Design a 44MHz rubber ducky antenna

I need a rubber ducky antenna working at 44MHz, how should I start? Is there any book discussing the design of this type antenna? Any equations to calculate the parameters such as coil diameter, number of coil turns, etc? Thanks in advance.

Do a Google search for Normal mode helical antenna, this is what you are looking for.
Peter

maolh said:

I need a rubber ducky antenna working at 44MHz, how should I start? Is there any book discussing the design of this type antenna? Any equations to calculate the parameters such as coil diameter, number of coil turns, etc? Thanks in advance.

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1. What is the gain you want to design ?
2. Rubber duck antennas is usually a OMNI directional antenna (monopole or dipole). Just because the enclosure is rubber or plastic based, the name "Rubber Duck" comes into picture.

here is a helical antenna calculator
https://www.daycounter.com/Calculators/Helical-Antenna-Design-Calculator.phtml

After the basic design here, you could as well simulate / optimize the structure to get best possible results.

vedagarbhamsandeep said:

1. What is the gain you want to design ?

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for 44MHz it isnt going to have any gain !!
the antenna, even with some loading coils is still going to be substantially shorter than anything actually resonant for that freq

Dave

davenn said:

for 44MHz it isnt going to have any gain !!
the antenna, event with some loading coils is still going to be substantially shorter than anything actyally resonant for that freq

Dave

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you mean to say we are to have a RLC circuit for 44MHz resonance wherein L constitutes the coil design and RC could be SMD components ?

But how are we to do a rubber duck with all this electronics inside ?

G4BCH said:

Do a Google search for Normal mode helical antenna, this is what you are looking for.
Peter

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Is that true?

Obviously the diameter needed for an axial mode helix is huge. I calculate it to be 2.17 m in diameter, since the circumference needs to be one wavelength. I also know that normal mode helix antennas are smaller in diameter than axial mode helix antennas, and radiate to the side more, not along the axis.

But a normal mode helix needs a decent ground plane. And as far as I am aware, there is a formula for computing the diameter, and whilst it wont be as large as the 2.17 m for an axial mode helix, it will be very substantial. Kraus is going to be the best source of reference on this.

I just fired up the HFSS antenna design kit. It can design a normal mode helix, so I got it to compute parameters for a normal mode helix at 44 MHz, where the wavelength is 6.818 m.

This is what the HFSS antenna design kit gives:

Diameter 341 mm (which equates to a diameter of λ/20)
Helix spacing 341 mm. (which equates to a pitch of λ/20)
Number of turns 1.8
Ground plane: 6.8 x 6.8 m (which equates to λ x λ)
Gain 2.96 dBi (based on perfect electrical conductors)
Gain peaks at 42 degrees away from the axial direction.

As such, I don't believe a rubber duck can be considered a normal mode helix, though I stand to be corrected. If I had Kraus to my side, I would take a quick look, but I don't.

I don't believe there are any real design equations one can apply to a rubber duck. One might as well make it as long as practically possible, given whatever you do, it will be electrically short. One could make it resonate, by adjusting the number and spacing of turns so the distributed capacitance resonates with the self-inductance, but I don't have any evidence to suggest that is optimal.



Deborah.

DeboraHarry said:

Is that true?
Deborah.

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Well that is how I have seen them described when they first started appearing on handheld radios and FWIW Wikipedia also refers to the term when used for this type of antenna.
They do not perform in the same way as an axial mode helix, but behave more like a loaded quaterwave vertical. I would expect the gain to be negative a few dB relative to isotropic. The lengths of these antennas is usually around 0.05 wavelenths and in an ideal world would have a good groundplane, but in a handheld radio environement that is obvoiuusly not possible.
I have seen some empirical derived design equations more years than I care to remember ago. I did design a couple of antennas for use at VHF and UHF using them and they resonated OK, but resonance was rather dependent on the surroundings, and I suspect that the resistance was more loss than radiation. As for radiation pattern that would be anyones guess especialy when used on a small handheld radios.
I've seen these antennas used on low VHF radios but they have tended to be quite large, a 40MHz version that is small enough for comfortable use on a handheld would have quite poor performance, but for short range use probably adequate.
As you say at the end all you can really do is get as much radiative length as possible and hope for the best as it is a sub-optimal situation, and we have not been given any idea as to the application.
A little RF wil go a very long way given the chance.
Peter

vedagarbhamsandeep said:

here is a helical antenna calculator
https://www.daycounter.com/Calculators/Helical-Antenna-Design-Calculator.phtml

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But that is going to be of no use to him, as it is based on an axial mode helix. The web page says:

"Where C is circumference, which is normally chose to be close to one wavelength."

That will give you a diameter of over 2 m. I know the axial mode helical antenna is a broad band antenna, but if you reduce the diameter by a factor of 200, to 10 mm, then it will not behave anything like an axial mode helical antenna. It is not that broad!

As I noted in an earlier post, I'm not convinced that a rubber duck antenna is a normal mode helical antenna. But I'm absolutely certain it is not an axial mode helical antenna, which is what that calculator is for.

Here is a link to a paper that gives some data on a design method for a dipole version of these small antennas.

ap-s.ei.tuat.ac.jp/isapx/2010/pdf/121.pdf

The design method looks similar to the article I recall from the early 70s. Simplistically take a quater wave of the thickest wire you can manage, wind it into a diameter that you can manage then stretch it unitl you get the best performance you can given any length contraints.
It is a non ideal situatin so you get what you get.

