Helix Antenna dimensions

[Douglas Decker]

I am still trying to build a working 21cm helix. Some might ask why. The answer is "mine is not to question mine is but to do or Die)I have no desire to die but just want to make one Helix that works and has the right frequency and SWR. Beam Width is also important.

So here goes. I have  found a program that allows me to 3d print a form in which to build the Helix on. See picture below.( I can only print a 7 turn with my 3d printer but am working on modifying the code in the GitHub link to be able to print it in two sections for a 10 turn antenna.)

The author\designer  lets the cad program do all the work if you enter all your requirements.

https://github.com/sgcderek/helix-antenna-scaffold

So now the questions?

So question number #1:  Does the length of the wire used to wind a 7 turn helix (Which is   57.11716037 inches)include the 1/4 wavelength matching section( 2.0794055673)or not?

Now to help I refer to John Kraus:

Item "A""= axial length which does not appear to include the matching section.?

So question number #2

So the dimensions I used for the helix are compatible with the next chart for beam mode.

Beam Mode is the hatched section

Circumference of 1 wavelength per turn, 15 degree pitch angle, and a spacing of.21 wavelengths.

I am just hoping that some person smarter then can help me verify the above.

Tks for any help. 73's

Doug

K5WMT. 

[Jeff Kruth]

One thing I always found interesting: A helix is a travelling wave antenna. The media in which the helical element in is not only air usually, there is some support structure. The coil form or supports affect the wave velocity due to the dielectric effects based on the material permittivity and thickness. That piece of ABS pipe or fiberglass adds stuff!. Also post style supports will affect it often times in such a way as to spoil the circularity (axial ratio). Most analyses do not take this into account: They assume the helix is a free standing coil with no support structure to perturb the fields.

You might say "Oh no big deal" then you wonder why the helix performs so poorly....

You can make all the forms you want, but until you that into account the material around the coil, you wont get the predicted answer. A wavelength in vacuum (or air) is different that a wavelength in fiberglass. Also the loss tangent of the material will affect the antenna efficiency so while the directivity may be close, the gain will be off.

The plastic foam supports used for flower arrangements (like in the Michaels craft stores) are a big air bubble foam that has a low dielectric constant. Experiment with air and foam supported designs to home in on this effect. 

J. Kruth

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[Alex P]

" A helix is a travelling wave antenna "  

so is a Yagi

The Yagi antenna consists of a single 'feed' or 'driven' element, typically a dipole or a folded dipole antenna.

Having the reflector slightly longer than resonant serves two purposes. The first is that the larger the element is, the better of a physical reflector it becomes.

Secondly, if the reflector is longer than its resonant length, the impedance of the reflector will be inductive. 

Hence, the current on the reflector lags the voltage induced on the reflector. 

The director elements will be shorter than resonant, making them capacitive, so that the current leads the voltage. 

This will cause a phase distribution to occur across the elements, simulating the phase progression of a plane wave across the array of elements. 

This leads to the (Yagi ) array being designated as a travelling wave antenna.

https://antenna-theory.com/antennas/travelling/yagi.php

Alex

[Jeff Kruth]

True, mostly. Another way of looking at it as a multipole resonant structure, like a band pass filter,Its response is measured in its spatial filtering ability (directivity) instead of its spectrum filtering ability (frequency bandpass).  More elements (poles), better spatial filtering (more gain). Travelling wave antennas tend to be broadband (LPDA's, Helixes. so on) with lower gain, wide frequency. Bandwidth & gain work inversely. So true travelling wave antennas are not as frequency dependent as a yagi where the bandwidth (chosen VSWR range) is usually < 10% while for a helix it is 100% (octave minimum, maybe as much as decade,10:1 bandwidth).

J.Kruth

In a message dated 1/11/2024 11:36:21 AM Eastern Standard Time, sara...@googlegroups.com writes:

 

" A helix is a travelling wave antenna "  

 

so is a Yagi (Not really, usually travelling wave antennas have broader bandwidth, not sharply tuned, and a resulting lower gain, as all element components do not "add" in phase, lower gain. A yagi is a resonant structure, narrowband)

 

 

 

 

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.

[b alex pettit jr]

Jeff,

Do you have info on Slow-Wave-Structures beyond what is in this link ?

https://lea.hamradio.si/~s53mv/cigar/sws.html

Thanks,

Alex

Thanks

[Douglas Decker]

So back to Helix antennas, some more information.

