This is a reasonably compact, linearly polarised and well behaved antenna, with a gain , when well made, of around 1dBD    ( 1dB more than a dipole). It is smaller than the Big wheel and the Skew Planar wheel, and I have used it very successfully on 868MHz on our SurVoyeur UAV.

It is derived from a 1/2wave ring, located some distance above a large conducting ground plane. The ring is at a point on its circumference shorted to the ground plane by means of a conducting copper strap. The coaxial cable feed line has the center conductor attached to the ring a short distance away from the shorting strap, and its braid shield connected to a corresponding point on the ground plane. directly below the center conductor connection. The Antenna radiates from the gap between the ring and the ground plane - a Slot radiator antenna. Since the ground plane is inconvenient, we can 'roll' it up into a cylinder, the same diameter as the ring, located coaxially directly below the ring. The antenna performance remains intact. However, the long cylinder is just as inconvenient, so we shorten it till its length is the same as the thickness of the wire forming the ring, and the end result is the picture above. Two concentric rings, spaced apart by plastic insulators, with a 4mm wide copper shorting strap, and the feed coax in place. When the dimensions are proportioned correctly, the antenna gives 1dB gain over a dipole, and when laid horizontally, has a vertical radiation pattern identical to the dipole, with a slight null in the direction of the shorting strap.It is linearly polarized.

For 433MHz, the antenna dimensions shown will give an SWR of 1.1:1 at 433MHz, and 1.25:1 at 429MHz and 437MHz


Seen from the side :

The rings are 100mm diameter, 2mm clean copper wire, no plastic insulation.

The plastic insulators can be Servo push rod tubes, or any plastic tube or solid rod, about 6 to 10mm diameter, and about 20 to 25mm long, with 2mm holes drilled in the sides, 15.5mm apart, to take the wire rings. 

To make the rings, take the copper wire, place one end tip in a vice, and with a pair of pliers clamping the other end of a 1meter long piece, pull hard and gently to stretch-straighten the wire. Find a former ( coffee tin, etc) exactly 100mm diameter, and wrap the wire around the former overlapping the wires. With sharp sidecutters cut through both overlapping wires, to make a butt joint in the ring.

Remove the wire from the former, slip over the plastic spacers ( at least 4 prefer 6) and then, using  a 10mm x 5mm piece of thin copper foil, wrap it around the but joint and solder the wire ends together this way. 

Make two such rings, feeding the second ring gently through the second set of holes in each spacer before butt jointing the ends.

Then position the two butt joints over each other and connect a shorting strap of copper foil, 18mm x 4mm wide, from the top ring to the bottom ring. solder the bottom connection, and then the top, ensuring the strap is straight and taut.

Now connect the inner conductor of the 50ohm coax to the upper ring, 9.5mm away from the center of the shorting strap position. then connect the braid of the coax to the lower ring, at the same place.

The shorting strap end of the antenna is low impedance and low voltage. the direct opposite end of the ring is a very high impedance, high voltage point, sensitive to the proximity of auy other objects, servos, wires, etc, as are the end tips of a dipole, for example. Objects close to that region will affect the tuning...Do not place any support plastic pillars  past the two pillars farthest from the coax feedpoint, as depicted in the last image at the bottom of this blog.

                                             Here you can see the shorting strap location.


No spacers past the two on the right of this image...


It is convenient to use tubes for the spacers, as you can fill them with hot melt glue to fix the spacers and rings in place, forming a rigid structure.

The antenna works equally well right side up, or upside down....

Tuning the antenna :

This is a sensitive process, and you will need and RF source at the right frequency, and a decent SWR meter.

Connect the rf source to the SWR meter, and the meter to the antenna via the intended cables.

There re two adjustment to be made - feedpoint impedance, and frequency of resonance.

