Foundations of Amateur Radio

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Fan Vertical Antenna

Posted: 10 Jan 2019 07:52 PM PST


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...
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3