Double 5/8 Flower Pot Antenna

Double 5/8 Flower Pot Antenna

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.

The double 5/8 is a co-axially fed variation of the 1¼ wave (vertical) dipole

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.

2m Double 5/8

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.

Piece of heatshrink acting as a separator at the radiator feedpoint

Otherwise, building the antenna uses the same techniques as used in the basic half wave and single 5/8 versions.

Bandwidth

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.

VSWR plot for the Double 5/8

Other frequencies

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).

Single 5/8 Flower Pot Antenna

Single 5/8 Flower Pot Antenna

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.

2m Construction

2m Single 5/8 Flower Pot

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.

VSWR curves for three antennae. 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.

Dual Band Half-Wave Flower Pot Antenna

Dual Band Half-Wave Flower Pot Antenna

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.

Dimensions for a 2m half-wave Flower Pot

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.

Half-Wave Flower Pot Antenna

Half-Wave Flower Pot 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).

Basic arrangement of the 2m Half-Wave version

Drill two holes into the side of the conduit for the choke coil. The ‘top’ hole will be approx 925mm from the end (this distance is the length of the radiator plus a small clearance between its end and the end-cap). The spacing between the holes should be such that the coil turns will be firm and secure. Actual hole diameter and spacing will depend on the cable brand and/or where it was manufactured. It will be close to being two 6mm holes spaced 45mm apart but wind 9 turns temporarily on the conduit and take measurements.

Then take a suitable length of co-ax (I make mine using the one piece of cable, about 5 to 6m long, to reach from the antenna to the transceiver – the length is your choice). From one end, strip off 457mm of the outer sheath and braid to form the top element. It’s not a big problem if you end up with a length that’s a bit short, because a another piece of wire or the discarded braid can be soldered to the top to make the correct length.

Using several “half-hitches”, tie a piece of fishing line (or similar, thin nylon line), say about half a metre long, to the top of the upper element. This line will be used to pull the radiator taut, it will clip over the top of the conduit and be clamped by the end cap.

Now measure 447mm down from the feedpoint (the point where the braid/outer sheath now starts); this is the distance to the start (or top) of the choke coil and mark this position on the coax with a piece of tape, string, paint spot, or whatever, so as to be a reference/stop point when inserting the cable into the conduit.

The antenna is assembled by inserting the radiating portion (together with the piece of nylon line) through the top coil hole and pushing it upwards until the aforementioned reference/stop point disappears into the hole.

Fish-out (pun intended) the nylon line and by pulling it taut, temporarily straighten the radiator to “set” the bend at the choke coil top.

The coil is then wound on the outside of the conduit and the remainder of the cable inserted through the bottom coil hole and pushed down. Using firm but careful manipulation, the cable is pushed and tugged through the exit hole until the coil is tightly wound and secure. This must be done without altering the bottom radiator length (you should continue to just see your ‘mark’ through the top hole.

At the top, cut a small (thin, narrow) notch in the edge of the conduit, pull the nylon line taut and nip the nylon line into the notch. Later, when an end cap is fitted, the cap will clamp the nylon line solidly in place and hold the radiator straight.

Fit a connector, measure the VSWR, if necessary trim the top element.

However, you should find that very little trimming, if any, will be necessary. If you dual band the antenna, the 2m resonance will appear to shift upwards slightly. So, don’t be too concerned if your antenna at this stage appears to have its VSWR curve dip a bit below 146 MHz. The VSWR plot of the 2m Half-Wave antenna should look like the following:

VSWR plot of the 2m Half-Wave antenna

When you are happy with the VSWR, finally, cap the top, securing the nylon line and the radiator in place.

Don’t block or seal the bottom end of the conduit. This is to allow condensation etc to drain away.

Tips

File the coil holes to ease the bends.

Heatshrink the feedpoint to seal against water entry. Also heatshrink the coil’s entry and exit points to minimise water entry.

Heatshrink the bottom end to provide a buffer for the exiting coax and neaten the base.

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!

Cocky Proofing

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
	25mm	32mm	50mm
4	-	160	-
5	150	136	85
8	142	106	65
9	135	100	60
10	129	95	57
12	117	84	52
15	105	75	47

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

To build a 6m version, use 50mm (OD) conduit. The dimensions are in the following diagram.

6m version

VSWR plot of the 6m Half-Wave antenna

Experimental Dual Band High Gain Flower Pot Antenna

Experimental Dual Band High Gain Flower Pot Antenna

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.

Experimental Dual Band High Gain Flower Pot dimensions

Assembly requires the same approach as used on the basic Flower Pot. The conduit is prepared and drilled as shown below.

Conduit Preparation Details

The co-ax is pre-cut and trimmed prior to assembly as shown below.

Coax pre-Trim Details

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:-
144 MHz	1.1:1
145 MHz	1.2:1
146 MHz	1.1:1
147 MHz	1.2:1
148 MHz	1.2:1
433 MHz	1.05:1
438 MHz	1.1:1
443 MHz	1.1:1
448 MHz	1.05:1

Простая антенна для 2 м и 70 см диапазонов

Портативная антенна из коаксиального кабеля для 145 и 435 МГц

Простая антенна для 2 м и 70 см диапазонов


Портативная антенна из коаксиального кабеля для 145 МГц и 435 МГц

Радиолюбители, работающие на двух УКВ-диапазонах (145 МГц и 435 МГц) знают, как удобно использовать двух-диапазонную антенну.

