The ‘Flowerpot’ antenna is an end-feed, coaxial dipole antenna designed by John Bishop VK2ZOI.
It is made using a single length of standard RG58c/u coaxial cable and has a resonant choke wound in the coax immediately below the lower element to provide RF isolation between the antenna and the feed line. The antenna is enclosed in a length of PVC pipe to protect it from the elements.
The antenna performs very well and the construction is simple, meaning almost anyone can construct one and have it running in an afternoon. All you need is some RG58c/u coax and a length of PVC pipe from Bunnings.
Since VK2ZOI is the designer and his website clearly covers the construction of a range of flowerpot antennas for 6m, 2m and 70cm , I wont be providing any detailed building instructions here except to describe mechanical differences in my version of the 6m version. I highly recommend you visit the VK2ZOI website at https://vk2zoi.com/flower-pots/ for full construction details. Simply follow the dimensions John has provided (as shown in Fig 1 above) and you can expect the antenna to be on frequency and require little – if any – further adjustment.
The 6m version that I built was based on John’s “Half Wave Flowerpot Antenna” construction article which covers construction of both 2m and 6m versions. The description and dimensions of the 6m version can be found towards the bottom of the article.
The main purpose of writing this is to show some minor variations in my mechanical assembly. Specifically, John’s 6m antenna was constructed using a length of 50mm conduit. He chose this diameter because the coaxial choke needs to be wound on a 50mm former. Since I was planning to mount this antenna on an un-guyed mast, I needed to keep the weight and wind resistance to a minimum which meant reducing the diameter of the PVC pipe as much as possible.
As a result I constructed mine using a 3 meter length of 32mm PVC pipe from Bunnings. This meant I was going to need a separate 50mm former for the coaxial choke. Rather than buy an additional length of 50mm PVC pipe which would be a loose fit over the 32mm pipe, I designed a 3D printed former with an outer diameter of 50mm and an inner diameter that would fit snugly over the 32 mm PVC pipe. The former is printed using standard PLA filament and is 90mm in length. I printed mine in white to match the colour of the PVC pipe.
Construction
For the design and general assembly instructions please refer to the VK2ZOI construction article. If using my 3D printed former, first drill a 3mm hole 12mm from one end of the former. Drill the hole right through to the inner core at an angle to match the exit angle of the coax to minimise stress to the coax during assembly where it enters the former. Re-drill the hole using various drill sizes until the hole is 7mm in diameter. As indicated in the VK2ZOI website, the use of RG58 coax incorporating a foil shield is not recommended as the foil may break under stress.
After measuring and cutting your antenna as per the VK2ZOI website you will have a taped mark on the coax where the top of the choke winding will start. Place this against the hole in your former and carefully wind exactly 13 turns of coax along the former. Mark the second exit hole location and drill the second hole – again angle the hole to minimise stress on the coax during final assembly.
Once the former is drilled you can slide it onto the 32mm PVC pipe and position it so that the tip of your antenna is around 100mm from the top of the 32mm pipe. Use the 3D former as a template to drill two holes through the 32mm pipe then remove the former and elongate the holes to ensure minimum stress for the coax where it enters the pipe.
Final Assembly
Fix a length of nylon fishing line to the tip of the antenna. This will be used to ‘hang’ the tip of the antenna from the top of the 32mm pipe.
Refit the former to the pipe and pass the tip of the antenna through the top hole, feeding the antenna towards the top of the 32mm pipe. When the tape is just inside the former’s top hole, wind 13 turns side-by-side then feed the remaining coax through the bottom hole and out the bottom end of the pipe.
Make a small cut in the top edge of the pipe and hook the nylon line through it, pulling it firmly so it supports the weight of the antenna. Fit a 32mm end cap to seal the antenna and hold the nylon line in place.
Performance
Using the dimensions provided in John’s article, I found the antenna had a minimum SWR at 53 MHz of 1.1:1 with a fairly flat SWR of less than 1.5:1 from 51.7 MHz right up past 55 MHz .
Since I was planning on using this antenna for the FM repeaters the tuning was perfect for me and no further adjustment was needed. If you wanted a better SWR at the lower end of the band, the bandwidth is such that you could easily lower the resonant frequency by 1.5 MHz and still get good SWR at the FM end of the band.
References & Website Links
VK2ZOI Website : View and build John’s Flowerpot Antenna designs.
Flowerpot Antenna Calculator : General calculator to determine the length of each element of a flower pot antenna. Refer to the choke table in VK2ZOI’s website for the number of coax turns and the diameter of the former required for various frequencies.
This is the STI file for 3D printing the 50mm former for the 6m coax choke.
Parts from Bunnings:
Holman 32mm x 3m PVC Pressure Pipe: I/N: 4750152 (AU$14.88)
Holman 32mm end cap: I/N: 3140489 (AU$2.65)