by: Scott Roberts (N6SJR) & Craig Roberts (W6CMR)
The “Flower Pot” antenna is a clever twist on the half-wave, end-fed dipole antenna design that incorporates the coax feedline directly into the construction of the antenna. All credit for the design and instructions goes to John Bishop (VK2ZOI). Learn more about the history and development of the antenna, together with detailed plans and instructions by visiting John’s website:
The project began with a desire to build and extensive research. Various designs ranging from twin-lead j-poles and center-fed dipoles to full copper slimjims were under consideration. Ultimately, portability and overall performance were the dominant criteria (along with affordability). Due to the awkward and bulky nature of end-fed monopoles with ground planes, antennas of that type were excluded.
While researching different designs we stumbled across a forum posting about an idealized dipole that was end-fed (instead of center-fed) by using the feedline as part of the antenna. While attempting to locate plans and dimensions for this project the Flower Pot antenna was discovered and immediately became the favored candidate.
After deciding on the Flower Pot design it was necessary to choose from the five variations in the design. We opted for the Dual-Band Half-Wave design because of its greater flexibility while still being relatively simple to construct. The direct application to EMCOMM was another big selling point.
The first challenge was to obtain all of the required materials for the project which include, among others: a suitable length of PVC pipe, coax feedline, various sizes of shrink tubing and copper foil. The latter two items proved more difficult to find.
While most of us likely maintain a healthy stock of shrink tubing ranging in size from ½” to 1/16”, the 1.5”+ size piece required for this project was less commonly available and fairly pricey. For our build it was purchased from a local NAPA auto parts store (not a NAPA product) and cost about $20 for a 4 foot piece which was enough for 2 builds.
The copper foil was also an elusive necessity. Several online sources were vetted but most made vague claims about their true composition and provided unconvincing product descriptions. Ultimately the freshly constructed Hobby Lobby was the right place to buy for this one. The quantity was more than required, and it was not cheap ($15), but it was the ideal size and thickness and made of pure copper. It should be noted that other materials such as aluminum foil would have been sufficient, but we were going for a “premium” build.
Like many antenna builds, there is considerable opportunity for per-unit cost savings in the construction of larger quantities. As an example, we opted to split a 75 foot length of RG-8X from AmateurRadioSupplies.com (for around $50 shipped) which had professionally installed, high-quality PL-259 connectors at each end. When divided, the end result was a 35.5’ length of RG-8X with one connector (all that is required for the build). Likewise for the PVC, which was purchased as a 10’ length and divided in half.
The instructions provided on John’s website are superb save for one (unsurprising) detail: they are entirely in metric units. While the conversion to imperial units is quite simple, we found ourselves repeatedly converting the same numbers and second guessing. This problem is compounded by our collective forgetfulness coupled with an unwillingness to write anything down. I would recommend making the appropriate conversion ahead of time, double checking them and then having them available as written figures with the plans.
Another deviation worth documenting was with respect to that coax with pre-installed connectors. Not being particularly apt at the art of coax termination, we were intent on keeping the very nice connectors shipped with our feedline. However, the design calls for feeding your coax from the center (through each end of the balun) because it is simply much easier to do it this way. We managed to get around this by feeding the coax through the bottom and then fishing it through the pre-drilled holes by use of a locking, curved hemostat. Doing so ruined approximately 1” of the end of the coax which was written off but ultimately worthwhile. Once we had (gently) fed enough feedline through the first segment of PVC we made our measurements and cuts and proceeded with feeding through the top segment of the pipe per the original directions. The balun was finished by back-feeding the coax through the bottom (gently) until it was tight and stable.
Having a proper heat gun is a must for managing 1.5” shrink tubing, but do not forget that PVC is also apt to soften or melt when continuously exposed to high heat. It is best to lay the PVC on a flat surface while activating the shrink tubing as opposed to being awkwardly supported in a vice where it has a tendency to form a subtle “bow” which sets when cooled.
The construction went off otherwise without any major hitch. As with any project, you should take stock of the materials and the tools required before you begin. While we are fortunate to have a rather decent set of tools between us, someone who is new to electronics or tinkering may be disappointed to find they lacked a critical resource. Lots of credit to John Bishop (VK2ZOI) for the incredibly clear instructions and thoughtful photos.
We have yet to activate the antennas in an EMCOMM scenario but look forward to having an opportunity to do so. Handheld at approximately 8 feet of antenna height we managed to make contacts on local repeaters using low power (5W) in town. While not a remarkable achievement, it does prove the basic functionality of the antenna is there. We have both built stands to support the antenna at greater heights (12-20 feet) for future use and look forward to more great signal reports.
While we have yet to put an analyzer to the antennas, preliminary test show good clear, quieting signals into the local repeaters. The length of RG8X used (35ft) has about 2db of loss but the antenna provides around 5db of gain for a Net Gain of around 3db on 2m.
As I mentioned, the antennas have not been deployed for field exercises yet but they are generally going to be as durable as a 5 foot length of PVC. We opted for the grey conduit PVC as per John’s recommendation which, unlike the white PVC, is UV stabilized. Only a small section of external feedline (the balun) is exposed but is well protected with shrink tubing.
The antenna is as portable as a 5 foot length of PVC (coupled with the mounting/support mechanism of your choice). The options for supporting a standard ¾” (or size of your choice) PVC pipe are practically without limit. It is worth noting that the minimum length of the antenna is just over 3 feet. If you have a compact vehicle or space, this antenna may be built fairly small and certainly beats having to install and remove ground plane radials (or risk losing, bending and/or breaking them).
Even if only to serve as a backup to your commercial antenna, this design is so inexpensive, effective, and portable it is a no-brainer for anyone seriously considering remote/field operations or use of amateur radio as a component of their disaster preparedness plan. The overall cost to construct two of these antennas worked out to around $40 per unit, including the leftover materials. Competitive products include the Diamond X30A ($70) or the Arrow Open-Stub J-Pole ($50). While both options may likely be more sturdy, they have their own drawbacks by being less flexible with mounting, bulkier, heavier and requiring a separate feedline.