The Science of Successful antenna design



As promised, James, Brad and Hughes, and anyone else interested, some info on turnstiles and the methods of antenna matching.

James made the following statement:

I came across this link  which explains how to build what they call a turn stile antenna.  Could anyone try and explain why this antenna would work as I had really poor performance with it.  I ended up using a simple cross dipole on the RX side with much better results.

I gave the start of an answer in a long reply which can be found in his post : 433 UHF LRS Antenna "Turn Stile"

Some further questions where asked regarding the methods of match the antenna, and the antenna tuning, so I will try to elaborate. 

First some fundamentals.

When I speak of antenna radiation it also includes the reverse, that is, the antenna 'collecting' radiated energy from the ether. 

Any piece of wire will radiate energy when connected to a source of RF ( Your transmitter or receiver). How WELL it radiates that applied energy  is dependent on only two factors-

  - That the antenna is resonant at the same frequency as the radio signal applied and

  - That the feedpoint of the antenna is matched to the impedance of the attached transmitter energy source.


Maximum energy tranfer between source and load ( the antenna) occurs ONLY when the load and source impedance's are equal.

How well the antenna radiates that energy in any or all directions, if the above conditions are met, is then only dependent on the antenna design, shape, or style. For example, a simple vertical 1/4 wave radiator will radiate a doughnut shaped pattern, in all direction of the compass, with low energy upwards and downwards, in the direction of the antenna element.

A yagi type antenna, on the other hand, ( such as your vhf or uhf TV antenna) focuses the energy in a single direction, as would a torch. Antenna do not have gain. They focus the energy to a greater or lesser extent, in a direction of design, but do this by robbing energy from other radiating directions - Your torch puts more light out the lens end, with nothing out the rear end. If you remove the reflector from behind the torch bulb, the light is radiated in all directions, omnidirectionally, but is much weaker at any distant point than the focused beam.


Antenna radiation is polarized; that is to say, the radiated electromagnetic wave has a net polarization plane. This is usually either linear or circular. Elliptical polarization is also found, but that is merely a mix of the two former types.

A 1/4wave vertical antenna will radiate linearly, with vertical polarization. Placed on its side it will radiate horizontal polarization.  A Helical antenna ( looks like a coil of wire wound in a screw fashion) wound clockwise when viewed from behind will radiate clockwise circular polarization, and vise versa.

For reception of maximum energy, the two antenna must be identically polarized. There is a massive loss of signal ( easily some 30dB, although the theoretical loss is infinite) if one attempts to receive a horizontally polarized signal with a vertically polarized antenna. Similarly, there are massive losses if trying to receive a circularly polarized signal with an antenna of the opposite circular sense.

The odd man out is that there is only a 3db loss between an antenna that is circularly polarized and one that is linear.

Why would one use circular polarization?

If the two antenna in question could not be made to maintain similar attitudes, such as one in a pitching , rolling aircraft, then there would be unacceptable signal losses as the aircraft banks and pitches. So you could use a vertical on the aircraft, and a helical or turnstile, or similar, on the ground segment. This way you would only ever experience a 3db maximum loss. ( all assuming good line of sight view). Or you could gain back the 3db loss by fitting a similar circularly polarized antenna on the aircraft, giving the best of both worlds. But you actually gain more than that with circular polarization at both ends.

Assume first that the two antenna are simple vertical monopoles, radiating vertically polarized signals. When you are flying, at the flying club, etc, you are probably near some metal structures, the 'hanger' , cars and other vehicles, etc. All these structures reflect the same energy you are trying to receive. In addition, when the aircraft is low and far, the RF transmitted by the A/C antenna follows two paths to your receiver - one directly, and one via a reflection from the ground, somewhat midway between you and the A/C.  What happens to the reflected wave is that the polarization is changed in unpredictable ways. Your receiver ( and antenna) does not know or care where the received energy comes from, so it receives this reflected energy as well. These multitude of received waves add constructively and destructively with the main received wave, causing large, short duration, signal drop-outs - a sort of 'flutter' in the signal.

