Dipole style antenna for 433MHz


Hugues and a few other folk have been looking at using dipoles on 433MHz, I presume this to be the telemetry or RCS radio frequencies used by them. 

I have been playing with some concepts here to try come up with a repeatable implementation with reasonable performance. The main aim is to obtain a good match to the feedline and RX/TX equipment, and to ensure that the dipole radiation pattern is preserved as best possible. The idea is to use a vertically orientated sleeve dipole on the ground as this could be housed in a plastic tube with the radio connector at the base of the tube. On the aircraft such an antenna is rather unwieldy since it is around 400 to 500mm tall, so a conventional dipole coaxed into a V form is suggested. This can be cut into the fuselage and vertical stabiliser foam, or simply taped to the foam on the outside for tests. It could also be taped onto the horizontal stabiliser, one element on each, with the coax down the center of the fuselage, although the signal polarisation to the ground vertical is not optimum this way. If this were the prefered installation, then a similar 'V' could be used on ground.

I have not indicated all the final construction details, such as the tube into which the sleeve dipole could fit, or a substrate onto which the V antenna could be fitted for robustness when used for ground antenna - I can help with further ideas and suggestions in this regard if need be - it is just a little difficult as I do not know what materials may be available to each in his part of the world...

Onto the antennae:

The sleeve dipole resembles a section of coax cable with a 1/4wave section of the braid folded back onto itself, over the insulation jacket. This leaves a 1/4wave section of the center conductor at the top, and then below the 1/4 wave braid.

This forms the dipole and the braid section forms both the one half of the radiator, and a balun to suppress unwanted current from flowing down the outside of the coax shield and degrading the radiation pattern of the antenna.

However, the performance and matching of such a 'folded braid' sleeve dipole is not optimum.  A more optimum sleeve is where the inside diameter of the sleeve is 2 to 4 times the outer diameter of the coax cable, and when around 4 times, the sleeve should be around 0.21 wavelengths long. With this in mind, an antenna was constructed and tuned to see if this would be easily reproducible. The first used the 'folded braid' method, and then a further three employed brass tubes as the sleeve, of 7mm, 9.4mm and 11mm internal diameter. In each case the sleeve length started out at 0.26 wavelength.

In each case the antenna was tuned by trimming the length of the top vertical element till resonant. Then an RF current probe with a spectrum analyser was used to measure the currents flowing in the braid of the coax , a 400mm long section below the sleeve on the antenna. Then the sleeve was trimmed in length by 2mm, and the top element re-trimmed for resonance, and the currents measured again. This process was repeated on for each diameter sleeve tube.

The results indicate that for the thinner diameter tubes ( folded braid being the 'thinnest') the SWR would remain above 1.5:1, and the common mode current on the coax outer shield was still high. As the tube diameter increased, the tube would need shortening, the top element lengthening, and the SWR would improve. Simultaneously, the coax currents began to reduce dramatically. The 9.4mm diameter tube showed excellent results, with SWR of 1.15:1 at 434MHz, a tube length of 145mm,( around 0.21 to 0.22 wavelength,)  a top element length of 182mm, and the coax current was less than 7%  of that measured on the folded braid version. The 11mm tube showed no worthwhile improvement, so the 9.4mm tube is chosen as the optimum.

Dimensions are indicated in the drawings below: 

The construction is as follows - the tube has a small brass nipple , or disc, with a hole in it dimensioned to pas the coax braid. This nipple is soldered into the top of the tube. Inside the tube are three plastic spacers, through which the coax passes, keeping the coax centered in the tube. The end of the coax is tripped of the insulation for about 2mm for the center conductor, and then the braid cut back to expose around 2mm on the insulation of the center conductor. The outer braid insulation is cut back about 6mm and the braid then enters the nipple, with the exposed center conductor protruding. The braid is then soldered to the nipple, and the top element soldered to the protruding coax center conductor.

different sized tubes with top nipples                                                  Nipple and coax prepared





Nipple soldered to coax and braid                                                       Exploded view



Plastic spacers fitted




The following images show SWR and Smith chart data for the final antenna. Note that this antenna is not fitted into any housing or tube. I fitted it into a length of 20mm diameter PVC conduit tubing to measure the effect, and it is quite dramatic. Therefore, if anyone wished to package is so, please let me know what the tubing that you wish to use is - I will try to obtain something similar, and retrim the elements to compensate for the tube shortening effect.

