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Flowerpot Antenna for 2m & 70cm (UK)
The Flowerpot antenna is an interesting yet simple design that is
constructed from one continuous length of RG58 coaxial cable. Although
there are many websites that feature this antenna, the dimensions given
seem to differ slightly from each other, so who's right? Being
essentially
a centre fed half wave dipole (this is always debated), I guess it all
depends on
things
like the coax cable used, coil diameter, number of turns and
positioning etc. Also, the 2m band in the UK is narrower than in some other
parts of the world, so dimensions will vary a bit depending
on location. To give credit where credit's due, it seems
that the term 'Flowerpot Antenna' was coined by John, VK2ZOI who was inspired by an article written by Ian, VK3AYK.
I hope I've got that right! The version described here is a little
unusual in that it's dual band and of open construction when usually the dual
band versions are housed in a PVC pipe. Being 'open' it will not be waterproof and so is best suited for hanging up in a loft space. The image on the right (not to scale) shows the dimensions that worked for me after much experimentation, but as with all things
homebrew, I can't vouch that they'll work for everyone because
situations differ.
RG58 is possibly the worst coax
cable to use for VHF and especially
UHF, but in this application just for the antenna elements it's fine.
So I suggest keeping the 'tail' from the base of the coil to
the RF connector as short as possible (about 1cm) and then use better
quality cable to the shack. Referring to the image on the right, it all
starts at the bottom of the antenna by
taking a two metre length of RG58 coax and fitting an RF connector of
your choice. I use an N type line socket not just because N's are best
but
because the antenna is fed with Messi & Paoloni Ultraflex 7
with an N plug at each end. The length of coax between antenna and
shack is not critical, but the shorter it is the better to keep losses
to a minimum. From the connector you've just fitted, wind 12 turns of
coax onto a 65mm length of 21.5mm PVC overflow pipe. 65mm was chosen
because RG58 is 5mm in diameter and 12 turns side by side is 65mm to
the extremities where a couple of cable ties are used to hold the turns
in place. I could have made the pipe a bit longer and fed the cable
through
two holes to keep the turns in place, but although this is neater, it's
possible for the coax to be damaged due to the very tight bends at each
end.
The antenna effectively starts at the top end of the coil, so from this point,
measure up 465mm and carefully score around the outer insulation with a
craft knife taking care not to cut into the inner conductor. Remove the
remainder of the insulation by slicing it along its length up to the
top and then peeling it off. The exposed braid isn't required and must
be removed. The newly exposed inner conductor is then cut to 475mm.
Left like this the antenna would work well, but only on the 2 metre
band
(with some slight trimming). To make it true dual band, a 240mm
long aluminium sleeve is added to the centre to
create a quarter wave phasing section which makes the antenna resonate
on
the 70cm band. An effective method of fixing the sleeve to the coax is
to fit a rubber cap at each end with holes punched in the centre for
the coax to pass
through. This holds the sleeve in place while still allowing it to
slide up and
down for tuning and also ensures that the coax runs centrally inside
the sleeve. A hanger can be made by taking a 20mm long M3 nylon hex
spacer and drilling a small hole through the side to thread a loop of
string for hanging. This little assembly is then screwed onto the end
of the coax inner conductor. |
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The
classic flowerpot
antenna design frequently found on the web tends to be shorter than
shown here. I think this is mainly due to the UK 2m band being
restricted to 146MHz where in many other countries around the world it
extends up to 148MHz, so the element lengths
here will be longer. The aluminium tuning sleeve is 5mm longer too
simply because I found it improved the VSWR a little on UHF
and every little helps as they say! I also
found that 12 turns on the coil works better
than the recommended 9 turns, but it is what it is! The tuning sleeve
is positioned at the centre of the antenna where the lower radiator
ends and the upper radiator begins. It is then moved up or down to
achieve the best match which for me was 15mm higher up the coax.
I found that 25mm (1") is the 'sweet spot' for the sleeve diameter with
smaller diameters not working so well. Also, the thinner the tube wall
the better (1mm was the smallest I could source). Regarding the coax,
Messi & Paoloni RG58 is perfect for this project
as the quality is consistant and being nice and flexible it's easy to
work with. And it doesn't cost too much either!
An unexpected bonus is that the antenna works well on the UK PMR446 band... receive only of course! (click here to enlarge)
Parts List
N type line socket Click here
RG58 coaxial cable Click here
25mm aluminium tubing Click here
24mm rubber end caps Click here
20mm M3 hex standoff Click here
A good description of how a flowerpot antenna works
can be found here
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