Axial-mode helical[edit]End-fire helical satellite communications antenna, Scott Air Force base, Illinois, USA. Satellite communication systems often use circularly polarized radio waves, because the satellite antenna may be oriented at any angle in space without affecting the transmission, and axial-mode (end-fire) helical antennas are often used as the ground antenna.Axial-mode helical antenna for wireless LAN communication, working frequency app. 2.45 GHz

When the helix circumference is near the wavelength of operation, the antenna operates in axial mode. This is a nonresonant traveling wave mode, in which instead of standing waves, the waves of current and voltage travel in one direction, up the helix from the feedpoint in a transmitting antenna and down the helix toward the feedpoint in a receiving antenna. Instead of radiating linearly polarized waves normal to the antenna's axis, it radiates a beam of radio waves with circular polarisation along the axis, off the ends of the antenna. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.

In radio transmission, circular polarisation is often used where the relative orientation of the transmitting and receiving antennas cannot be easily controlled, such as in animal tracking and spacecraft communications, or where the polarisation of the signal may change, so end-fire helical antennas are frequently used for these applications. Since large helices are difficult to build and unwieldy to steer and aim, the design is commonly employed only at higher frequencies, ranging from VHF up to microwave.

The helix of the antenna can twist in two possible directions: right-handed or left-handed, the former having the same form as that of a common corkscrew. The 4-helix array in the first illustration uses left-handed helices, while all other illustrations show right-handed helices. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the emitted wave. Two mutually incompatible conventions are in use for describing waves with circular polarisation, so the relationship between the handedness (left or right) of a helical antenna, and the type of circularly-polarized radiation it emits is often described in ways that appear to be ambiguous. However, J.D. Kraus (the inventor of the helical antenna) states "The left-handed helix responds to left-circular polarisation, and the right handed helix to right-circular polarisation (IEEE definition)".^[2] The IEEE defines the sense of polarisation as:

"the sense of polarization, or handedness ... is called right handed (left handed) if the direction of rotation is clockwise (anti-clockwise) for an observer looking in the direction of propagation"^[3]

Thus a right-handed helix radiates a wave which is right-handed, the electric field vector rotating clockwise looking in the direction of propagation.

Helical antennas can receive signals with any type of linear polarisation, such as horizontal or vertical polarisation, but when receiving circularly polarized signals the handedness of the receiving antenna must be the same as the transmitting antenna; left-hand polarized antennas suffer a severe loss of gain when receiving right-circularly-polarized signals, and vice versa.

The dimensions of the helix are determined by the wavelength (λ) of the radio waves used, which depends on the frequency. In order to operate in axial-mode, the circumference should be equal to the wavelength.^[4] The pitch angle should be 13°, which is a pitch distance (distance between each turn) of 0.23 times the circumference, which means the spacing between the coils should be approximately one-quarter of the wavelength ( λ /4).^{[citation needed]} The number of turns in the helix determines how directional the antenna is: more turns improves the gain in the direction of its axis at both ends (or at one end, when a ground plate is used), at a cost of gain in the other directions. When C < λ it operates more in normal mode where the gain direction is a donut shape to the sides instead of out the ends.

Terminal impedance in axial mode ranges between 100 and 200 Ω, approximately^{[citation needed]}

where C is the circumference of the helix, and λ is the wavelength. Impedance matching (when C = λ) to standard 50 or 75 Ω coaxial cable is often done by a quarter wave stripline section acting as an impedance transformer between the helix and the ground plate.

The maximum directive gain is approximately:

^[5]

where N is the number of turns and S is the spacing between turns. Most designs use C = λ and S = 0.23 C , so the gain is typically G = 3.45 N . In decibels, the gain is 

The half-power beamwidth is:

^[5]

The beamwidth between nulls is:

The gain of the helical antenna strongly depends on the reflector.^[6] The above classical formulas assume that the reflector has the form of a circular resonator (a circular plate with a rim) and the pitch angle is optimal for this type of reflector. Nevertheless, these formulas overestimate the gain by several dB.^[7] The optimal pitch that maximizes the gain for a flat ground plane is in the range 3–10° and it depends on the wire radius and antenna length.^[7]

[Douglas Decker]

So back to  question number #1:  Does the length of the wire used to wind a 7 turn helix (Which is   57.11716037 inches)include the 1/4 wavelength matching section( 2.0794055673)or not?

The answer appears to be yes.

Now on to question Number 2:

So question number #2

So the dimensions I used for the helix are compatible with beam mode. (Beam mode can also be called Axial Mode).

[Douglas Decker]

I meant the answer is you have add it to the 7 turn length.

I will get it right in a minute.

I keep getting interrupted about my clothes choices for our 52nd  weeding anniversary dinner  tonight.