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 dow
n 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 (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. The tip can be threaded through two knife cuts made in a short length of sleeving to form a strap that can be pinned to a rafter, or it can used mobile 'hung from a tree', as it can be folded up and carried in a ruck sack


A good description of how a flowerpot antenna works can be found here