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ZOI Flowerpot Antenna For 2m And 70cm

By Stephen Stebbing 2013-11-17 17:16:08 2 comments

The ZOI Flowerpot is an end-fed half wave antenna for 2m made from nothing more than RG58 coax and 25mm PVC pipe. With the addition of some tinfoil and sticktape, it can double as a 70cm colinear.

The design is by John VK2ZOI, who has excellent documentation and a step-by-step construction guide on his website here.

As I see it, the antenna works by having the signal travel through the feedline as usual until it reaches the centre where the coax outer conductor ends. At this point, the signal on the inner-conductor continues on up and is radiated. The signal on the (inside of the) braid outer-condutor has nowhere to go but back down the outside of the braid and radiate. The unradiated part of the signal that reaches the coil cannot continue due the coil’s resonant high-impedance and is hence reflected back towards the center.

Anyway, my old made-from-thick-wire double J-pole for 2m needed replacing as it was getting tired after have been up for a couple of years, and doubling as my 70cm antenna it gave marginal performance at best. I decided that it was time for something new and thought that I would give the flowerpot a try, and have to say that I am very pleased with the result.

1. Construction

The coil with coax feeder exit hole beneath it

The coil is just 9 turns of the coax around the pipe.

The bottom section of the pipe was strengthened with the insertion of some wooden dowel

Instead of running the feeder coax down the pipe as in John’s design, I bought it out through a hole as I put a piece of wooden dowel inside the pipe below the coil to make things a little more rigid.

A piece of corrugated PVC irrigation pipe was used to sheild the coil from birds.

For bird protection purposes, the coil was covered by a piece of PVC corrugated irrigation pipe and wrapped in duct tape.

The coax feed line is protected by being run through 1cm pipe.

Cockatoos will try to eat pretty much everything and what they can’t eat they will destroy, and are especially fond of coaxial cable but, for some reason, not black irrigation pipe.

Top cap is made from a beer bottle top.

My major contribution to the design was the realisation that a beer bottle top can be used for the end-cap, although I’m not sure how having a piece of metal so close to the top of the antenna affects performance.

BNC socket connected to coax and encased in hot-melt glue.

The BNC socket connection was wrapped in electrical tape, sealed against moisture with hot-melt glue and covered with several layers of duct tape.

In my experience, hot melt glue works well for this purpose but if exposed to the Sun, it will start to become brittle after a few months, and crack apart after a year or so. Shielding it with duct tape seems to work well, and a test piece is only just starting to become brittle after more than two years in the weather. Silicone would probably be better but I didn’t have any.

2. Testing

The antenna being given an initial SWR test.

The initial SWR test gave a reading of less than 1.1 from 145MHz to 148MHz as predicted in John’s table, and at 146MHz to 147MHz the needle barely moved at all.

It seems that if you use the specified grey coloured conduit and match the measurements as precisely as possible then this antenna will work first-time, every-time.

The antenna in position on the roof.

The antenna was mounted on the chimney using a discarded beach umbrella pole that is visible at the bottom of the photo on the left.

I connected the rig, checked the SWR again which had not changed, and was astounded with the results. I immediately made contacts on several repeaters that I had never been able to even hear before, and had a simplex QSO from the other side of the city. It was only when I had a simplex contact with Newcastle, some 150km away and thought to check the Hepburn Charts that I realised that I was testing during a period of highly enhanced conditions on VHF.

Anyway, under more normal conditions the flowerpot is a big improvement over old J-pole and I am able to reliably use several repeaters that I could not previously. Ten watts is enough to cause one of the digital TVs to stop receiving on VHF channels, and this never happened with the J-pole. The old phased-array VHF TV antenna is just visible at the top right corner of the photo; it’s about five or six meters away and looks to receive in both directions along its line and is probably pretty good at working on 2m. Now that the TV stations have stopped using the lower VHF channels, I can replace it with a dual VHF/UHF unit which will be pointing away from the flowerpot and should solve the problem. I suspect that the J-pole was radiating pretty much straight up!

3. 70cm Dual-Band Version

Being very happy with the new antenna and having 2m of PVC pipe left over, the next day I decided to build another one, this time with the foil sheath added so it could be used on 70cm as well. It only took it only took around 90 minutes now that I knew what I was doing, including mounting it on the roof. The SWR was pretty much flat on UHF, but a little increased on VHF compared to the single band version, but still nothing to worry about.

Performance wise, I am now able to reach several repeaters that I couldn’t previously and on those that I could, the signal strength has improved by 30 or more percentage points (my old PRM8025 shows signal strength as a precentage).

4. Conclusion

I couldn’t be happier with my two Flowerpot antennas, and will dual-band the 2m only one next time I am on the roof. Thanks again to John for sharing his hard work in refining the antenna into such a cheap, easy to build and reproduce, and easy to hide design. Next up, I will give the double-length dual-band colinear version a go.

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