flower pot antenna. The antenna is constructed as a co-axially, end-feed antenna using co-ax cable feedline as the antenna elements with a resonant choke formed in the co-ax line to provide isolation between antenna elements and the feedline and, the antenna enclosed in PVC conduit or similar plastic pipe.
flower pot antenna Some Background
I first constructed one of these antennas in 1993 based on an article by Ian Keenan, VK3AYK published in AR Magazine of May 1986. But to get VK3AYK’s dimensions to resonate and to broaden its response across the 2m band, I had to add a small capacitor hat to the top element. With this modification, many of these antennae were built by HADARC members and have given excellent service for over ten years as both home QTH and portable antennae.
Having a strong interest in 6m FM, I also attempted at the time to adapt the design to this band; however, I was unable to directly scale the design to work (ie, achieve a low VSWR) on 6m.
During the 90s, I regularly revisited my attempts to scale the antenna to 6m but was always unsuccessful. Further inspiration came from another magazine article by Rolf Brevig LA1IC in CQ August 1999 entitled “Feedline Verticals for 2 and 6 Meters”.
This article gave a set of formulae for co-axial dipoles. These were ‘bare bone’ construction, ie they were not encased in any type of radome. Encasing the antenna in PVC conduit or plastic water pipe can pull the resonance down by 2 to 3% (depending on material), so allowance for this has to be made when encasing this style of antenna in a radome.
Adapting VK3AYK’s original design and using the information provided in LA1IC’s article, I was able to achieve a half-wave antenna design which will readily scale to any frequency. I have also developed two higher-gain versions (a single 5/8 and a double 5/8) of the antenna which also scale to any desired design frequency.
About 5 years ago (during 2002), I described the half-wave and higher-gain versions to a number of Sydney and NSW Country Clubs and have now since collected sufficient feedback from builders to verify that the designs and their expected performance can be readily replicated.
They have proven to be robust designs, very easy to build, rugged and, because of both the inherent simplicity and replicity, are very suitable as an antenna project for Foundation Licensees (the entry-level amateur radio licence that has been introduced in Australia).
More recently, I have designed a simple dual band modification for the half wave antenna.
Half-Wave Flower Pot Antenna
The basic 2m half-wave version of the antenna.
The diagram below shows the basic arrangement of the 2m Half-Wave version of the antenna. To construct the antenna, first select a suitable length of grey 25mm conduit (as a minimum 1m but if you make it longer, you will have more room below the coil to attach to your antenna support).
The VSWR plot of the 2m Half-Wave antenna should look like the following:
The type of Co-Ax is Important. Use braided co-ax only.
Do not use co-ax with a foil shield as the foil tends to break during assembly especially at the sharp bends at the choke entry/exit points. Obviously if this happens, your antenna will not work!
To protect the choke coil from bird attacks especially from the White Cockatoos, the coil needs to be covered with a ‘Cocky’ shield. An empty Silicone Sealant cartridge (enlarge the hole at the top and cut the barrel to length) neatly fits over a 2m antenna coil. A PET soft-drink bottle can be used for larger coils which, when heated with a hot-air gun (but don‘t melt the conduit), will act like heat shrink tubing and become a very tough shield. Before fitting the shield, wrap PVC tape over the coil and the entry/exit holes to minimise water entry.
Using something other than grey electrical conduit
To the purist and his microwave oven, grey electrical conduit is considered lossy. It is, however, very UV resistant. The design compensates for the affect of the conduit by shortening the elements (by about a 2% factor) but otherwise the conduit appears to have little effect on the radiation efficiency.
If you use orange (HD) conduit, irrigation pipe, Telstra conduit, GRP, etc, the element lengths will be different. An unenclosed antenna will have longer elements (probably 2% or maybe 3% longer). Similarly, an antenna enclosed in something that is very much loaded with conductive filler will be much shorter (but, of course, don‘t ever use a material like this for an antenna).
Scaling to Other Frequencies
The above design will scale to other frequencies, the limitation being the mechanical properties of the conduit. To make an antenna for other frequencies, a suitable choke coil can be determined from this table.
|RG58 Co-ax Self Resonant Frequency (MHz)|
|Coil Turns||PVC Conduit Former Diameter|
As a suggestion, construct a series of graphs from the data to make it easier to interpolate. Ideally, the choke should consist of unit turns. Half turns are OK but do not wind a choke coil using other than full or half turns. If your design is for a single operating frequency (or very narrow band) then chose the lowest half turn (ie the choke frequency is closer to the operating frequency); if, however, a broader-band antenna is required, chose the nearest higher half turn.
