Foundations of Amateur Radio

Foundations of Amateur Radio

One of the single most recurring topics within our community is that of antennas. Everywhere you look is a story or a photo or a website or a contact about an antenna that came into being because somebody had an idea.

Now if you've been in the ideas field for a while you'll have learnt that having the idea is often just the start of the process. After that there's planning, sourcing, building and testing. If you're lucky you'll end up with something and a story to tell. If you manage to persist you might even end up with a working antenna.

The other day I managed to have an idea that I'd not seen anywhere else. As it turns out and perhaps not unsurprisingly, I'm not the first to have this idea. Despite that, what struck me is that I'd not seen or heard of this combination of antennas before.

As you might recall, one of my earlier forays into antennas consisted of purchasing a set of mono-band antennas. I intended to use these on my car while operating mobile, but despite countless unsuccessful attempts at making them work, the project ended up being abandoned and written up as a learning experience.

That said, each of these antennas works just fine on a roof, just not on the roof of my car.

Recently I'd been reading about how much separation is needed between antennas that are resonant on different HF bands and my research unearthed the idea that while they might affect each other to some degree the overall effect appears to be not that large. Combing that with an antenna called a fan dipole, I wondered if I could do the same with some vertical antennas. As it turns out, yes you can. It's sometimes referred to as a fan vertical.

Before I get too carried away. A fan dipole is an antenna that consists of a set of dipoles that are all fed from the same feed point. Imagine three or four dipoles, each for a different band, with each centre connected to the same balun. Each of the legs are spaced apart so they're not touching. After a bit of tuning you'll end up with a combination antenna that works on each band. The beauty of this is that it takes up the same amount of space as the largest dipole and you'll only need one feed line, rather than several. You'll also only need two sky hooks, so you won't have to plant a forest before setting up your antenna farm.

For all those reasons I wondered if I could make a single feed point for all my vertical antennas and get the same benefits. At one point I got so excited that I started modelling this in cocoaNEC, an antenna modelling tool based on NEC2, but my learning curve exceeded my skill set, so I had to postpone that in order to actually do some income generation instead.

Discussion with fellow amateurs encouraged my tomfoolery, unearthed prior work and assured me that it would work and since then I've started down the procurement phase and have now got some SO239 connectors, a piece of metal and ideas to space holes evenly with a central socket to connect my coax to. I plan to solder all the connector centres together with some thick copper wire and use the metal plate to connect all the shields together.

The only fly in the ointment at this point is my unhealthy relationship with drills. You might remember that I managed to drill a hole in my hand a while back - all healed, I was incredibly lucky, a delightful scar to remind me - so if at all possible I'd like to avoid such a thing. Last time all I wanted was to make a single hole bigger, this time I've got four 16mm holes to drill.

You'll be pleased to learn, just as my partner was, that I'm now able to use a drill press and I even splurged and added a vice, so if I'm not too clumsy, I should be able to avoid stitches this time around.

What I'm hoping to achieve is a little group of vertical antennas, connected to the same coax, mounted on the metal roof of the house, all but invisible to our neighbours without needing to swap antennas in and out like I currently do and actually use those lovely mono-band antennas I purchased so long ago. I may have to experiment with radials and tuning and no doubt there's still a gap between theory and reality, but I'll let you know how I go.

My question to you is, what antenna project are you working on?

I'm Onno VK6FLAB