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

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.

• Top section (core of the coax) 1315 mm
• Bottom section (braid with plastic cover in place) 1255 mm

Design Theory

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):
Source: http://www.vic.wicen.org.au/?page_id=1353

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.

De-Coupling:
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)

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.

• The overall Correction Factor for Velocity Factor or End Effect on the half wavelength is probably 0.88 to 0.885 for flowerpots in grey conduit and about 0.89 for flowerpots hanging in free air.

• The %OCF which is the Upper ¼λ as a percentage of the ½λ is about 44.7 percent enclosed the grey conduit and 45.6 percent in free air.

Forum Discussion

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: http://collinsradio.org/archives/ssb_fundamentals/Fundamentals%20of%20Single%20Side%20Band-one-file.pdf


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.

Source: https://www.worldwidedx.com/threads/coax-cobra-wire-antenna.135781/

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Choke Design

VNA Analysis

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;

• Half Wave Length: 300 / 53.525 / 2 = 2802.5
• Physical Half Wave: 2802.5 x 0.885 = 2480
• Quarter Wave Top (stripped)= 2480 x 50.56 = 1254
• Quarter Wave Bottom (unstripped) = 2480 x 49.44 = 1226

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.



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Tuning adjustments

• Cut the antenna long and pruned it to a match.
• Uncoil the choke and push a bit more coax up inside the tube, then rewind the coil.
• Place the coil in series with a noise source and plug it into my VHF/UHF spectrum analyser.
  This gives a clear picture of the "trap" function at resonance.
  Any close objects or body parts will pull the tuning.
  The trap is quite low Q however, so this is probably not of huge consequence.

Skin Effect

References