More 433 MHz antenna - the 1/2wave Ring Radiator

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

Joe

Views: 8105

Comment by Gustav Kuhn on July 27, 2015 at 1:39pm

Oops, did I say 915 MHz ?

3DR modems, easier to get.

Will be using your Planar Ring design shortly, just received my miniVna Tiny :-)

With 15000 mAh @ 16 m/s my Talon just might do 120 minutes, but that is with only a GoPro :-(

Love to see some pictures of your new plane....

Regards,

Gustav

Comment by Kev W. on July 28, 2015 at 12:37am

 

Hi Joe and fellow antenna dabblers

I did make a couple of the 1/2 wave ring radiator antenna's, I was aiming for 459MHz, but the results were not what I was after :(

A couple of issues, first being trying to locate a cylindrical former with the exact OD that I could then use to create the wire 'rings'. Whilst I could get a degree of consistency between the two rings by adjusting the circumference by +/- 0.5mm when soldering the ends of the wire - it was not perfect and I found that even the slightest difference in diameter between the two rings adversely affected the performance of the antenna.

The second problem, which Joe highlighted in his prior posts, was the necessity of maintaining a consistent air gap between the two rings at the correct measurement. In my case I was trying to use plastic 'posts' as per the CAD drawing at the beginning of this blog - to be honest I found it nigh on impossible to keep the rings equal distance apart around their entire circumference - again adversely affecting the performance of the antenna.

I am not giving up, but it is mid summer here in the UK and to be honest sitting in my man cave at the solder station is not that appealing when the sun is shining, so this will be an Autumn/Winter project :) 

Comment by Gustav Kuhn on August 3, 2015 at 11:13am

All Joe's Fault....

After I figured out that a "SlimJim" (Google is your friend) might just be the easiest omni antenna for the "test instrument deprived" to make, I wanted one for my new XUAV Talon.

The base station one was kind of "in the ball park", using online data, but for Talon, it needed to be solid, no wires.

So it had to be strip lines, on printed circuit board, half buried inside Talon. (In theory, the foam has hardly any effect, but it has...)

And this is what I came up with, except, remember, SlimJims are end fed halfwave dipoles, and as such, have a 5 to 8 degree uptilt, cool if the bank angle is towards you..........

Still runs circles around those rubber duckies you get with your modems !

Some pictures, no guarantees......

Comment by Gisela & Joe Noci on August 4, 2015 at 4:13am

Looks Good Gustav!

What did you use for the dielectric constant for the FR4 to start with? I find its normally around 4.2 to 4.4. I do prefer the Jpole - easier to set up, and it is easy to add a phased section on top to get another 2 to 2.5dB  - does get a little longer though..What VNA are you using?

I have a ( long in the tooth now..) Rhode&S 20GHz 4 port VNA which still is perfect, but weighs around 40kg - mains only, not really meant for any field work...So I just acquired the 6GHz FieldFox combined VNA/Spec analyser/tracking gen from Keysight ( old Agilent) - Very nice, and very portable..Use it a lot for field antenna patten measurements. Toys,toys, toys...

As long as we can have fun!

Joe

Comment by Gustav Kuhn on August 4, 2015 at 8:00am

Hi Joe,

I tried to calculate what the physical size on FRG would be, but gave up, and just made the first one as per free space measurements.

That one came out at about 750 MHz. From that I factored a correction in, and it came out on 868 MHz !

As you can see from the shorting straps top and bottom, I just cut this one down about 18mm to get to my design frequency.

The plot is with the antenna in the body of my new Talon.

The idea is, as with the VHF SlimJim, that if you copy my dimensions accurately, the antenna should work quite ok.

My VNA, well, it is that little blue box in the pictures !

Just google "miniVNA Tiny", from mini Radio Solutions

I know, not lab quality, but not lab prices either......

And the freeware software is really cool :-)

Hey, he who dies with the most toys, wins!

gk

Comment by Jakob Schmidt on March 3, 2017 at 12:55am


I have been fascinated by this design for a while, mostly due to it's compact size and I now have a project where range is not hugely important, but compactness is, so decided to give this a shot. This one is just roughly assembled to test the assembly process. I printed the mount from PLA, which has an extremely low melting point, but it actually held up well...now I just need to build a SWR meter for 915mhz! 

Comment by DaBit on February 18, 2018 at 3:11pm

Hi Joe,

Thanks a lot for the beautiful antenna designs and explanations behind them! I am very new to antenna design, and I am struggling a bit.

I tried to build this antenna to accompany a motorbike-to-motorbike device I am building (which is using Semtech SX1278 LoRa radios) since they would fit better than a dipole, but with some slight modifications since the bare metal rings without housing are not acceptable.

I built one with a solid 1mm thick copper disc instead of the bottom ring, and one with two 98mm diameter wire rings housed in a circular ABS housing.

Using a simple VNA (sdr-kits) I am able to tune both to resonate at the 433MHz with a return loss of -16..-28dB. However: the antennas are extremely sensitive to their environment. Just picking up the antenna and putting it down in another spot can shift the resonance several MHz and reduce the return loss to -7dB or so. 

To reduce this I tried both a 1/4+3/4 wavelength coax balun and coiling up the coax a few turns as a sort of commonmode choke, but without succes. Touching the feed coax does not shift the resonance or return loss significantly either, so I guess that a radiating coax shield is not the culprit.

I noticed in your pictures that you used PCB material with wider tracks. As I said: I am new to antenna design, but I suppose this makes the resonance wider. Would that help in desensitizing the antenna? If yes, do you have suitable starting dimensions for a 433MHz antenna in a close fitting ABS housing?

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