My 6 Meter Flowerpot Antenna

  1. Design Inspiration
  2. Design Theory
  3. Survey of Constructed Flowepot Antennas
  4. Forum Discussion
  5. Choke Design
  6. VNA Analysis
  7. The Second Build with Center Frequency at 53.525 Mhz
  8. Tuning adjustments
  9. Skin Effect
  10. References

Related Pages:
  1. Construction Photographs


Design Inspiration

To those who have gone before, I follow in your footsteps and try to add a little more to the understainding of the flowerpot antenna tradition.


VK2ZOI Flower Pot Antennas for 6 & 2 metres by John Morrissey VK3ZRX:

SOTA – 6m 52 MHz Coaxial Dipole Antenna by Andrew VK1AD - 16/08/2015:

I quote form this page:

"I have no plans to enclose the antenna in a plastic tube therefore according to John’s notes each radiating element must be 3% (38 mm) longer than the length shown in his article. An interesting point is to compare the lengths specified by John against the formula for a 1/4 wave element at 52 MHz = 1370 mm long (300/52 x 0.95 x 0.25 = 1.37metres).

You will note the bottom half (braid section) of the coaxial dipole antenna is considerably shorter than a 1/4 wave length most likely due the velocity factor of the cable.

Taking John’s figures and adding 38 mm to each element, the element lengths are: To start out I cut the top half element to 1.370 metres and after trimming the element for a low SWR, the 1/4 wave element is 1315 mm long. I didn’t alter the lower 1/4 wave element leaving the length at 1255 mm."

The Coaxil Dipole Design was further developed as the Flowerpot Antenna by John Bishop (VK2ZOI).


Design Theory

Design Ojective:

The design objective is to produce a vertical antenna for the six meter band, centered on 52Mhz with a 50 Ohm reactance and reasonable performance from materials I have available at home.

This image of a 2 meter flowerpot shows the two quarter wave sections.

An Inexpensive Vertical Antenna for 2m by Mark A. Dods VK3XMU (Now VK3ZR):


The Design Principle:
This design uses a single length of coax to form a choke and two 1/4 wave radiating elements forming a single 1/2 wave centeJohn Bishop (VK2ZOI)r fed vertical dipole. The asumption is that the upper quater wave radiates from the core on the coaxial cable and the lower section radiates from the shield.

This antenna works due to the skin effect. The RF travels out the quarter wave radiator section of the coaxial cable on the inside of the shield. When it hits the end of the shield it travels back on the outside forming the "other half" of the dipole. The RF is stopped by the choke wound into the feedline.

The Western Carolina Amateur Radio Society-Smoketest-August-2005:

Source: WCARS Smoketest August 2005

The Quarter Wave Dipole:
The radiating elements act as a centre fed quarter wave dipole. The quarter Wave lenght can be calculated as (300 x 106 / 52 Mhz x 0.95 x 0.25) = 1370mm.
where:John Bishop (VK2ZOI)
Speed of light = 300 x 106
Velocity Factor = 0.95
Centre Frequency = 52 MHz
Quarter Wave Lehgth = 0.25

The Unknown Correction Factor:
The big question is; What are the correct velocity factors for the upper and lower quarter wave sections? Given the construction method, the stripped upper consisting of core wire and diaelectric and unstripped lower consisting of a section of complete coaxial cable, and PVC couduit that acts as a radome, all have an dielectric effect.

The purpose of the Radio Fequency Choke is to de-couple the antenna from the feedline in order to prevent common mode currents reaching the transceiver. My understaining is that to achieve maxiumium "chocking" the coil should be designed to have maximium resonance at the centre frequency. However, John VK2ZOI in hte section on "Scaling to Other Frequencies" states, "The choke needs to be resonant about 5 to 6% below the desired operating frequency."John VK2ZOI in the section on "Scaling to Other Frequencies" states, "The choke needs to be resonant about 5 to 6% below the desired operating frequency."Frustratingly, no explanation is given for this adjustment.

He gives us the following table:

RG58 Co-ax Self Resonant Frequency (MHz)

RG58 Co-ax Self Resonant Frequency (MHz)
Coil Turns PVC Conduit Former Diameter


The Velocity Factor:

The upper and lower section of the dipole are slightly different lengths, preseumable due to the differing velocity factors as a result of having or not having the outer protective layer.