Одну из таких антенн нидерландского радиолюбителя Франка Бремера (PA0FBK), разработанной специально для 2-х метрового и 70 см диапазонов мы сегодня рассмотрим.

Она полностью изготовлена из коаксиального кабеля RG58 и является переносной, что иногда очень удобно.

Антенна представляет собой запитываемый с конца 1/2 волновой диполь для диапазона 2 м и 3/2 волновой диполь для диапазона 70 см.

Описание изготовления антенны

Она выполнена из одного отрезка коаксиального кабеля RG58. В частях «A» и «C» (каждая длиной 360 мм) были удалены внешняя оболочка и экран, в частях «B» (230 мм) и «D» (288 мм) — все оставлено. В верхнем конце части «E» (35 мм) внутренний проводник и экран соединены.
Части «D» и «E» внизу соединяются с разъемом BNC или другим, подходящим для вашей радиостанции.

Работа антенны

Эта антенна работает следующим образом: части «A», «B» и «C» вместе являются полуволновым излучателем на 2 м. Часть «B» резонируют на 70 см и ведет себя, как коаксиальная связь между «A» и «C», которые являются половинами волнового излучателя на 70 см.

Для оптимальной адаптации к 50 Омам есть трансформатор — части «D» и «E». Длины «D» и «E» важны, изменяя их, можете улучшить SWR, в случае необходимости.

Эта антенна столь же хороша, как и «J» антенна и имеет то преимущество, что работает и на 2 м и на 70 см.

Я выбрал эту антенну потому, что её легко сложить в сумку и использовать при работе в командировках.

В гостинице, закрепив эту антенну на карнизе в комнате на втором этаже, я проводил постоянные сеансы радиосвязи портативной радиостанцией на расстояние 75 км. У моего корреспондента использовалась антенна GP на крыше пятиэтажного дома.

Длину отрезка E (короткозамкнутого) нужно взять на несколько сантиметров больше и закорачивая оплётку с центральной жилой иголкой, настроить антенну на минимальный КСВ.

От антенны до радиостанции я использовал кусок коаксиального кабеля RG58, около метра длиной, с четвертьволновым отсекающим стаканом для 435 МГц, выполненным из оплётки коаксиального кабеля. Место соединения оплёток я не паял, а выполнил бандаж из «кроссировки», предварительно сняв изоляцию и обмотал всё изолентой.

Справка. Коаксиальный стакан

Самый эффективный на УКВ способ отсечки тока — 1/4 волновый стакан, который известен всем по картинкам в букварях антенн в виде трубы длиной 0,24 λ и диаметром втрое больше диаметра кабеля, нижний край которой имеет дно, соединенное с оплеткой кабеля. Его обычно называют симметрирующим, но дело не в терминах. Его сопротивление ВЧ току около 5000 Ом, но для антенн ВК в таком исполнении он слишком громоздок. Более технологичное, без затратное и не требующее настройки устройство для отсечки тока можно выполнить в виде стакана из оплетки от более толстого кабеля, плотно лежащего на внешней оболочке кабеля.

Длина стакана зависит от диэлектрической проницаемости оболочки кабеля и равна 1/4 λ х К укор. Для полиэтиленовой (ПЭ) оболочки кабеля К укор 0,667, для поливинилхлоридной (ПВХ) — около 0,59.
Сопротивление z стакана можно рассчитать по формуле: z = ρ / tha L, где ρ — волновое сопротивление коаксиальной линии, образованной оплеткой кабеля и стаканом, а — затухание в неперах на 1 метр (1 непер = 8,686 дб),tha — гиперболическая функция а, L — длина стакана в м.

Если учесть,что при затухании меньше 0,21 непера (2 дб/м) tha и а практически равны, а нп заменить на дб, получится z = 9ρ /a L, где а — затухание в привычных дб/метр. Стакан работает с высоким КСВ в нём. При этом потери в нём будут примерно втрое больше, чем рассчитанные при КСВ 1,0 и формулу z стакана можно записать так: z = 3ρ /a L, где а — потери в дб/м, рассчитанные для коаксиала стакана при КСВ 1,0 в нём.

Длина стакана на кабеле с ПЭ оболочкой для 145 мгц 345 мм, для 435 мгц 114 мм, z стакана 1500 Ом.
На частотах ±2% от частоты резонанса (142 и 148 МГц; 426 и 444 МГц) он снижается вдвое.
Стакан может работать и на частотах, где его длина составляет 3/4λ, 5/4λ и т.д.,
но его z и соответственно эффективность отсечки тока будут падать примерно в V¯3, V¯5 и т. д.
Иными словами, стакан длиной 345 мм на ПЭ оболочке будет достаточно эффективен и на 435 МГц

Встречалось описание этой антенны, где на кабеле снижение на расстоянии 195 мм от разъёма для подключения антенны изготавливался ВЧ-дроссель из 6-ти витков кабеля на каркасе диаметром 20 мм.