The choke needs to be resonant about 5 to 6% below the desired operating frequency. Closer spacing will sharpen (and deepen) the VSWR response; wider spacing flattens but raises the VSWR. curve.
6m Half Wave Flower Pot Antenna
To build a 6m version, use 50mm (OD) conduit. The dimensions are in the following diagram.
Dual Band Half-Wave Flower Pot Antenna
Modified half-wave Flower Pot to dual band the antenna for operation on a band that is the (approximate) third harmonic of the fundamental resonance.
The basic half wave version of the Flower Pot antenna can be readily modified to dual band the antenna for operation on a band that is the (approximate) third harmonic of the fundamental resonance.
Operation on the third harmonic is achieved by using a sleeve technique so as to form quarter wave phasing sections (at the higher frequency) to end feed two half waves in phase at the third harmonic.
This arrangement provides useful gain (3dBd) on the higher band. The sleeve technique maintains the impedance matching for both bands and (probably fortunately) there is sufficient longitudinal impedance in the choke coil to provide the required isolation at the third harmonic.
The sleeve is applied after the basic antenna has been constructed.
Dimensions shown are for the (basic) 2m half-wave Flower Pot. The modification involves placing a co-ax phasing sleeve around the outside of conduit, positioned as shown.
The sleeve material can be aluminium (kitchen) foil, copper foil, brass shim, roof/building alfoil sarking or salvaged co-ax braid.
Before fixing the sleeve in place, check VSWR on 2m The sleeve should cause little if no change to 2m VSWR although it may appear to very slightly raise the resonant frequency; With the sleeve fitted, the VSWR should not be greater than 1.15:1 across the FM portion of band).
Then check VSWR across the 70cm (430 – 450 MHz) band. Expected VSWR readings will be less than 1.2:1 at band edges and less than 1.1:1 in band centre.
If VSWR is outside these limits, adjust position of sleeve ( /- 5mm max) and, if necessary, trim sleeve length to lower VSWR. When trimming sleeve length (dimension B) adjust dimensions A and C accordingly to keep centre of sleeve adjacent to feedpoint of the inner 2m dipole. However, little, if any, adjustment to the sleeve should be necessary. When satisfied with the VSWR, fix in place and protect the sleeve with UV protected PVC tape or heatshrink.
Methods of dual banding the other versions are being developed and will be added to this website when available.
Single 5/8 Flower Pot Antenna
Substitutes a 5/8 wave-length section for the top quarter wave of the basic half wave antenna design.
The Single 5/8 version of the Flower Pot simply substitutes a 5/8 wave-length section for the top quarter wave of the basic half wave antenna design. The arrangement is shown in the sketch below.
The 5/8λ radiator uses a 0.2λ (shorted) co-ax phasing stub to resonate the 5/8λ element. In a conventional 5/8λ mobile whip, an inductor is used to bring the 5/8λ element to resonance; however, in this Flower Pot style of antenna, using a co-ax phasing (or delay) stub suits the construction technique and has the advantage of being able to be precisely determined and cut at the construction stage.
The antenna configuration is similar to, but slightly shorter than, the “Gain Sleeve” antenna described in the RSGB Hand-book (6th Edition – figure 13.99, which itself is derived from the reactance – or shunt – fed 5/8λ monopole antenna at figure 13.84 of the handbook).
The Gain Sleeve antenna achieves an effective radiating element length of one wavelength and, since the aperture is twice that of a half wave dipole, a theoretical gain of 3dBd (gain over a dipole) could be achieved.
However, note that the Handbook indicates that in practice, the Gain Sleeve antenna would realise about 2.5dBd. The effective radiating element length of the Single 5/8 Flower Pot is 7/8λ suggesting it would have somewhat less than 2.5dBd gain.
Construction of the Single 5/8, whilst a little more involved than the basic half-wave antenna, is again fairly simple.
From the top of the co-ax, measure off an approximate 5/8λ distance to locate the position of the feed point. Make this distance slightly longer than the exact 5/8 length (say 10mm) as you will trim the top element to length later.
At the feed point, cut away the outer sheath and braid so as to form a 2-3mm gap.
From the edge of the gap, measure off the distance for the 0.2λ section. For solid polyethylene dielectric cable, this is 276mm for 2m and 755mm for 6m.
From this point, expose sufficient braid to be able to make several pigtails to be soldered to the inner conductor and then the braid and outer sheath for the remainder of the top element length can be stripped off. At the 0.2λ point, cut into the inner dielectric to expose about 3mm of the inner conductor and solder the braid pigtails to the inner conductor.
Trim the top element to length. It will be most unlikely that you have to further trim the antenna later but you could leave a small, extra margin to allow some later adjustment if desired; however, builders of this antenna have reported that further trimming was unnecessary so you should have confidence in cutting the element to length at this stage.