Feedpoint impedance is adjusted by moving the coax connections to the ring ( the inner conductor and the braid) either closer to or further away from the shorting strap. Why does this work? Well, the shorting strap is a short, so impedance at the strap is 0 ohms.  The opposite side of the rings circumference is a high impedance point, many thousands of ohms, from an RF standpoint. So, somewhere in-between 0 and 'thousands', will lie 50ohms - that is the point you seek, and it is very close to the short.

Frequency adjustment is done by squeezing the end of the two rings opposite to the shorting strap together to lower frequency, or apart to raise it. In other words, the spacing of 15.5mm is reduced or increased appropriately.  If you find you need to increase this spacing by more than 2mm or so at that quadrangle, then the rings are to long in circumference. Cut 2mm out at that point, and join with the wrapped foil trick again, SOLDERING WELL...and tune again.

When you achieve lowest SWR by squeezing, then adjust the coax spacing to the short, try closer or further, for lowest SWR. Work back and forth between these two adjustments, till no more improvement occurs, and you should strive for an SWR better than  1.4:1. 1.2:1 is easily achieved.

This antenna works very well. I have achieved nearly 30km with 300milliwatts on 868MHz, with one of these on the aircraft at 100meters AGL, and a sleeve dipole on the ground at 2meters AGL.

Here are some pics of the 868MHz implementation , the rings made from FR4 PCB material.


This is the datalink and video tx module used on our SurVoyeur aircraft. The ring of copper is the 1/2wave ring radiator ( the other ring lies below it)

The Skew planar wheel is for 2.4GHz Video TX.

To the left of these antenna are the datalink modem and the video TX module.

                                       Here are views of the shorting strap and coax connection

                                                                    to the rings



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  • Hi Kev W - 

    As mentioned in the description, the diameter ( center to center of the wire you use) is 52deg, and the spacing ( also C to C) is 8 deg.

    so 52deg = 52/360 parts of 1 wavelength = 0.1444  wavelength    and

          8 deg = 8/360 parts of 1 wavelength = 0.0222 wavelength.

    So @ your freq of 459MHz - 1 wavelength - 300/495 = 0.606m (606mm)

    therefore Diameter = 0.1444 X 606 =87.5mm

                  Spacing = 0.0222 X 606 = 13.45mm

    Note that increasing the spacing will increase the resonant freq and vise-versa. However, do not change spacing by more than 1 to 1.5mm - large changes will affect the radiation pattern and efficiency.

    Tuning can be done by  means of the tuning 'tab' a indicated above - simply a small capacitor at the end opposite to the shorting stub and coax feed. 

    The spacing of the coax shield and center from the shorting stub should be varied in very small increments - start with the shield connected directly to the short, and the coax center as well, and the tuning tab spaced midway. Check the SWR, then move the coax center away from the short stub by 0.5mm, and recheck, If it improved, move the shield similarly, then the center, etc. keep at it till the swr no longer improves, or worsens. Then move the trim tabs closer or further to see if SWR improves. Oscillating between these adjustment will bring you easily to an SWR of 1:1, or very close. Since you have a vector analyser, set it to sweep from , say 430MHz to 500MHz, and see where the SWR reduces to the lowest - do not worry about the actual frequency at this point - just the SWR to the lowest you can by the described process. When you have achieved that, see what the frequency is where SWR is 1:1. If way to low or to high  ( 5MHz or more maybe) then you need to reduce or increase the ring diameter a few mm. If 2MHz or so away from desired , you can try to adjust the tuning tabs to get it centered ( bring the tabs closer will lower Freq, further apart will raise Freq). This will alter SWR, so then you can adjust coax shield and  center relative to the shorting stub to improve SWR - 1.5:1 or less is fine.

    The construction MUST be firm and rigid, Use wire maybe of 2mm or so - stretch a length of wire between vise and a strong pair of pliers to straighten the wire, and work harden it ( copper wire , please..) The spacing between the two loop affects frequency and SWR radically - the spacing must not easily change in handling, etc. Partly why I went to PCB type construction - was just more rigid, but was not easy - the diameter of the loop is also dependant on the base material ( PCB) dielectric constant. Stick with Wire spaced in air to start...