If both antenna are circularly polarized, however, the picture is quite different. When the circular polarized waveform is reflected , it REVERSES its polarization. When this reversed polarized signal arrives at your receiving antenna it is largely rejected and hugely attenuated, so interfering minimally with the main received signal.



On to Issues of resonance and matching.

To repeat a little in my post to James:

Most simple linear antenna are either of the monopole or dipole form. A single monopole ( 1/4 wave vertical for example) or a single dipole will only radiate linear polarization.

Any antenna is only resonant when it is exactly the correct length AT the frequency of operation.( this does not apply to the class of broadband antenna, such as helical antenna, etc. The helical will easily cover an ocatve with good performance).

At resonance the antenna will exhibit its characteristic feedpoint impedance. Feedpoint impedance is expressed with two terms, the pure resistive part, and the reactive ( j operator) part.

Most transmitters and receivers terminal impedance are made to be 50ohms resistive, or very close to that. So it stands to reason the antenna must also be 50ohm resistive to have max energy transfer.  However, none of the antenna are that obliging, so we have to do some feedpoint matching to meet the criteria.

A 1/4wave vertical monopole over a ground plane has a resistive feed point of around 75ohms. A half wave dipole is around 72ohms. As with resistors, placing two dipole in parallel as in the IBcrazy turnstile, will result in a feedpoint impedance of 35ohms.

A 75ohm feed connected to a 50ohm coax and transmitter will exhibit a 1.5:1 SWR ( the ratio of power going out to power reflected). A 1.5:1 SWR means that approx 3% of your transmitter power is not being radiated. ( 30milliwatts for a 1watt transmitter). That is not so bad, and we can live with an SWR of 1.5:1 in most cases.

The turnstile antenna is a pair of crossed dipoles, fed 90deg out of phase with each other, thereby generating circular polarisation. You CANNOT simply connect the dipole in parallel at the coax feedpoint though. Apart from the halving of impedance ( which we decided we can live with) the radiation pattern and polarization of the antenna will be totally destroyed by unwanted radiation from the coax cable. The RF energy, at the dipole connection point, 'leaks' out and currents then flow down the outer shield of the coax. As mentioned previously, any piece of wire will radiate RF energy, and so the coax radiates this energy, and the radiation again adds constructively and destructively with the main antenna radiation, causes complete distortion and signal nulls in the pattern. This radiation from the coax MUST be prevented.

This is done by means of a Balun transformer. - which is is an acronym for 'Balanced to Unbalanced transformer'.

A dipole is a balanced device - it is electrical equal along each element, outwards from the feedpoint. It therefore requires that the feedpoint be fed in a balanced fashion. Coax cable is an an balanced feeder - the shield is at ground potential, while the inner core carries the energy. This effectively ( oversimplifying a little) connects the one dipole half to the 'live' core, and the other half to 'ground' unbalancing the dipole. This causes currents to flow on the coax outer shield, and distortion of the dipole radiation pattern.

Baluns can be constructed from coax cable, but the accuracy required in coax cable length ( they are normally length multiples of 1/4 wavelength) is very critical, especially in the GHz range - 0.5mm can have a great effect.

The turnstile is not new - it is some 50 to 60 years old, and is well researched and published. Up to the VHF and lower UHF region , the coax balun, with embedded impedance match transmission line transformer, is used, along these lines:


For the higher microwave frequencies, a plumbing type version is more appropriate. This is called the spilt tube or split sheath balun, and looks like this when used as a feed for a pair of crossed dipoles.



The balun and feed match consists of an outer and an inner tube. The ration of diameters D/d is chosen to give the desired impedance:

D/d = 1.86 for 75ohms, and 1.5 for 50 ohms.

Typically the outer tube would be around 8mm for use at 2.4GHz.

In order to obtain circular polarisation, I mentioned that the two dipole have to be fed 90deg apart ( phase quadrature).

