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.
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.
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!