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. If you want it to work properly on 70cm, making it true dual band, this can be achieved by the simple addition of a phasing sleeve placed over the centre section of the 2m version, which will be explained later.

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.

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 (the rubber end cap is tight enough to hold it in place). The section of the antenna between the bottom of the coil and the N socket is not critical and can be any length you want. The antenna is fed with 12 metres of Messi & Paoloni Ultraflex 7.

NanoVNA plot from 144MHz to 146MHz (click here to enlarge)

NanoVNA plot from 430MHz to 440MHz remembering this is not a UHF antenna (click here to enlarge)

Parts List

20mm electrical conduit Click here
20mm rubber end caps Click here
RG58 coaxial cable Click here
N type line socket Click here
75mm aluminium foil (for dual band) Click here
Short M3 bolt - have a rummage!

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

Make it Dual Band (2m/70cm)

It's very easy to convert the 2m flowerpot antenna into a dual band antenna that also works on the 70cm band. Everything remains the same as the 2m version except for a 235mm sleeve of aluminium foil which is wrapped around the outside of the PVC conduit, centred at the point where the coax ends and the stripped inner conductor (radiating element) begins. Although everyday kitchen foil works fine, I found that 75mm wide sticky back aluminium foil was more convenient to fit and looks quite neat if time is taken to cut it accurately and apply it while smoothing out any air bubbles (75mm is wide enough for the circumference of 20-22mm pipe). I did find that the 2m tuning moved up a bit when the sleeve was fitted, but the VSWR improved slightly, so I didn't feel the need to change anything. I know the title of this page suggests that the dimensions are biased towards UK frequencies, but in fact with the sleeve fitted it covers 144MHz to 148MHz with 146MHz bang in the middle, so perfect for all regions!

144MHz to 146MHz / 430MHz to 440MHz Dual Band Flowerpot Antenna (UK)

NanoVNA plot from 144MHz to 146MHz (click here to enlarge)

NanoVNA plot from 430MHz to 440MHz (click here to enlarge)

An unexpected bonus is that the antenna works well on the UK PMR446 band... receive only of course! (click here to enlarge)

So, does adding the sleeve make much of a difference? Well... to say the improvement in performance is remarkable would be an understatement. Fitting the sleeve is absolutely worth doing if you use 70cm, as it transforms a pretty good antenna into an exceptional antenna! Take a listen below.

Audio sample of a test transmission using 4 watts on 70cm over a distance of 27 miles using the unmodified 2m flowerpot antenna - Click here

Now with the sleeve fitted - Click here