This can be done as in the coax balun version above ( inserting an extra 1/4wave length of coax in the leg to one dipole gives an extra electrical wavelegnth of 90 degerees). 

Or, this can be achieved by slightly lengthening the one element ( becomes more inductive) and shortening the other( becomes more capacitive) - this also introduces the required phase difference between the elements.

This can be seen in the images above - the one element is typically around 0.21 wavelength per half, while the other is around 0.25 wavelength. One short and one long element penetrate the outer tube and are connected to the inner tube, while the opposite pair of elements are connected only to the outer tube. The outer tube is split or slotted ( 0.5mm width slot). The slot is approx 0.23 wavelength long.



The relationship in length between the two dipoles is critical, typically this would be measured on a network analyser and the feed impedance of each element set to say R+j45 ohms ( longer dipole) and the other to R-j45 ohms. This will give the correct phase relationship between elements. A half mm variation can have a great effect, turning a good antenna into a mediocre one..

The last image above shows a teflon tube - this is inserted in the tube from below, and fits snugly inside the outer tube, and over the inner tube. This is then slid up and down to adjust the 'R' part of R+-jX, till the match is a good 50ohms. This does not affect the antenna radiation pattern or characteristics. Obtaining a 50ohm impedance match can be done by trimming the element lengths as well, at the same time destroying the antenna radiation pattern and circularity.

And that is why it is not so simple to do at home, and why the 'Hobby King'  et al variants sold everywhere are mostly trash..You will probably achieve a few km range with those- remember, any old piece of wire will radiate -  I easily  achieve 15km with 500milliwats at 2.4GHz using two split sheath balun , properly matched and trimmed, crossed dipoles..

For those interested:

References are - RSGB VHF/UHF Manual - page 8.45

Modern Antenna Design - Page 255

Here are some images of my split sheath balun crossed dipoles..




The Nampilot.

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  • Joe,

    Thank you for the long detailed reply. Several points to cover, but first a misunderstanding...

    "Why is a 915MHz dipole bigger than the IBcrazy turnstile?"

    "I would not recommend a plain dipole fitted horizontally to your aircraft"

     I meant to say that because an IBCrazy Turnstile can be laid flat under the wing its size is less of an issue than a dipole that needs to be mounted vertically. I failed to explain that a long rigid antenna protruding vertically out of the top and bottom of the wing makes it very cumbersome and vulnerable when breaking down for transport. The vertical stabilizer is perfect for securing a dipole, and if I can wisely fit two antennas there that would be a good solution.

    Here is my 910MHz Turnstile which I recently learned is flawed by the parallel feed line...



    Where have you mounted the 433MHz dipole on the stab? Can you give me an idea of the stab shape and size? It is not to difficult to possibly mount both dipoles on the stab since they are very different in size and not related in frequency harmonics. Perhaps I can find a way for you to do that?


    I have two versions of the Skywalker one with the 433MHz on the tail, and the other with the 1280MHz VTx.




















    So the above 3 images show each antenna I need to find a home for. Of the 3, the 433MHz is the least critical because the DragonLink range is far beyond the other transmissions. However it is also the hardest to find a home for since its so long. The 1280MHz video is usually the limiting factor, but its a close tie with the 915MHz telemetry. The problem with loosing telemetry is that it often takes a long time to re-establish the connection, whereas the analog video transmission returns instantly when the signal improves.

    I suggested the CP helical on ground ( tracking if need be) and vertical linear...

    I really want to try to stay away from the complexity of antenna tracking. I would prefer to stick with a beam width similar to that of the Patch antenna. A 3-5 turn helical would fit that, but I tried to avoid the 3db penalty. I'm giving that second thought after reading your response, but I still don't fully understand why you are recommending the Helical over a linear antenna like the Patch or Bi-quad. Perhaps the small penalty is to avoid the larger penalty when at steep bank angles? My steep bank angles tend to be only momentary as I change flight lines.

    The down side of a 5/8 antenna is that it must work over a ground plane, so you would have to install it over the fuselage and use some adhesive copper tape on the fuslage to create a ground plane.

