frank