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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  • Sorry, I have PCB !

  • And PCB !

  • Oh, and measurements!

  • Hi Joe,

    Yes, have lathe(s), have use of the saltmine 3GHz Anritsu network analyzer :-)

    Am using APM, with both Xbee and 3DR telemetry on 868MHz...

    Can you PM me some pictures?



  • Hello Vladimir, That will be fine - let me know how it goes, and if you have problems I will build what you have and help you make it work!!



  • Moderator

    Dear Joe.

    I make a join very accurate. :)

    2 mm brass tube - is OK for replacement solid wire?

  • Hi Vladimir,

    Forgive me, I messed up - my mind ran ahead of me last night- This statement at the end of my reply to you is nonsense:

    And you cannot afford a big blob of solder there - it will affect the inter-ring capacitance as well.

    Since the join in the wire loop should be at the end where the shorting strap is placed ( not the high voltage end..), a larger blob of solder here is not critical. However, the electrical integrity is, the current flow is highest at the shorting strap, so the joint must be good.



  • Oups, I meant "The polarization of a horizontally orientated slot is vertical...", 

    Thanks for all the time and information, :) Its inspired me to dig out my antennas books and try and get my head round slots,


  • Will, 

    It is perhaps not easy to get your mind around the slot antenna...In a conventional dipole, wire, or tubular elements, the antenna current flow is in the metallic elements, establishing the radiated electromagnetic fields.

    In a slot antenna, such as described, there is no current flow in the slot, as there is no conductive paths, so the current flow is in the  flat sheet, or conducting material, around the slot. The field established at the slot periphery by the surrounding current results in radiation form the slot.

    Also, be careful not to confuse things - the polarization of the EM field created by the slot is not vertically polarised...The polarization of a horizontally orientated slot is vertical, and vise versa. It makes for neat antenna in a metallic fuselage, for example - simple cutting a slot in the fuselage, of the vertical stab, etc, gives you your antenna


  • Gustave, my 868MHz RIng/slot antenna:

    The PCB version- these are machined on my CNC router, from normal single sided FR4 PCB material, 1.6mm thick, copper is tin/lead plated ( sorry to all those anti-lead greenies out there...)

    The copper ring outer diameter is 45.5mm, inner is 41. The rings are place on to of each other with the PCB laminate inside, and the copper ring away from each other - they are space apart by 5.5mm, so the outer dimension from copper surface to copper surface = 1.6mm +5.5mm + 1.6mm = 8.7mm.

    I purchase a large sheet of FR4 and made a number of test rings of different diameter, to calculate the dielectric constant of this sheet of FR4 - it turned out to be 4.3. That makes the rings smaller in diameter than what they would be in air only. Once I had the size, I made many from that sheet - the next sheet will probably be different...

    I also made a few non-integrated antenna, ie, just the ring pair which I could position anywhere, and connect with coax to the TX/RX. The size is very similar for the implementation.

    these are made from 0.8mm tinned Cu wire. I machined a cylinder from a thick bar of polypropylene - the cyclinder is 12mm tall, 46mm outside diameter and 42mm inside. On the outside I made two grooves, 0.8mm deep, 0.8mm wide, spaced 7mm apart. Two rings of the wire are tightly wrapped into the groove, with a small overlap of 2mm at one end, and soldered. at the junction, the two rings are shorted together with the short copper foil strap, 2mm wide. The coax feed is attached at the same end as the short, spaced away from the short by 1mm. At the opposite end, two small copper foil tabs - 1mm wide by 3mm long, are soldered to each wire, pointing outwards from the loops. These two tabs can be bent out or in, towards or away from each other, to tune the antenna.

    Somewhere in the zillions of post I have posted photos of this - if they are lost and you want to see, shout, I will post again..

    How will you use this though? I think you are using APM autopilots - not sure those can work with the xBee 869 xBee stuff - protocals, setup, etc??


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