    I'm having a hard time picturing this antenna and its installation in a model airplane, but it sounds doable. Do you have any photos or drawings of what you mean?  I have copper tape for controlling slugs in the garden :)




  • Moderator

    Well that's why we are trying to cherry pick the really good stuff into the wiki and have been holding out for Ning 3 the new version of this platform. But that is already way late. Ning 2 is showing its age and it bugs me!

    The best quote from this thread so far IMHO

    I do not acknowledge the existence of multi rotor contraptions

    Loving this thread Joe it ticks loads of boxes for me.

  • David, no magic, just a dash of science...

    But still we manage to work the world on a few milliwatts and a piece of wire...


  • To All..

    Funny how this post reached top of the hit parade, till the blog dropped of the bottom of the DIYd main page - pity, There have been many gems of blogs posted by many folk, holding info of great value, simply lost in the black hole of blogs....

    there must be a better way for people to see and access this type of info.


  • Joe, some of the confusion about antennas I and many others experience goes like this:
    A 50 ohm resistor will match to your finals which will be very happy @SWR 1:1. Almost no RF will be radiated. The misunderstanding people experience is the difference between a "matched" antenna and an efficient radiator. I have a LC network that will match a curtain rod, but it is a crappy radiator.
    Those before us discovered we need the overall impedance of the antenna and feed-line to be a suitable load for the RF stage and an efficiently radiating antenna, at the resonance you want, with the Q you need. I've spent 40 years trying to get my head around the first principles of antenna design and have decided that Physics is a weird and magical thing. And like the man says, "There's no free lunch"
  • Hi Joe,

    Excellent post!.   

    Wilfred, ON4NDO

  • @ iskess;

    You do have a disadvantage on antenna size at those frequencies, for sure. 

    To cover your comments in sequence-

    Why is a 915MHz dipole bigger than the IBcrazy turnstile? The turnstile element ( both of them..) are dipoles, so must be the same or very similar dimensions to the plain dipole..

    I would not recommend a plain dipole fitted horizontally to your aircraft. There is a large signal null off the ends of the dipole, ie, sighting along the length of the dipole you will be in a signal null, so each time the aircraft  turns such that the dipole is in line with the receiver on ground, you can lose signal. The dipole must be vertical - that waythe null is above and below the aircraft, and the null below is of little consequence, since if the A/C is above you, it will not be very high ( 1000meters??..naughty boy..) and the null is not that sharp that you will lose all signal at 100meters up..

    Where have you mounted the 433MHz dipole on the stab? can you give me an idea of the stab shape and size? It is not to difficult to possibly mount both dipoles on the stab since they are very different in size and not related in frequency harmonics. Perhaps I can find a way for you to do that?

    I suggested the CP helical on ground ( tracking if need be) and vertical linear ( 5/8wavelength base loaded vertical)

    for the following - 

    The 5/8 base loaded vertical has around 1.2 to 1.5dB more gain than a dipole. A helical antenna of  6 turns @ 915MHz will give around 10dBi gain, and will be around 450mm long, and 110mm diameter. The 3dB loss due to cross polarization is more than compensated for by the higher gain of the helical, and the extra gain of the 5/8 wave vertical. 

    The down side of a 5/8 antenna is that it must work over a ground plane, so you would have to install it over the fuselage and use some adhesive copper tape on the fuslage to create a ground plane. However, fitting a dipole to the vertical stab will only lose a max of 1.5dB and the system will still work better than the Turnstile with poor coax orientation, so I would go for the vertical dipole in the tail.

    Regarding the 'double Bazooka' Dipole - the link you refer to - An controversial  Dipole, also been around for donkey's years, since around 1940 or so. Its only redeeming factor is that it does not need a balun, but it has so many other bad traits, that it is not really worth it - If wanted , I can expand on it and explain, but the basics are that it obtains it greater frequency bandwidth and easy to achieve low SWR by virtue of its high internal impedance losses  - sort of akin to fitting a pure resistor across the antenna feedpoint, at the coax feedline - this give a good match, over a large frequency range, with poor signal radiation - although this text:

    is aimed more at the low frequencies ( HF, sub 30MHz), the explanations hold true for all frequencies, and losses are  worse at the higher frequencies.