As Deborah says they are not what most people would immeadiately recognise as a helical antenna, but that is what they were called by someone back in the early days, and for better or worse, probably the latter the name seems to have stuck.

Peter

To radiate exactly in normal mode, the current must be constant along the helix, requiring an end termination, see Ballanis, Antenna Theory, Chapter 10.3.1 Helical Antenna.

Or the current distribution must be symmetrical, simlar to a λ/2 monopole, which sounds hard to achieve for a short antenna.

Ballanis is designating a rubber duck-like antenna as helix monopole:

A helical antenna operating in the normal mode is sometimes used as a monopole antenna for mobile cell and cordless telephones, and it is usually covered with a plastic cover. This helix monopole is used because its input impedance is larger than that of a regular monopole and more attractive for matching to typical transmission lines used as feed lines, such as a coaxial line.

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I think, you can see it as an electrical short monolpole with a built-in series inductor.

Thanks everyone, it seems I was wrong, and the rubber duck is considered as a normal mode helical antenna.

A place I used to work made rubber duck antennas, and adjusted the length so that it showed a dip in return loss whilst the antenna was mounted on a small metal box. To my knowledge, nobody designed them, and nobody ever measured any parameter other than the return loss on a metal box. But it was the worst company I have ever met for quality control - many items had exagerated specifications, which were justified because the customer would be unable to measure them. Published gain figures, radiations patterns etc, were all just invented. SWR was measured, but if devices did not meet the spec, unless grossly out, they would be shipped to customers.

It would be interesting to perform a simulation in something like HFSS, using a realistic size for the ground plane and finite conductivity materials, then determine if maximum realized gain does occur when the return loss is a maximum. I did sometimes wonder if a simple parallel tuned circuit was formed, since I very much doubt anyone ever designed them.

Deborah

A friend of mine used to work for a mobile radio company and I remember him tellling me about the antenna designer there would only measure these things on a large groundplane in the open with no one anywhere near. I expect he would be horrified if he realise how they were going to be used in real life; close to a lossy body, with the ground plane the skimpiest of metalwork surrounded by the lossy body's hand.
With such a variable evironment I'm supprised that they work as well as they do.
Peter

G4BCH said:

A friend of mine used to work for a mobile radio company and I remember him tellling me about the antenna designer there would only measure these things on a large groundplane in the open with no one anywhere near. I expect he would be horrified if he realise how they were going to be used in real life; close to a lossy body, with the ground plane the skimpiest of metalwork surrounded by the lossy body's hand.
With such a variable evironment I'm supprised that they work as well as they do.
Peter

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haha Peter thats funny huh

A rubber duck antenna is, AT BEST, a poorly radiating dummy load

The only time you can get any useful gain from them is if they are used on VHF, UHF and SHF where their physical size is such that you can start playing with colinear arrangements.

Dave

Thank you all for the great inputs. Especially thank Peter for pointing me to the right direction - the nature of normal mode helix for this type of antenna.

Like you guys pointed out, I don't expect any gain from this antenna. It's more like just make it resonate at 44MHz so it can radiate more efficiently. Am I right to say that? Like most mobile application, our device doesn't provide any solid ground plane either.

We are currently using a 17' long rubber ducky antenna (44MHz). I'd like to get the length down to 6' to 10', is it possible to achieve this goal? Which direction should I go in terms of adjusting the geometry of the structure, such as increase or descrease coild diameter, increase or descrease wire diameter, etc.?

Thanks agian for all your help!

When you say "resonance", how are you going to test for resonance? It does not necessarily follow that best return loss (VSWR) will occur anywhere near resonance. Neither is it necessarily true that best return loss or resonance will give maximum radiation.

It you have a few remote signals you can receive, I'd be tempted to make a number of antennas of different lengths, then see what gives the best signal.

What you want is maximum realized gain in your particular setup. I believe the best way to do that does not include needing a network analyser or SWR meter.

Others might disagree, and believe resonance is important. I look forward with interest to any responses.

Dave

I Assume that the dimensions are inches, the ' abreviation means feet, " is used for inches. Potentially confusing unless you have been brought up with the units, one good reason for using metric measurements.
I would try to keep the wire as thick as you can, the thinner it becomes the greater the loss you will have, and Dave's comment about these antenna being a poorly radiating dummy load is not far short of the mark.
As Dave who just repiled said don't panic too much about resonance, Try too keep the losses to a minimum. Whatever power you get intot he antenna will be radiated if it is not absorbed by losses.
If you want a very short antenna just put as much wire into the helix as you can ( or jut use a straight piece of wire, with or witout a loading coil at the base ) and then match into what ever you get when it is in your system. Bearing in mind that the impedance will be all over the chart as you change the way the radio is held or mounted, make the output amplifier as robust as you can so that it can withstand any impedance without oscillating or destroying its self, then tune it into the antenna under the conditions it will be used and hope for the best.
There are no hard and fast rules, make one and see how well or otherwise it works.

Peter

G4BCH said:

I Assume that the dimensions are inches, the ' abreviation means feet, " is used for inches.

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Sorry, my bad. It should be " (inches).