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Flowerpot Antenna for 2m & 70cm (UK)
The Flowerpot antenna is an interesting yet simple design, being
essentially a centre fed half wave dipole (this is often debated)
constructed from one continuous length of RG58 coaxial cable. Although
there are many websites that feature this antenna, the dimensions given
can vary due to things like the coax cable used, coil diameter, number
of turns and
positioning etc. Also, the 2m band in the UK is restricted to 146MHz
where in other parts of the world it extends up to 148MHz so the UK
versions will naturally have slightly longer elements. To give credit
where credit's due, it seems
that the term 'Flowerpot Antenna' was first 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 cable to use for VHF and even more so for UHF, but in
this application for the antenna elements it's fine.
For this reason I suggest keeping the 'tail' from the base of the coil to
the RF connector as short as possible (about 50mm) and then use better
quality coax to the radio. 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 onto one end. I used an N type line socket not because N's are the best
(though they are) but
because I already have a length of good quality coax terminated
with N plugs going between the loft and shack. The length of coax isn't
critical, but the shorter it is the better to keep losses to a minimum.
Next, 12 turns of coax is wound onto a 100mm length of 21.5mm O.D. PVC
pipe, but first, two 7mm holes must be drilled 60mm apart for the coax
to pass through (because RG58 is 5mm in diameter 12 turns laid side by side works out to be 60mm). Once the holes are drilled, pass the end of the coax up through one of
the holes, tightly wind 12 turns and then pass it back down through the
other hole.
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 470mm.
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 fine 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
length of the aluminium tuning sleeve is usually given as 235mm, but by
experimentation I found that 240mm improved the VSWR a little on UHF
and as they say, every little helps! I also
found that 12 turns on the coil works better
than the recommended 9 turns so for whatever the reason for this I just
went with
it. The tuning sleeve
is positioned at the centre of the antenna where the lower radiator
ends and the upper radiator begins. It can then be moved up or down
slightly to achieve the best match.
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 is fine). Regarding the coax,
Moonraker 'Mil Spec' RG58 works well and doesn't cost the earth. Try to
avoid cables with an aluminium foil sheild as they don't like being
wound into a tight coil. An added bonus is that this antenna can also
receive well on the UK PMR446 band and also the VHF marine band, so all
in all, it's a very versatile piece of
wire!
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
21.5mm overflow pipe Click here or Here
A good description of how a flowerpot antenna works
can be found here
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