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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. So first of all I needed to make a quick and easy antenna for 2 metres, and it wasn't long before a bit of Googling led me to the 'Flowerpot' antenna, which is an end fed half wave dipole (debatable) 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 a lot, so who is correct? Apart from frequency, I guess it depends on things like the coax cable, coil diameter/turns, the plastic pipe it's housed in (if you choose to do this) and of course, location. To give credit where it'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 have that right

So I got some coax and a PL259 plug and constructed one using the most popular dimensions and surprise, surprise the SWR was way off. Obviously these antennas do work, but maybe it's just the variables mentioned earlier that are responsible for the differing results. So I 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 the elements were too short. After increasing the element lengths slightly the SWR was pretty much spot on, but strangely the dimensions were now longer than any of the examples I had seen online. But hey, they work for me so I've posted the results here and hopefully they'll work for others too

Construction: It really is very easy to make. Select a length of good quality RG58 coax and cut away 470mm of the outer insulation and also the braid, leaving just the inner conductor with its foam (or whatever) covering which will be the radiating element. Then measure 445mm down from the last cut and make a mark to indicate the starting point of a coil (choke) that will be wound from the coax itself onto a section of PVC pipe. Finally, fit a plug to the other end to suit your rig and you're ready to go, though to finish it off properly it'll look much nicer if housed in a length of PVC tubing. I used 1100mm of white 21.5mm overflow pipe, which is common here in the UK (available from B&Q or Screwfix). For end caps, I found that 22mm (7/8") rubber ferrules for chair legs do the job well and are readily available from eBay. If you make a couple of small holes in the top end cap, a zip tie can be threaded through to form a hanging loop for the loft. The bottom end cap just needs a single hole for the coax to exit. The pipe that the antenna is housed in also doubles up as the coil former. Simply drill two 6mm holes 60mm apart (for RG58), insert the coax into the top hole up to the mark, wind 12 turns of coax, then feed it back in again so that it can exit through the bottom end cap. I found that 12 turns gives a much better match than the 9 turns usually recommended. The image on the right shows the antenna dimensions and the image below shows the overflow pipe dimensions (both not to scale)






To keep the coaxial cable straight and to stop it from flopping around inside the pipe, a piece of fishing line is usually tied around the end, pulled over the top and under the end cap to hold it tight, but as I didn't have any fishing line I used zip ties instead. Before the coil is wound, insert the coax through the upper hole so that it protrudes from the top of the pipe. This is to enable the zip ties to be fitted. Once fitted, the coax can then be pulled back to the correct position, leaving the tail of the zip tie ready to be bent over the top of the pipe and secured by the top end cap. See photos below