To complete construction, follow the same procedure as for the half-wave antenna. As in the half wave version, use a length of nylon fishing line to pull the radiator taut and clamp it in place with the conduit cap. The length of the conduit above the choke can be 10 to 20mm longer than the total length of the quarter wave and 5/8 wave elements.
The Single 5/8 has a slightly sharper VSWR response than the basic half wave Flower Pot and, although a VSWR of less than 1.5:1 across the 2m band can be achieved, the antenna can be cut to favour the high FM portion of the band or the lower packet portion. The dimensions derived during my experiments for 2m are given in the following table; these dimensions have since been validated in further builds of the antenna.
|Desired Portion of Band||Upper 5/8 element||Bottom “λ/4”||Choke Coil|
|Across the Band||1228mm||465mm||9 turns on 25mm former|
|FM & Repeaters||1224mm||465mm|
|Packet low band||1236mm||480mm|
The VSWR curves for the three antennae are shown in the next figure. Note that the “Across the Band” curve purposely favours the higher end of the 2m band.
6m Single 5/8 Version (and using the Antenna at Other Frequencies)
The physical/mechanical properties of conduit are not conducive to building a 6m or a lower frequency version of the single 5/8 antenna because conduit is not sufficiently rigid to maintain straightness and it retains a set after a hot day.
I have, however, built a 6m single 5/8 by terminating the top end of the phasing stub onto a standard mobile base mounted on a conduit cap and using a plain (braided) mobile whip for the remainder of the top 5/8 element. This way the conduit length is approximately halved and is less susceptible to bending.
If you want to try building one doing this, the length of the whip will, depending on its diameter, probably be marginally longer than the equivalent length if it was made using the co-ax inner. Also when using a whip, the overall length of the 5/8 element will need to include the length of the phasing section. Otherwise, the dimensions readily scale from the 2m antenna.
The antenna can be scaled to any operating frequency using the choke data given previously. Note that the 2m single 5/8 is close on 2m long; 25mm conduit is mechanically OK for this length and this suggests that the ideal application of this style of antenna is for frequencies around 2m, ie boating, aircraft and the VHF two-way communications bands.
When working out the phasing stub length for other frequencies, don’t forget to take the velocity factor of the cable into consideration.
Double 5/8 Flower Pot Antenna
Extension of the Single 5/8 and uses a 5/8λ element for both the top and bottom radiators.
The Double 5/8 is a natural extension of the Single 5/8 and uses a 5/8λ element for both the top and bottom radiators.
The double 5/8 is a co-axially fed variation of the 1¼ wave (vertical) dipole shown in the adjacent diagram.
This antenna should not be confused with an in-phase 5/8λ over 5/8λ collinear. If it was horizontal, made of wire and cut for HF, an old-timer might call it an extended double Zep. However, in addition to having gain over a half wave dipole, it has a predictable 100 Ohm feedpoint impedance which is transformed close to 52 Ohms by a 75 Ohm quarter wave line transformer. About half of the line transformer is formed into a choke to act as a current BALUN to allow co-axial cable feed.
2m Double 5/8
We fashion this antenna into the Flower Pot co-axial design by constructing the antenna using RG59 75 OHM (solid dielectric) cable and bringing the feed co-axially down through the bottom element as shown in the diagram below.
The choke performs the dual role of providing isolation of the high impedance at the end of the bottom element and acting as a BALUN. Seven quarter waves of 75 Ohm cable are required for the bottom 5/8λ element and the coil winding. Seven 1/4 wavelengths of solid dielectric cable at 2m is 2.36m, to this add the 1.225m length needed for the top element to give a total length of 3.585m of 75 Ohm cable to construct a 2m antenna.
There are two ‘fiddly’ parts in making this antenna
The first is forming the 0.2λ section at the feed point of the bottom element. I ran a piece of braid on the outside of the cable sheath, carefully soldering this to the coax braid at the 0.2λ point and used heatshrink to hold it tight against the sheath. I assumed a velocity factor of 0.66 for this section. Care is needed when soldering to the coax braid (and this dictates the use of solid dielectric cable as foam or aircell dielectric will collapse away with the heat of soldering).
The second is ensuring that the braids don’t short at the feed point and I found a piece of heatshrink solved this. The sketch opposite also shows the detail at the radiator feedpoint and the piece of heatshrink acting as a separator.
Otherwise, building the antenna uses the same techniques as used in the basic half wave and single 5/8 versions.
The antenna provides a low VSWR (less than 1.2:1) across the 2m band. But, if you like operating close to 1:1, a small variation in the bottom radiator length gives favour to either the high or low end of the band.
