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Flowerpot Antenna for 2 Metres (UK)
A couple of my recent projects have been ham radio related, but as
I've been away from the hobby for more years than I care to remember, I
thought I should rekindle my interest. First though, I needed to make
a
quick and easy antenna for 2 metres, and it wasn't long before a bit of
internet searching led me to the 'Flowerpot Antenna' which is a centre fed half
wave dipole made from a single piece of RG58 coax.
Although you can take your
pick from the many websites that describe what is essentially the same
thing, the
dimensions
given can differ. So who's correct? I guess it all depends on
things
like the coax cable used, coil diameter, number of turns and antenna
positioning. Also, the 2m band in the USA covers a wider range of
frequencies than in the UK, so website information will vary 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!
So I obtained some coax and a PL259 plug and constructed one using the
most popular dimensions and surprise, surprise the VSWR was way off.
Obviously these antennas do
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 had noted that my first attempt was resonating at a
higher frequency than I wanted (I was aiming for 145MHz), so it was a bit too
short. After increasing the overall length slightly the VSWR came down and with the antenna pinned up in the loft space using the dimensions
shown on the image to the right (not to scale), the antenna achieved a 1.1:1 VSWR.
I have since found an online flowerpot antenna calculator which seems
to agree (within a few mm) with my experimental dimensions, so I'm
happy with the results. The calculator can be found here.
Construction
is really simple. Select a length of RG58 coax and cut away
470mm of the outer insulation and also the braid, leaving just the
inner conductor with its covering. This will be the radiating element.
Then measure 445mm down from the last cut and make a mark to indicate
the starting point of the coil (or common mode choke to be more precise).
The coil is wound on a 100mm tube cut from a length of 21.5mm PVC
overflow pipe. Drill two
6mm holes 60mm apart then push the coax through the top hole from the inside of
the tube up to the marked position. Wind 12 turns of coax and feed it
back in again so that it exits through the bottom of the tube (I found that 12 turns gave a better VSWR match than the 9 turns
usually recommended). Finally,
fit
a
plug to the other end of the coax to suit your radio and you're
ready to go! As a finishing touch, I fitted a silicone rubber end cap
to the tip. For mounting, the tip can be threaded through two knife cuts made in a
short
length of sleeving to form a strap which can be pinned to a rafter. |
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