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I am designing an antenna for a device and was hoping to gain some insight in the correct design direction for the antenna.

The Idea: A receiver & antenna that fits in a 6" diameter by 3" deep cylinder.

Known: Frequencies range from 162.400 to 162.550 MHz. The signal will not be coming directly from the top or bottom.

Goals: Design the “best” antenna I can given my knowledge of the direction the signal will be coming from, the size constraint of the enclosure and the frequency.

Any advice, recommendations, and/or help would be greatly appreciated. I am specifically interested in the physical layout/design of the antenna.(For example coil wire cylinder)

Thoughts: My gut feeling is to enclose the maximum area possible with the antenna. From my initial research the length is not as important as I had previously thought which lead me to start to ponder this even more.

enter image description here Notes: I like math if you are so inclined :)

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  • $\begingroup$ Your design looks like a "Disk Cone Antenna". Google images show many. strutpatent.com/patent/07286095/… $\endgroup$ – Optionparty Sep 29 '14 at 21:49
  • $\begingroup$ Does too, but if I interpret the areas where he is expecting the strongest signals from it looks like a quarter-wave vertical closely matches the omni directional radiation pattern shown above. To come up with something that performs well and fits inside the 3" by 6" available space would be a challenge.. $\endgroup$ – captcha Sep 29 '14 at 22:22
  • $\begingroup$ @Optionparty - by design you mean what exactly? $\endgroup$ – Josh Cox Sep 30 '14 at 12:22
  • $\begingroup$ Cross-post with Electronics $\endgroup$ – Josh Cox Sep 30 '14 at 13:26
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Given the shape and desired band, I think what you want is a small loop antenna. This would simply be a coil of wire wound in the shape of your cylinder, preferably with a capacitor wired in parallel to make it resonant at the desired frequency and match impedance to the receiver.

Small loops have the type of radiation pattern you are looking for (in the plane of the loop). What makes it a “small" loop is that it is small, both in radius and total length of wire, compared to the wavelength; we can see that a radius of 3 inches (0.07 m) is smaller than your wavelength of 1.85 m but I don't know whether it'd be better to make it even smaller than that. If the loop is too large, comparable to the wavelength, it will have the opposite of the pattern you desire.

Small loops are inefficient antennas not suitable for transmitting, which is why transmitters don't use them to avoid having long antennas sticking out, but since you are receiving that doesn't matter unless your desired signals are particularly weak.

They are commonly seen as antennas for AM broadcast receivers, either as external wire loops on a frame (oriented vertically to have some directionality; in your case, you would orient the loop horizontally because you are asking for an omnidirectional pattern), or very small internal “loopstick” antennas (wire coiled around a ferrite rod).

(Disclaimer: I have made no systematic study of antenna theory; I have never built a small loop antenna; there's probably some caveats I haven't mentioned. I'm writing this answer because I haven't seen a better one yet and I'm reasonably confident the basic idea is OK.)

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  • $\begingroup$ So judging from your disclaimer you have no reason to believe this would work well outside of the radiation pattern being similar correct? Hey I appreciate the answer and the attitude! I will investigate this type further and come back with my results. $\endgroup$ – Josh Cox Sep 30 '14 at 13:08
  • $\begingroup$ Exactly how do you imagine the loop positioned? I think this would yield the exact opposite of what I am needing? $\endgroup$ – Josh Cox Sep 30 '14 at 13:51
  • $\begingroup$ The loop would be in the same orientation as the cylinder in your diagram. As I already stated, as long as it is a small loop, the radiation pattern will be oriented towards the "sides" of the loop, which is what you want. $\endgroup$ – Kevin Reid AG6YO Sep 30 '14 at 14:53
  • $\begingroup$ Heres my source of confusion - Why do standard AM radio antenna sit up right? Why not lay them down if the radiation pattern is in the plane of the coil AM antenna $\endgroup$ – Josh Cox Sep 30 '14 at 15:28
  • $\begingroup$ @JoshCox That makes it a directional antenna. You can orient the loop towards (edgewise) the station you want to receive and cut out noise sources that are perpendicular. This is good when the transmitter and receiver are stationary and there's no noise source directly above or below you. If your receiver is in motion, then you likely want an "omnidirectional" configuration, so the loop would be horizontal instead. $\endgroup$ – Kevin Reid AG6YO Sep 30 '14 at 23:41
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Try a rubber ducky antenna. Heavily loaded vertical whip like you see on handheld transceivers.

Something like this:

enter image description here

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  • $\begingroup$ I'm limited to 3" depth so I'm not sure how to best wind the coil to increase the quality of the antenna. $\endgroup$ – Josh Cox Sep 30 '14 at 0:16

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