The VSWR plots are shown below.
The Double 5/8 will scale to other frequencies, however the physical size and the mechanical properties of the conduit suggest that the design is more suited to the high VHF band.
Relative gain measurements between the designs
I do not have a means of accurately measuring antenna gain but set up each antenna with a switched attenuator in the feedline to a receiver. The attenuator was not ideal for this purpose, it had only 3,6 10 and 20 dB steps. However, using a local 2m beacon as a signal source and the basic l/2 Flower Pot as a reference and, within the limits of available accuracy and resolution of the steps of an S meter, I determined that the Single 5/8 had about 2dB gain over the l/2 antenna and the Double 5/8 was discernibly in excess of 3dBd gain (but, of course, much less than 6dBd).
Experimental Dual Band High Gain Flower Pot Antenna
An extension of the basic Half-Wave Flower Pot, higher gain is achieved by adding an additional half wave element.
This experimental version is an extension of the basic Half Wave Flower Pot Antenna. Higher gain is achieved by adding an additional half wave element at the fundamental frequency (2m) coupled by a half wave phasing line to drive both half-waves in phase. Theoretically, this should provide an antenna gain of 3dBd at 2m.
The antenna is dual banded to operate on 70cm using the same sleeve technique as used on the dual-band basic Flower Pot. This results in the antenna operating with four half-waves in phase on 70cm and provides a theoretical antenna gain of 6dBd on 70.
Dimensions shown are for a 1st build prototype from a concept drawing and have yet to be refined. It started out as the 2006 Christmas break project but, unfortunately, I haven’t had time to refine the design. However, several members of my local radio club have built one of these and have reported success.
Assembly requires the same approach as used on the basic Flower Pot. The conduit is prepared and drilled as shown below.
The co-ax is pre-cut and trimmed prior to assembly as shown below.
The antenna radiating elements are constructed out of RG58 co-ax. It is essential that braided RG58 is used rather than foil-shielded co-ax (like that supplied by some of our local electronic hobbyist shops) as the bends required at the coil entry and exit points will likely damage and split the foil resulting in the antenna failing to perform.
The Sleeves can be made from any available high conductivity material such as aluminium kitchen foil, building (roof) sarking, disposable ‘baking’ trays, copper or brass shim etc. Materials such as steel or stainless steel are not suitable. The sleeves are fixed in place and protected from the weather and mechanical damage by covering them with UV resistant electrical tape or heat-shrink.
See original Flower Pot articles regarding protection of the coils from “White-Cockatoo Attack”.
VSWR as Measured on the Prototype
I gained my amateur licence in 1971. As a young electronics and communications engineer, I found amateur radio ideal for playing with things that interested me rather than what interested my employer. At the time, I joined the hobby of amateur radio for the technical aspects but, after 35 years plus playing amateur radio, I have since found that there is a much wider aspect to amateur radio than just the technical side.
Probably the first time I experienced this wider aspect was during a work posting to the UK in 1979 where amateur radio became more a means of meeting people; during that year I also held the callsign G8RUU and I have maintained several friendships over the years from contacts made during that visit.
VK2ZOI and VK2DB
We are also, partly, a family of radio amateurs. My treasured, life-partner Dot was first licensed as VK2NVQ in 1978 and now holds the callsign VK2DB. Two of our ‘boys’ also hold callsigns: Peter VK2ZCU and Roger VK2FOTO. My hope is that they will experience as much fun as I have from playing the hobby.
I built my first transmitter and built much of my first station. My favourite band then was 6m and I thoroughly enjoyed the opportunity to build and experiment with various equipment, techniques and antennae. However, as I have become older, I probably build less equipment – other than specialised test equipment- but have maintained an intense interest in antennae. Experimenting with antennae has now become my main interest.
This website describes several of my antenna projects which I believe others might be interested in reading about. In October 2005, an entry level amateur radio licence (the Foundation Licence) was introduced in Australia. So I have especially focussed the projects described here at antennae which a Foundation licensee can build and have work first time. This hopefully will provide a positive experience as Foundation licensees develop those skills necessary to move them toward upgrading to the higher licence levels.
Future Additions: I intend to describe several versions of HF antennae based on the Windom style of antenna which I have developed for both home and portable use. Why Windom? Simply because the Windom is a simple but most efficient and effective multi-band antenna. There will also be several articles on various antennae related test equipment but this material requires much refinement before it can be published on this site.
Before I sign-off on this page, I would like to thank my son Ben for his much appreciated effort in designing and compiling this website. No, he is not an amateur yet but his mother and I are working on it.
73 de John VK2ZOI