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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  • @justin,

    To measure is to know..

    I think a lot of folk would like to know what they have purchased. I must admit I find it odd that one can purchase capability and performance against simulated specifications..

    For all the other elements of RC/UAV endeavours, most of the parts used by developers comes with some sort of specification - brushless motors have specs related to all pertinent parameters, kv, power, impedances, and inefficiencies ( on the better motors anyway), servos give voltage ratings, torque, ESC's give voltage ratings, max amps, etc, and these tend to be proven and measured..Antenna should be the same, no? 


  • @iskess,

    Not really a perfectionist, but easy to be picky when you have the knowledge AND the test equipment, so forgive me for coming across badly - that is definitely not my intention. I suppose I like to start from the point of doing things 'perfectly' and then letting the inevitable compromise detract therefrom, rather than starting out with a compromise..

    I have no solution for your problem with the thin wing - even with the ferrite beads at the antenna connection, if the coax lies in the same plane it simply behaves as another antenna element, and will distort your antenna pattern completely. In fact, the beads will make it worse with such a configuration, because they 'absorb' the rf energy by, as 'chokes' . Simulation of this configuration indicates the antenna becomes directional in as follows -

    Imagine the antenna in the X shape, ie 90deg between each leg.  Now let the coax be connected to the center feed and allowed it to bisect the 90 deg angle between two elements, while lying in the same plane as the elements. Imaging laying this on a table, and view the antenna along the coax, ie, coax towards your eye centerline. The radiation pattern has a deep null in the direction of you eyes, ie, in the direction of the coax, while showing a 3 to 5 dB increase in radiation from the antenna side AWAY from the coax, with a sort of herring bone pattern to the sides - nulls of up to 2dB deep. So what was an omnidirectional antenna is now full of nulls, dips and peaks, which will appear as the a/c turns.

    Can you not fit the antenna over the thicker part of the fuselage somewhere? Obviously there should not be any avionics, etc below or above it..

    With the options you have with that antenna, you would obtain far better results using a simple 5/8wavelength vetical omni on the aircraft and an 8 or 10 turn helical on the ground - you will easily achieve 10 to 12km with 500milliwatts at 5.8GHz, and the same distance with 300milliwatts on 2.4GHz.

    If interested I can give you some design info on those options.


  • ? Ive got an antenna tracker and i cant help but wonder that something similar on the flyer would make things so much better and more efficient.

  • Ready made RC sent a bunch of antennas to be characterized at Ohio state university in February but they have not released the results from this work. 

  • Joe,
    My plane is foam, so I gather it's safe to mount the turnstile flush with the surface.
    However I also gathered from your response that my wing is too thin to allow for a long enough coax feed. So I will have to stick with IBCrazy's side feed method, however I will add tight ferrite beads for 1/4 wavelength along the coax from the solder joint.
    Are there any other recommendations that I could practically apply.
    I can see you are a perfectionist and this lipstick on a pig process must be really uncomfortable for you ;)
  • Thanks David Scobie, I do enjoy fiddling with antenna, esp in the microwave region!

    73 de V51NC

  • You are bang on with your explanation; thanks for taking the time to write it out. Antenna-tion to detail when making for VHF-UHF-SHF. It helps to have a microscope, steady hands and a vector network analyzer.
    73 de VE3BOW
  • @Sgt Ric, 

    Yes , I have seen the various postings by 'IBcrazy', and that is one of the reasons I was prompted to post this blog. I admire what he has done, but I fear that , since he has 'broken' most or all of the rules on the 'crosshair' and turnstile antenna, that the performances of said antenna are well below parr. I have nowhere found any actual measurements made by him on these antenna with respect to radiation patterns ( E and H planes) and to circular polarization performance ( axial ratio). These measurements require use of an anechoic chamber, some pricey equipment, and known reference antenna. I have only ever seen graphs and plots of computer simulations of his antenna, and that is not the same thing at all. The most difficult parts of antenna design to model tend not to be part of the antenna at all, but the feed mechanism and the antenna surrounding environment. Most inexpensive antenna simulation software does not take into account pattern distortion by feedline radiation, for example.

    So, my jury is out on that one..


  • Thank You Joe I will make a page out of it (which will also link to this blog) and I will add some of the info to it you have already added here.

    It was my thought also that this would be a good time for a stable page that could be augmented as time went on on this increasingly important to us topic.

    I will put a link to it here when the page is ready.

    Best Regards,


  • @ Artem;

    50ohm was simply the easiest impedance to achieve in most solid state amplifiers, and so it kind of stuck - in the old days of valve transmitters and amplifiers, 300ohm was the impedance of choice to some extent, with balances feeder instead of coax cable - balanced feeder is simply two wires spaced apart by a certain distance - that distance and the wire diameter defined the characteristic impedance of the feeder. This was great, since this could be connected directly to a FOLDED dipole - a folded dipole looks like a paperclip, splt in the middle of one of the long legs, and each point hen connected to the balanced feeder. A folded dipole has a characteristic impedance of around 300 ohms, so everyone wins - there are no antenna currents flowing on the feeder, because we have a balanced feeder feeding a balanced antenna, and the impedances are matched. But the feeder is a problem, in that it cannot lie against any metallic object, unlike coax cable which can, etc. So, balanced feeder died out with valves, and we are saddled with coax, 50ohms, and unbalanced to balanced problems...

    But don't dispair, remember what I said about impedance mismatches - unless huge, they can be ignored to a large extent. A 72ohm diople connected via 50ohm coax to a 50ohm TX will show a standing wave ratio of 1.5:1, which is ONLY a 3% loss of signal - very small. BUT, what you must NOT do is try to get the SWR down to 1:1 by trimming the antenna element lengths. The antenna element lengths must be the correct length for RESONANCE, not for matching.


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