The antenna system

Foundations of Amateur Radio

Several years ago I participated in a local contest. Over a 24 hour period I activated my mobile station in about 30 different locations. On my car, my vertical antenna screwed into a boot-lip mount connected to an antenna tuner or ATU, and my radio. I used rope to guy the antenna, threaded through the rear windows and held tight by closing the car boot.

Setting up consisted of parking the car, triggering the ATU to tune the antenna system and calling CQ. Moving to the next location consisted of driving there and setting up again.

Although this worked really well, I'm skipping over what I'm interested in exploring today.

The phrase "triggering the ATU to tune the antenna system" hides a lot of complexity. It was a surprise to me that there were several locations where the ATU just wouldn't tune. Despite my best efforts I was unable to get the system to a point where the radio was happy. In some cases I tuned off frequency and put up with a poor SWR. In others I physically had to move the car and park somewhere else.

In every case it was completely unknown if a particular location was going to be a problem. I recall for example parking in an empty nondescript car-park and having to drive around to find a location where my set-up would work. Afterwards I considered that the car-park was potentially built on top of an iron ore deposit, an old industrial area, or a pipe-line, all of which were a good possibility.

The point of this is that an antenna doesn't exist in isolation, it's called a system for a reason. We talk about the theoretical isotropic antenna and add disclaimers about that it cannot physically exist because it's infinitely small. One often overlooked aspect of an isotropic antenna is that it's in free space.

Free space is defined as space that contains no electromagnetic or gravitational fields and used as a reference. It's a theoretical place. On Earth there is no such thing, there's a planet under your feet, but even in outer space there are both gravitational and electromagnetic fields that impact on an antenna and its performance.

Staying nearer to home, recently we had a discussion about how close two antennas can be together. A suggested rule of thumb was that they need to be at least one banana or 30 cm away from each other.

Similarly when we erect a dipole, there's recommendations around needing to have it mounted more than half a wavelength over the ground. Some sources say higher. I'll ask the first obvious question. Is that dipole completely straight? In other words, should the centre be half a wavelength above the ground, or should the ends, and how far should the ends be from their mounts?

My point is that every antenna exists within the context of its environment and together it's a system. Some environments help the performance of your antenna system and some don't. Depending on frequency, this might not be the same for any location, or antenna design.

To be clear, an antenna system consists of the antenna, the feed line and the clips that hold it, the tuner, the radio and its power supply, the mount and the space around it, the radials, the tower, the pigeon poop on the wire, all of it.

Until recently my process to get any antenna to perform in a reasonable manner was to set it up, connect an antenna analyser, scan the appropriate range, tweak the antenna, scan again, rinse and repeat until it arrived at something approaching useful, or until it was good enough.

If you recall, I recently added some loading coils to a telescopic antenna to attempt to make it resonant on 10m, so I could connect my Weak Signal Propagation Reporter or WSPR beacon to it directly and leave it running independently from my main station.

I used the antenna analyser method, got it to the point where I had an antenna with a nice dip right at the required frequency and then watched it go completely sideways when I mounted the antenna in the window.

Having spent several hours getting to that point, I walked away and left it for another day. Today was that day. I again started on the floor of my shack and got nothing but an infinite SWR and no amount of tweaking could fix it. Right until the coax fell out of the SMA connector I was scratching my head.

After removing the faulty coax lead, I again tweaked the antenna and instead of using my antenna analyser, I fired up my NanoVNA, a tiny handheld open-hardware Vector Network Analyser or VNA. If you're not familiar, it's a standalone palm sized device with an LCD display and battery which will allow you to test most of your RF equipment. This little box came to me via a generous gift from a fellow amateur. It can repeatedly scan a range of frequencies and report in near real-time what's going on. Instead of waiting a minute after each adjustment, I could wait less than a second and immediately see the effect.

This has been a game changer.

I could mount the antenna against a metal surface and immediately see what the impact was. I could see the difference between it being mounted horizontally, where it would sag, to it being vertical where it stayed straight. I could see the steepness of the SWR plot, see how the low point moved around, up and down the band, see what the depth or lowest SWR was at any point. I could see my hand approaching the antenna, how nearby metal objects affect the antenna, what made it better and what made it worse.

The reason that I'm talking about this is because it's the very first time that I was able to actually get a feel for what affects an antenna, in what way and by how much.

To describe an analogy, it's like watching someone play a theremin and hearing how their hands affect the sound. If you're not familiar, a theremin is an electronic musical instrument controlled without physical contact by the performer, named after its inventor, Leon Theremin who patented it in 1928. From the outside it looks like a metal antenna that you bring your hands near to change the field. The changes are converted into sound.

The NanoVNA gives you the same level of feedback, but does so visually in a quantitative way, providing you with the insight to adjust your antenna to your liking and taking into account its entire environment.

Does this mean that I'm telling you to go out and buy one today? Well, that's not up to me, but I am intensely grateful for it arriving at my doorstep.

I'm Onno VK6FLAB