    All the adjustments ( coax relative to shorting stub, and the tuning tabs) affect each other, so make small changes and 'sidle' up to the correct frequency and lowest SWR.

    Shout if I can help...



  • Hi Joe,

    I am looking to create a compact Rx antenna for mounting on a RC plane/UAV where UHF is used for control not datalink.

    In particular, I am based in the UK and we are limited to using 459MHz, I would be very grateful if you were to confirm the relevant dimensions for a 1/2 wave ring antenna at this frequency (i.e. the OD of the elements and the spacing between them).

    Also, your example for 433MHz states 2mm dia copper wire elements, what would be the optimum dia for the elements at 459MHz?

    I have access to a large range of various diameters of solid copper wires 1.00mm, 1.25mm, 1.50mm, 2.00mm, 2.64mm, 3.25mm, 4.00mm, or for the larger dia could possibly use 1.8mm 2.0mm 2.5mm 2.8mm copper refrigeration tube.

    I note that you changed to a PCB type trace element for your 868MHz, would it be better if I also used copper tape on a non conductive substrate in lieu of wire/tubular elements?

    I have use of a basic VHF/UHF vector antenna impedance analyzer (Times Technology T100+), so can measure VSWA, impedance, return loss etc.

  • Moderator

    Vladimir, now you have me interested...

    Why not? :))) Join forces :)))

  • Ah, thanks Joe, now I have a very good starting point.



  • Vladimir, now you have me interested...


  • Moderator


    May be you need a good flying plane? :))))

  • Hi Gustav,

    Here are some details for you to try out the slot radiator.

    First is the PCB version - dimension are given etc. The PCB version is the most difficult to make as the PCB dielectric constant plays havoc with the ring dimensioning. You will see that the copper ring is surrounded by PCB laminate, rebated below the ring level - even reducing or increasing the depth of the rebate by 0.2mm can shift the ring resonance by 10MHz - not an easy one to cope with. In essence the ring slot radiator is a very high Q element and as such sensitive to dimensions and substrate.

    In order for you to start somewhere when scaling for other frequencies, here are some guidelines:

    Note, the dimensions are in mm, but the actual dimensions are given in terms of degrees where 1 wavelength = 360 degrees.

    You will see from my indicated dimensions the for the 868MHz antenna, both PCB and 'plastic ring' version, the dimensions differ from these guidelines - this is due to the dielectric effect.

    In degrees: ( for wire ring approx 0.7mm to 1mm wire diameter)

    Ring Diameter = 52 degrees

    Height between Rings = 8 degrees

    Match = distance from coax connection to shorting strap = approx 5 degrees.

    So for the PCB version for 868MHz:


    Here are3701752912?profile=original the PCB rings from underside ( non copper side):

    The small tuning tab is a piece of copper foil, about 2mm wide and 3 to 4mm long, soldered to each ring, and bent towards or away from each other to tune the antenna.

    The coax connection to the ring is at the same time moved closer or further from the short and this iteration will bring the SWR below 1.1:1


    The tuning tab is seen here:


    The rings viewed from the copper side.

    This is the plastic ring versions:


    This shows the ring fitted with the two wires, and the short between the two rings:


    This shows the two small wires attached at the end opposite to the coax attachment end, and tuning is by moving the wire tips closer or further apart.


    Hope that helps - shout if you need more info.


  • Ok Gustav, I will prepare some pics with all the dimensions, etc.

    Should I maybe just use email - less pain maybe - if so, pm me with your email and will do dat...



  • Hi Joe,

    I would like to try both options, pcb, and plastic cylinder.

    your dimensions would save me a lot of time !



  • Hi Gustav,

    I will send pics, justwant to know which avenue you wish to pursue - the PCB version ( no lathe needed, but CNC router needed...) or the wires rings on plastic cylinder version...?  Alos, the 868 version can easily be made as the 433MHz version is described in this blog - I can give you dimensions for that as well if you wish - point me...

    Great on the network analyzer - that makes life a breeze...


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