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I am attempting to design an antenna for a 162.4 MHz omni receive only application and am confident it is going to be difficult if not impossible.

I have very tight specs on space(6" diameter by 3" deep cylinder) so the first antenna I came across was a quarter wave antenna.

162.4MHz length is 17.291 inches. Given my enclosure height this is not going to work.

The next antenna to look at is the rubber ducky. From wiki I find, "Rubber ducky antennas are typically 4% to 15% of a wavelength long; that is, 16% to 60% of the length of a standard quarter-wave whip.

So 17.291 inches in the quarter wave becomes 17.291*(.16)=2.76656 inches in the rubber ducky. Now I know having antenna this small will hurt the range terribly, but it meets the size requirement.

So I am attempting to understand how to design such an antenna. I have access to some equipment and will attempt to acquire the tools I need, but I lack the design flow to accomplish this.

Currently I have access to a spectrum analyzer w/ tracking and can get signal to noise ratio(SNR) and Received signal strength indicator (RSSI) values.

I have no issues with the impedance matching and am purely focused on how to design the physical antenna for my design.

Any suggestions or information would be greatly appreciated on how to go about designing a rubber ducky.

Additional information:

When I was looking to choice an antenna type: https://electronics.stackexchange.com/questions/131323/antenna-design-for-specific-directions

Can Rubber ducky be modeled: How much loss for 1 to 1.5 inch Coil (rubber ducky) Antenna at 70cm band?

Details:

  • The enclosure is made from PLA plastic.
  • Other then a ground place the diameter of the enclosure it will be in free space.
  • There is a AC power cable running out of the top of the enclosure.
  • The receiver circuit board is inside the volume.
  • The orientation of the cylinder is 3" vertical & 6" horizontal
  • The direction of the signals will be horizontal.
  • I can ideally cope with a directional design, but this is not prefered.
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  • $\begingroup$ With antennas, the environment at least 0.5 wavelengths in all directions matters a lot. Please add some information: What is around the antenna volume you describe? I'd there any metal nearby? What is the box made of. Any cables hanging out of the box? Or is it a completely self-contained product, like a large pager? Is it fed with a coax cable, or is the receiver circuit board inside the volume? Also, what orientation is the cylinder in, and which direction will the signals be coming from? $\endgroup$
    – tomnexus
    Feb 10, 2015 at 5:23
  • $\begingroup$ Tomnexus - I have replied to your questions in the Details segment of the question! $\endgroup$
    – Josh Cox
    Feb 12, 2015 at 1:20
  • $\begingroup$ This sounds like an X-Y problem. You need a small antenna. There's no reason it needs to be a rubber ducky. $\endgroup$
    – Phil Frost - W8II
    Feb 12, 2015 at 12:32
  • $\begingroup$ Phil Frost - I'm open to that, but I need an antenna that I can determine how to build. My current thinking has led me to a rubber ducky... My lack of antenna knowledge is why I'm here trying to find my way to an answer. $\endgroup$
    – Josh Cox
    Feb 12, 2015 at 13:01
  • $\begingroup$ Well, the more appropriate question is then "what kind of antenna fits in a 3" x 6" cylinder and works well enough for receiving at 162.4 MHz?" $\endgroup$
    – Phil Frost - W8II
    Feb 12, 2015 at 15:21

1 Answer 1

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The rubber duck antennas are monopole antennas that have their length of wire turned into a helix. The helical shape allows you to cram the same length of wire into a smaller volume, just as you correctly noticed. The "rubber duck" comes from the fact that these helices are often sealed in rubbery stuff to keep the shape intact and the water out. Monopole, GP (ground plane), and quarter wave antenna all refer to the same basic idea of replacing the other half of a dipole antenna with a ground plane.

I see a few challenges in your design: the height of 3" is really litle for the intended wavelength and you also want to put your electronics PCB into the same container. The antennas do not generally like conductive parts near them as the electric field generated by the antenna is coupled into the conductor such as your PCB. The same goes for your AC cabling: it might negatively affect the performance of your antenna.

Normal design flow would probably go as follows: first you get an idea of how the antenna should look like, then you simulate it with 3D electromagnetic simulation tools, optimize it and later include a conductor sheet to represent the receiver PCB and a short length of conductor pole to act as the AC cable. In this case, however, I would go experimenting: with your measuring equipment you can hack together a working prototype very quickly and find out if the result is good enough for your application.

And finally to the real content: I would try to do a dipole antenna that has a 90 degree turn in the middle and whose arms have been shortened by wrapping them as coils (just as they do in the rubber duck antennas). The antenna arms would be in horizontal plane of your enclosure. The 90 degree turn in the middle will prevent you from having any radiation nulls in horizontal plane.

I hope that the size of your receiver is relatively small part of your box, otherwise the following won't work. Does your receiver PCB have a coaxial connector or are you going to solder the antenna leads directly onto the receiver? In either case:

  • take a coaxial connector that can be connected to your VNA and your receiver
  • take two little over $\lambda/4$ (20") wires that keep their shape but can be formed with hand.
  • roll the your antennas around of a plastic pipe with 2-3" diameter so that you will end up with a helix that a) fits inside the box, b) is as long as possible
  • solder the arms of your dipole to the coaxial connector to 90 degree angles.
  • Connect the connector to your VNA and hope that you see a dip in S11 somewhere below your intended frequency.
  • shorten the helices with wire cutters one piece at a time as long as your dip is close to the intended frequency. Make both arms equally long.
  • connect your device into the receiver and hope for the best.

This is a workflow I would try, it might not work for you, depending on your application, signal strenght requirement, and especially the size of your receiver. Let us know how it went.

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    $\begingroup$ I think the purpose of coiling the wire in a rubber ducky antenna is to add inductance, not to fit the same amount of wire in a smaller space. The length of a rubber ducky antenna, stretched into a straight line, may not necessarily be a quarter-wave. $\endgroup$
    – Phil Frost - W8II
    Feb 15, 2015 at 20:29
  • $\begingroup$ Hi Phil, that is true: the coil in the antenna acts as inductance and just cramming the same length wire in any random shape would not have the same effect. However, even in this coil antenna, the total length of the wire is crucial to get the resonance. The inductance of the coil really shortens the actual wire as well, but only slightly, compared to the effect on the physical size. $\endgroup$
    – OH2FXN
    Feb 16, 2015 at 5:54
  • $\begingroup$ My biggest hurtle is A) How to pick the diameter & B) How to pick the length...Can I ask why you chose the diameter & length you did? $\endgroup$
    – Josh Cox
    Feb 25, 2015 at 15:08
  • $\begingroup$ @JoshCox The main idea was to make an antenna that is almost like a dipole but where the two arms are perpendicular and twist the wire into a coil to make it fit in your chase (or increase inductance). The "normal" dipole arm length is $lambda/4$ so take that as the starting point. It is easy to shorten the antenna wire until you are at the correct frequency so 20" is just a starting point. Also the diameter of the helix was chosen just to be something that fits your chase and is not too small. This should be something that you can prototype quickly and then decide if it is good enough. $\endgroup$
    – OH2FXN
    Feb 25, 2015 at 18:53

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