In my attempt to understand the correction factor (velocity factor?) used to determeine the length of the upper spripped section and the lower unstripped section for each centre ferquency, I have compiled this survey of website professing expertise in the flowerpot contruction method.


Survey of Constructed Flowepot Antennas

Theory is a good way to understand how an antenna should be built, however how they are actually built is the real test. So I gathered as many measurtements of dipole actually used by constructors. Unfortunalty, most constructors who published, did not alway quote their measuremnts. This is the results of what I could track down. A big thanks to those who published details.

NoAuthorBandFreq¼λUpperUp VFLowerLo VFUp + Lo½λVF%OCFNotes
1John VK2ZOI2m146.0005144570.8894470.87090410270.88050.55
2Mark VK3ZR2m146.0005145301.0313510.68388110270.85860.16Inconsistent. See page link
3Andrew VK1AD2m146.2005134600.8974500.87791010260.88750.55
4Andrew VK1AD6m52.000144213150.91212550.870257028850.89151.17No conduit used so 3% (38 mm) added
5John VK2ZOI6m52.700142312730.89512450.875251828460.88550.56Freq interpolated from graph
6Michael VK4MWL6m52.535140113000.92812500.892255028020.91050.980No Conduit - In Free Air

What can we learn from this small dataset?


Forum Discussion

This forum has some interesing insights into the theory behind the Flowerpot Design.

Andrew VK1DA/VK2UH - The End Effect:

The wavelength is calculated from the formula L = 300/f where f is in Megahertz and L is in metres. Each part of the antenna is approximately a quarter wavelength, but reduced due to the end effect of a wire antenna with an open end. Subtract 5% for end effect.

According to The Fundamentals of Single Sideband, published by Collins Radio,

"Resonance occurs when [a dipole antenna’s] length is a half wave length or multiples thereof. A practical rectilinear conductor will resonate when it is slightly less than a half-wave in length due to the end effect. End effect is due to a decrease in inductance and an increase in capacitance near the end of the conductor, which effectively lengthens the antenna. End effect increases with frequency and varies with different installations. In the high-frequency region, experience shows that the length of a half-wave radiator is in the order of 5% less than the length of a half-wave in free space. The greater the diameter of the conductor, the greater the difference between its electrical and physical length." Source:

David G0EVV Half Wave with Offset Matching:

The clever bit is the choke, this in not just any choke, this is a self resonant choke. That is to say its inductance and inter-turn capacitance is parallel resonant at 145.5 megs. I use RG174 for my flowerpots and that includes the choke. I find that 8.5 turns close wound on on a 22mm dia plumbing fitting is just right. To get the correct dimensions, make a coil, as per above then cut off the tails. Then use an aerial analyser with 1 turn coupling loop and look for resonance. It should be 145 ±1MHz. if high, try with a another 1/4 turn etc.

The distance from the top of the coil to the tip should be 1/2 an electrical wavelength, allowing for end effects and the tube dielectric. I use glass fibre fishing rods to mount it in. The offset “feedpoint” is to provide 50 Ohms impedance. Resonance is affected by the tube dielectric. So it must be set up inside the tube.

You will find the feeder is dead as far as RF is concerned, when you touch it the VSWR does not change, the choke is doing its job.

Spud on the Worldwidedx:

Spud on the Worldwidedxforum offers us this charming caricature for 11 meters. Note upper ¼λ at 103 inches (2616 mm) is shorter the the lower ¼λ at 108 inches (2743 mm) which indicates some doubt on the efficacy of these diamensions.


Choke Design


For further references see:
Short Tutorial: RF Choke vs. Inductor:

Wikipedia - Choke (electronics): [Top][Home]

VNA Analysis

This first VNA results:



The Second Build with Center Frequency at 53.525 Mhz

The second six meter antenna was designed for 53.525 Mhz. The diamensions I used were based on a velocity factor of 0.885 and off center feed point of 50.56 &pcnt; to 49.44 &pcnt;

I wasn't as lucky this time. The actual center frequency is about 53.760 Mhz and the inpedence about 45 Ohm. Still closer enough. Maybe I can fine tune th eformular a little further.



Tuning adjustments


Skin Effect



Alan Yates - VK2ZOI Flower-Pot Antenna - 2007-01-14:

Glenn Lyons VK4PK
Ver:gnl20200109 - pre published v1.1