    The length of the coax used to make the bazooka antenna elements  must also take into account the coax velocity factor (due to dialectric effect as a result of the insulating material in the coax cable). This factor varies from 0.6 to 0.85 for different cable, so the calculated 1/2wave length must be multiplied by the cables factor for the actual length to cut to.


    Your second post - Always Feed the dipole at 90degrees, with the coax at 90deg for the greatest distance possible - try for 1 wavelength if you can.  The number of ferrites fitted should make up a total length of at least 1/4 ( quarter) wavelength, not 1 wavelength - I just checked what I said in an earlier post in case I made a typo, but 1/4 it is. You can wind a few turns through a toroid, butI do not like doing that at these frequencies - the space close to the elements feedpoint is critical for  placement of 'stuff' that can disturb the electrical balance, and winding the coax into a coil, or around a ferrite ring, will end up occupying to much of that area - difficult to predict the detrimental effects then. A further disadvantage is that the coax ends up being forced around a small, tight radius, which distorts the inner insulation, creating impedance 'bumps'  in the coax, upsetting the antenna matching again..Neat, smooth, and straight...

  • Ok, so you've convinced me to switch back to dipole antennas on the plane. So how do we build these best to avoid rf traveling down the coax? Feed at 90 degrees and use ferrites? 1 wave length is a lot of ferrites, how about a couplet turns in a torroid?

    What do you think of this 50ohm dipole design?
    It is very interesting, and rather easy to build.

    Thank you so much for sharing your knowledge.
  • No Joe, you don't come across badly at all.
    I don't use those high frequencies. I use 1.3G video, 915 Telemetry, and 433 RC.
    So I'm dealing with big antennas. I use linear polarized dipoles for 1.3 and 433, and a turnstile for 915 telemetry. The advantage if the ibcrazy turnstile is being able to mount it flush with the bottom of the wing. I could go back to a dipole for this too, but 915 makes for a pretty big dipole and the vertical stabilizer is already taken for the 433 dipole.
    Why do you suggest using a CP Helical on the ground with a LP omni on the plane? I'm using LP patch antennas on the ground to avoid the 3db hit. Do you feel the 3db penalty is worth it?
  • @mP1;

    I am not sure if I understand your meaning correctly - you would like to place a tracking antenna on the aircraft? If that is what you mean, yes, that would for sure help. It is the cheapest , most efficient way of obtaining more range.

    However, the complexity , volume and weight problems associated make it a nightmare on the smaller planes. ( I do not acknowledge the existence of multi rotor contraptions ...).

    As an example -

    a 500mW 2.4GHz video tx consumes about 1.5watt from you aircraft battery - that is normal, class C transmitters/amplifier can have efficiency as low as 30%.  Let us assume that if we double the transmit power we can double the range ( NOT true but let it be so for now..). So for a 1watt output we now consume 3watt from the battery, and so on. Doubling the TX power is an increase of 3dB in TX power.

    If the ground antenna is a 3dB 'gain' antenna, and if the A/C antenna is the same, then we achieve some range, say Xkm.

    Now let us fit a 9dB gain helical on ground. That is 6dB more than the original, which is equivelent to doubling the TX power twice, ie, 500mW to 1watt to 2watt. At 2watt the TX consumes 6watts from the battery - that is 500milliamps from a 12volt battery. 4watts of that is simply heat, which you have to get rid off else the TX will fry..

    So with a simple antenna change you 'doubled' your range twofold, with 100% efficiency, no extra battery power, no heat, etc. If you fit a tracker antenna on the A/C with say 6dB gain, you 'double' you range again, but now there are servos to power, space to occupy, etc.

    Obviously there are disadvantages, a 9dB helical will have a beam width of around 55 to 60degrees, so you would need to track the aircraft.


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