3689597275?profile=originalSWR is 1.15:1 with no plastic overtube.


SWR is 1.04:1 at 425.35MHz - a big change with the plastic over tube.

SWR at 434MHz is now 1.8:1 with the overtube.

The V Dipole is made with a 1.4 wave section of the same coax serving as a balun to suppress the common mode coax currents. It is seen as the parallel section in the photo at the beginning of this blog.


here the antenna is taped to a tall block of polystyrene, which does not affect the antenna characteristics, while taking measurements. The balun is clearly visible.


   3689597327?profile=originalhis shows how the balun is terminated at the top -

the main coax shield connects to the left element.

The main coax center conductor connects to the shield of the balun and to the right hand element. The other shield end of the balun section ( photo above) connects to the main coax braid at that point.

The length of the balun is the same as one half dipole element, close to a 1.4 wave. Measurements were taken with the current probe with and without the balun - with the balun current levels were almost 22dB less, a significant amount.


This shows the balun shorted to the main coax at the balun bottom end.


Here are SWR Plots:


SWR is 1.01:1 at 434MHz


In this case the bare copper wire dipole elements was replaced with 'servo lead wire' - wire covered in plastic insulation - the resonant frequency has moved down considerably.

SWR is now 1.07:1 at 422MHz.

You cannot just put any plastic over the wires without re-tuning.

Also, 2mm removed from the wire ends shifts the frequency up by 1MHz - it is sensitive to adjustment! 


This Smiths Chart plot shows the excellent match of this antenna - no reactance and a good 50 ohm match.

The 50 Ohm match is achieved by the V shaped elements - bending them into the V form lowers the feedpoint impedance in this antenna. The element show very good match for inter element angles between 100 and 115 degrees, typical of V antennae.

The elements can be bent upwards or downwards ( away from or towards the coax feeder), as required by the installation.

Tapping the antenna elements to an EP or polystyrene aircraft frame or wings will have no effect on the element length or SWR. However, placing the elements against any fibreglass or plastic ( PVC, etc) surfaces will affect the tuning detrimentally. The shrink iron-on cover materials used on model planes will have no effect either.

I hope this will be of some use to all - if anyones ends up building any of these ideas, let me know if I can help with re-tuning for you choice of materials and mounting methods - I will try!


The Nampilot.

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  • 100KM

    Thanks Joe, sorry for the late reply - I've been thinking about it :) Perhaps our measurements were not considering the permittivity - a PCB antenna is so convenient - we might give it a shot again. The video transmitter is only 600mw. Thanks for the insightful answer.

  • Hello Hein,

    Could be due to a number of factors:

    At 1.2GHz the RF Skin depth is approx 0.001mm ie, 1um. The copper thickness on the PCB was probably at least 2oz, ie, 35um, so no detrimental skin effect issues here. The fact that it is flat strip - from a conductivity point of view  - a 4mm wide track would equate to a wire OD of  approx 1.3mm. What the flat strip does do is broaden the bandwidth somewhat. 

    Why did it not perform? Well, first of, normal PCB material ( FR4) is very lossy in the UHF and SHF frequencies, basically because of the epoxy resin in the substrate, and the quality of the glass fibre. For example, a dipole at 1.2GHz, made from RF4 PCB, would start charring along the elements with input powers of 100watts. A similar dipole made from thin tubing would still work fine with 1000watts input...

    Secondly, the V antenna would end up much shorter than its simple wire or tubing counterpart, because of the Permittivity of the fibreglass material - around 4 to 4.2:1. This means the element would be almost half as long, and more sensitive to adjacent wiring, electronics, etc.  I would suggest the losses played a bigger role maybe.



  • 100KM

    Hi Joe - you mention the "skin effect" - this might be applicable to a problem I've had before. I had a PC antenna made in the shape of a V for 1.2ghz. The element is therefore not a thin tube / wire shape, but a flat track shape - if that makes sense.

    The result was good reception nearby but as I would increase distance the signal would deteriorate quickly - sooner that you would expect a badly tuned antenna.

    Would that be because the element was flat 4mm track in stead of 0.5mm copper wire?

  • Moderator

    Very helpful Joe, as always.

  • Hello Trung.

