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If a 460MHz quarter wave antenna is only 6" long, why are they all helicals?

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Rred

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The typical UHF low band h/t has a rubber whip nearly 6" long. Which should be long enough for a simple quarter wave antenna. But, from the ones I've seen cracking open from old age, they all seem to be a helically wound brass core, not a plain wire.

What's wrong with this picture? What am I not understanding about this?
 

ab5r

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Not sure what you mean by "brass core." If you straighten out the helical, what would it measure? Would it be 1/2 or 5/8 wave?
 

nd5y

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The whip type antenna isn't helical. They are usually built with brass or copper coated steel wire wound in two directions like the inside of a speedometer cable. The wires are all touching each other which electrically shorts them together, not like an inductive coil.
 

jonwienke

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The wires are all touching each other which electrically shorts them together, not like an inductive coil.
Actually they typically ARE insulated, with a thin coating similar to the stuff used to insulate magnet wire, which may be clear. Expecting the coils to short out consistently given the flexing, stretching, and twisting encountered by typical antenna use would be ridiculous. If the design intent was to be shorted out, you wouldn't use a helix/coil, you'd use a braided structure like coax shield.
 

nd5y

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Actually they typically ARE insulated, with a thin coating similar to the stuff used to insulate magnet wire, which may be clear. Expecting the coils to short out consistently given the flexing, stretching, and twisting encountered by typical antenna use would be ridiculous. If the design intent was to be shorted out, you wouldn't use a helix/coil, you'd use a braided structure like coax shield.
All the ones I have disassembled were not insulated. They have at least two layers of wires wound in opposite directions so that the wires criss-cross eachother. Electrically it would be the same as braid.
 

ab5r

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Not the same??? If it walks like a duck and quacks like a duck, maybe it is Rubber Ducky?

Perhaps a photo would help.

73,
Jerry
 

jwt873

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What brand(s) of HT are you discussing?

I've seen UHF only radios that come equipped with dual band (VHF UHF) antennas. One example is the Icom ID-31 UHF radio and the ID-51 dual band radio. They both use the same antenna. I'm pretty sure this is true with the Chinese Baofengs, PooFlungs etc..

I'm guessing that if a company produces VHF and UHF radios, they won't have to manufacture and inventory separate antennas for UHF and VHF.

And.. small point.. 460 Mhz antennas aren't for amateur use.. They'd be more for the APRS/GMRS service.
 

nd5y

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Not the same??? If it walks like a duck and quacks like a duck, maybe it is Rubber Ducky?
You don't understand the difference between a helical rubber ducky and a straight whip made from flexible cable that is typically about 3/16" diameter, which I think is what the OP was referring to.
 

Rred

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The antennas I've seen coming apart (from age and abuse) are Motorola and Kenwood commercial. According to their part numbers, these are UHF wideband only, not dual purpose, and they do specify other antennas for dual purpose. I didn't do a full autopsy on them, I just noticed that they appeared to be a brass helical coil, very definitely not criss-cross construction. I thought that was a curious way to build an antenna, using a "coiled spring" which would be very good at keeping a stiff but flexible antenna, but more complex in terms of how the coil would behave, and more expensive than other options. (Perhaps not, springs can be cheap.)

Interesting Wiki article, especially the conjecture that the coil *may* be shorted out and acting as a linear conductor with possibly complex design issues. That would help explain why some brands of antennas seem to outperform others while all of them are "just" a simple quarter wave.

Still, it doesn't seem to be of any performance benefit to use a helical in any way, compared to a 1/4 wave of the same overall height. Given the durability of modern materials, maybe the ducky really is past its time?
 
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It's to provide a matching circuitry for the radio. A simple 1/4 wave antenna is easy but it does you no good if there isn't adequate radio chassis to act as a ground plane. Even a user's hand changes the impedance match. The antennas are designed to fit multiple radio models which vary dimensionally.


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jonwienke

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You don't understand the difference between a helical rubber ducky and a straight whip made from flexible cable that is typically about 3/16" diameter, which I think is what the OP was referring to.
No, the OP specifically mentioned the internals being a helix. So we (everyone else but you in this thread) are talking the typical rubber ducky here.
 

RFI-EMI-GUY

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I am confused. Most UHF 440 to 470 MHz helical antennas are about 3.5 inches tall while a quarter wave antenna is about 6.5 inches tall. There is definitely a shortfall in performance with the helical. The popularity of the helical is that it is more compact.

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