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Flowerpot Antenna for 2 Metres (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. 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 first coined by John, VK2ZOI who was inspired by an article written by Ian, VK3AYK.
I hope I've got that right! Although this antenna will work on the 70cm band, it won't
radiate too well. The VSWR on UHF isn't too horrendous though, so it should be
fine for local contacts. There are dual band versions that work by adding a phasing sleeve over
the centre section, but this one is 2m only.
So I ordered some RG58 coax and an N type inline socket (my connector of choice) and constructed one using the
most popular dimensions and surprise, surprise the VSWR was way off.
These antennas obviously work, but maybe it's just the variables mentioned earlier that are
responsible for the differing
results. I then decided as I usually do, to just experiment and see
what
happens. I intended to use it in the loft space, so to avoid having to
keep going up and down the ladder to do some 'trimming', I started my
experiments in the lounge with the prototype hanging from the light
fitting by some insulating tape! Luckily, it wasn't too long before my
NanoVNA reported a near perfect match (I was aiming for a centre
frequency of 145MHz) so up in the loft it went.
What? It was now resonating below 144MHz! I made sure that it was
placed away from any metal objects, so maybe the roof tiles were
affecting it. Anyway, it needed to be shorter. The dimensions
shown on the image on the right (not to scale) are what I found worked
best when the antenna was enclosed inside a length of PVC pipe up in the loft. There's an online flowerpot antenna calculator that indicates that my numbers are more or less in the ball park!
Construction
is as follows: Cut a 1100mm length of 20mm O.D. white PVC electrical
conduit and then drill two 7mm diameter holes 60mm apart and 90mm from
the
base. That's the enclosure done! Although the N socket can just be
attached to the end of a coaxial 'tail' emerging from the bottom of the
pipe, I wanted it mounted actually in the pipe, and it just so
happens
that the sockets I use fit perfectly. To do this, a 2.5mm hole is
drilled in
the side of the socket which is then tapped to 3mm. A corresponding
hole is drilled near the base of the pipe where an M3 bolt is used
to
secure the socket. On to the antenna: Take a piece of RG58 coax 2
metres long and fit the N
socket onto one end. Pass the other end into the base of the pipe and
up through the first 7mm hole until the connector lines up with the
hole in the pipe, then fit the screw to secure it in place. Wind 12
turns of coax around the outside
of the pipe and when you get to the second 7mm hole, mark the cable
with a pen or piece of tape. From this mark, measure up 450mm and with
a knife, carefully remove the outer sheath and braiding from the
remaining coax. The coil will unravel but it doesn't matter as we're
still in the process of measuring. |
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From
the point where the
centre core is now exposed, measure up a further 470mm and cut off the
excess (this will be the radiating element). Now the coax can be wound back onto the pipe and passed down through the second 7mm
hole and up
the tube. Bunch up the windings making them nice and tight. It's a good idea to put a bit of tension on the cable once
inside the tube to keep it straight, so before feeding the coax back into the tube, tie a piece of
thin heat shrink sleeving (or similar) to the end of the radiating element which can be used to
pull the coax tight and then be folded over the top of the pipe where
a 20mm rubber end cap is fitted to hold it in place. The cap also
finishes
off the top neatly. Job done!
The distance between the
7mm holes is 60mm because RG58 is 5mm in diameter, so 12 turns x 5 =
60. The holes for the coax are 7mm not 5, because 7mm makes it
much easier to push the coax through.
If
you don't have a
3mm thread tapper for the 2.5mm hole that was drilled in the side of the N
socket, a tapered 3mm bolt and a strong screwdriver is usually enough
to create a thread (use a
toughened bolt because the last thing you want is for it to shear off in the hole!).
Most flowerpot
designs favour 9
turns of coax for the coil, but I've always found that 12 turns gives a
much better match. It's best to use the type of RG58 that doesn't have
an extra layer of foil, as it may split during the winding process. I
recommend Messi & Paoloni RG58 as it's very flexible.
To stop
the
heatshrink sleeving (or whatever you used) from sliding off the end when
pulled, a small rubber cap can be fitted as shown on the left. A cable tie can't be used unfortunately, as it won't fit through
the 7mm hole! A 20mm rubber end cap secures and hides the folded over sleeving.
The antenna is hung from
a loft rafter using a 22mm pipe clip, with the rubber end cap being tight
enough to hold it in place. For the feed line I use 12 metres of Messi & Paoloni
Ultraflex 7 coax.
Parts List
20mm electrical conduit Click here
20mm rubber end caps Click here
RG58 coaxial cable Click here
N type line socket Click here
Short M3 bolt - Have a rummage!
A good description of how a flowerpot antenna works
can be found here.
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