    You are quite correct. Tubing is more often used in the lower frequencies, since the element lengths are greater, and to have sensible L/D ratios, so the diameters must increase. The only way then is to use thin wall tubing to keep weight down. Wall thickness from VHF frequencies up can be very thin - 0.5mm and less above 1GHz is fine. RF currents travelling along a conductor exhibit a phenomena known as the skin effect, which simply means the RF current in fact travels in a very 'thin' region, on the conductor outer periphery, ie, in the 'skin' of the conductor. This can in some cases be a few um to a few 100 um deep.  Thats also why its best to try use the most conductive materials to hand, since the current will travel in the thin outer layer only. If the material has a high resistivity to starts, say, stainless steel or spring steel, then the losses will be even higher when the current travel is limited to a thin skin layer.

    And yes, in all case the D is the outer diameter.

    If you maybe have access to these books -

    RSGB VHF-UHF Manual, chapter 8 page 5


    RSGB Radio Communication Handbook, Chapter 13 page 6

    These are easy to use graphs that help in computing element length variation due to L/D ratios.  Not that inany case these are always 'perfect' computations, implying elements in free space, etc. This is seldom so, with antenna close to the rest of the systems electronics and wiring. So, in all cases, compute lengths, correct for L/D, and then add 20% to that, and then trim for resonance with an SWR meter, etc.



  • 100KM

    Hello Joe,

    I have seen some dipoles (and yagis) use tubes as elements rather than solid rods.  What advantages or disadvantages are there?  (besides improved rigidity)  Would the same length to diameter rules apply?  Would the tube OD be used for this calculation?  I would like to make a 1280mHz version of your sleeved dipole and was thinking of using a small dia tube instead of rod for the top element.  Both modifications to your above antenna would yield lower L/D.



  • Hi Ignacio,

    I looked at your posted results - Maybe unscientific you say, but it got the job done! And in the end, and increase in observed signal is an increase, as long as the test environment was the same for all test cases. Well done!  I would suggest that for reliability maybe cover each of the soldered joints with a dollop of two-part epoxy , just to help keep it all together. I presume the coax you used is a PCV type insulation - ie, the soldering iron heat melts the plastic? That makes it difficult to solder a good joint, so I would suggest try get some Teflon/PTFE coax, RG316 type, or similar. That makes it so much easier to solder.

    Regarding the Moxon - I have made a few of them, used specifically as a tracking antenna in the belly of a larger plane, used for tracking the 433MHz electronic tags fitted to animal tracking collars. The antenna was swept side to side, about plus and minus 60degrees from the down looking position. The Moxon works fine, but the tuning is a bit tricky and the coax feed needs some ferrite beads to create a balun. For my application we eventually moved to two 3 element Yagis, phased and stationary inside the belly, at a slight outwards looking angle. We did get better range, but there is no reason the Moxon would not work as well. 

    The Moxon is neat in that the dimensions are reduced from those of the Yagi, so easy to handle, etc. I would suggest to test the antenna though, esp the direction of 'gain' versus the rear direction - if the antenna is not done properly, you may find the gain rearwards and forward the same or reversed...

    And again, Brass pipe is not a given, if you can get copper pipe, use that - it is better, although you wont really measure any difference. 



  • Hello Joe!

    Yesterday I built two vees, and today I tested them against very bad rubber duckys and a moxon rectangle I had previously built (by the way: what do you think of the moxon rectangle as a TX antenna?)

    I posted the results here: http://www.rcgroups.com/forums/showthread.php?t=2686238

    As soon as I get my hands on some brass pipe I will try your first antenna.



  • Hello Guys-

    Hughes, you are correct - the thicker the elements, within reason, the more broad band the antenna will be. However, the element length to diameter ratio also comes into the equation, so making the element thicker will require retuning.. Generally, the thicker the element, the shorter is needs to be trimmed to achieve resonance again. 

    For example, a length to diameter (L/D) ratio of 100 will require the element to be shortened to approx 0.92 of the computed length and a ratio of around 10,000 will require shortening to around 0.98 of computed.

    Ignatio, copper is perfect, better than brass or bronze - the skin resistance is far less - I use and easily refer to brass as that is what we can easily obtain here! Copper is very limited in size choices in my part of the world!


  • Ok, last question I hope! I cannot find a brass (bronze) tube anywhere. I can easily find copper pipes of similar dimensions, not bronze...

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