M7HFY and my Flower Pots

The VK2ZOI flower pot antenna probably needs no introduction but when I started making these using the scrap i had in the workshop and zoi’s drawings I was frustrated by not having the right materials at hand so made it from what I had available; it didnt work.

But everyone says they are easy and they always work I hear you say… maybe but not if your PVC tube is the wrong size, or the coax is an odd size or even not 50ohm!

What was needed was research!

I was convinced the problem with my non resonant 2m flower pot could be solved if i understood the math behind the critical three dimensions required to make an effective flower pot.

Before we get to that, the flower pot antenna is a half wave dipole and a choke usually constructed from one continuous length of coax.

The three critical elements are;

• The radiator or top half of the dipole.
• The counter poise or bottom half of the dipole.
• The RF choke that isolates the feed from the antenna.

It was at this point i realised my understanding is not good enough but i did find this online calculator specifically designed for the job!

I will ask David G3UNA to add an explaination here!

Practicalities

The construction suggested by zoi is to use a continous length of RG58 and a length 25mm od PVC pipe to provide the structure. This is exactly what I did for the first one I construced that worked. Success builds success and sure enough it was just a matter of hours before I had one for the home base and one for the workshop (both of which are still in daily use after a year) but a question to my self… could i build one for the car?

This is where the fun starts.

Im a fan of building from what I have within arms reach and what I had was;

• a sharp knife,
• something to measure with,
• something to mark with,
• RG58 (about 2m)
• Westflex 103 (RG213 equivelent, about 0.5m)
• 15mm pipe (about 1m)
• a dead CB antenna whip (at least 1m long)
• soldering iron
• connector (RG58 to your radio)
• heatshring tube, lots of sizes and colours!

For my 2m antenna i get the following lengths

• Radiating element; 462.8mm
• Counterpoise; 452.4mm
• Choke; 735.2mm

I suggest you start with what is potentially the most challanging part, getting the CB whip up the middle of the 103.

• cut around 500mm of coax
• pull the core from it, just the conductors, not the dialectric
• slide the CB whip into the coax so around 20mm of the thick end is pointing out the bottom and the rest out the top.
• trim back around 10mm of insulator from the base of the coax and make a tail with the braid so that you can splice it to the RG58
• strip around 30mm from the RG58 and splice and solder the braids of the RG58 to the 103
• strip around 15mm of the RG85 conductor, tinn it and wrap it around the base of the CB whip. Then use a crimp over the pair to secure the connection. I use ferule sleve crimps for this.
• Make this area a neat and compact as you can and sleve it in heatshrinkappropriately.

With this done you will need your length of 15mm pipe. Cut whatever length you need but I would suggest a minimum of 300mm. Drill a 8mm hole about 200mm down from the top.

Measure and mark the RG58 for the choke ; measure from the junction where the two coax’s are spliced then cut off the coax from the reel allowing as much tail (feeder) as you need in addition to the marked length for the choke.

Now feed the RG58 into the top of the tube and out the hole you just drilled, keep feeding untill the junction of the two coax’s is directly under the hole and the entire length of RG58 is outside the tube.

Now wind the choke around the tube and mark the tube where you reach the mark on the RG58.

At this point you need to decide if your feeder is staying outside the tume or going out the bottom, tie it off or drill another hole and feed it back in and out the bottom.

Your flowerpot is now complete! All we need to do is terminate the coax, connect the VNA and tune it.

I will write a seperate article on this subject.

Stephen M7HFY.