Helical Antenna "Spin" Determination?

Bennett Prescott

Bennett Prescott

Just This Guy, You Know?
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Jan 10, 2011
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How do I determine what "spin" or polarity circularly polarized antennas have? I see a lot of the Professional Wireless Systems helicals and the occasional Sennheiser A5000CP. My understanding is that, when using these antennas on the send and receive ends, I need to use the opposite spin on one as otherwise they will be out of polarity and there will be cancellation. In practice I only see about a 6-10dB loss of signal from using the same polarity at both ends, but that's still pretty significant. Life is easy when I'm only using the helical on one end or the other, but sometimes I need to get the most range possible.

I'm talking about these:

PWSHelical.jpgsennheisera5000cp.jpg

When I'm working with a company that has both polarities, of course, I can just look at the antennas. But when I need to rent in antennas of the same or opposite polarity, how do I know how to label them? PWS labels their boxes "Left" or "Right", but not their antennas, so once the antenna has come out of the box it's anyone's guess. I can just look at the spinny antenna bit, but I don't know which way is supposed to be which... from the front or the back? On the Sennheiser, I guess I'm just fucked? Is it even helical? It's circularly polarized, but it has a much wider pattern than the PWS... perhaps a function of its (the PWS antenna's) length?

I just want to know how to label these when I see them so I can better coordinate amongst the companies I work for regularly.
 
Re: Helical Antenna "Spin" Determination?

You want the same direction or spin on both antennas. Think of it like nuts on a threaded rod.

Sadly, I can't answer your main question about how to tell. It's obvious enough with the PWS helicals, but you can't see inside the A5000CP.
 
Re: Helical Antenna "Spin" Determination?

Scott Helmke said:
So what's the spin on a standard A5000CP, now that I'm curious?
Click to expand...

As the element gets farther from the back of the antenna it goes clockwise if you are facing the same way as the antenna. The A5000CP is a different antenna design, but also circular polarization.

Where opposite twist becomes a benefit is when you have RF mic receive antennas on both sides of the stage, and IEM transmitters on one side. Having the IEM the opposite twist will help keep that energy out of the mic antenna.

Mac
 
Re: Helical Antenna "Spin" Determination?

Bennett Prescott said:
OK, I was confused about this but then Phil Graham called me and now I know... many things. I think I'm pretty set on the polarization issue.
Click to expand...

Polarization is a hard thing to understand. Transverse waves like EM radiation (i.e radio -> microwave -> infrared -> light -> xray -> gamma radiation) have more complicated mathematical formalism than the longitudinal waves of sound.

I was 20 years old and had talked myself into an internship at an optical materials lab group at Penn State, having never had a formal class on anything in optics. I spent many evenings in the lounge of my Penn State dorm working to digest the classic introductory text for this topic "Introduction to Electrodynamics" by Griffiths.

Now, more than a decade later, I have two friends with PhDs in antenna design, and I (mostly) manage to not embarrass myself with them because of that time at Penn State.
 
Re: Helical Antenna "Spin" Determination?

Bennett I think what your partly remembering is something Henry Cohen mentioned at one of the expos in Wachussett where he said something like you should use opposite "spin" antennae if you were using two different signals eg. mics and iems in the same environment ie anti clockwise for one and clockwise for the other. G
 
Re: Helical Antenna "Spin" Determination?

Justice C. Bigler said:
Gghhaaahh.....Now my head is spinning. If only we could get Henry to write all this stuff down in a nice concise 400 page book, maybe it would be easier to learn and understand.
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Justice,

First let me say that Scott's statement about polarization, and his analogy are correct and decent.Further the books on this already exist and have for more than half a century, but without a comfort level with vector calculus, they are going to be hard to parse. Evidently, though, Henry knows of a resource, see below.

Finally, everyone from college professors down to Bob McCarthy will tell you writing a technical book through a conventional publisher amounts to a vanity project. There are some cases where I feel publishing makes sense, but that is a long, tangential post to the topic at hand. Henry is certainly the most knowledgeable person I am aware of on this topic for A/V, and I assume he keeps his consulting calendar very full. Frankly, that's the best dollar/time proposition for someone in his shoes.

Maybe when he retires ;)
 
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Re: Helical Antenna "Spin" Determination?

Scott is correct: Same sense (right hand versus left hand) for a point to point application. Note that it's not left hand or right hand polarization; it's circular polarization with either left or right hand sense.

The accepted method used by most commercial antenna designers and manufacturers for determining sense (left or right hand) is to view the antenna from the rear and note the direction of the antenna element as it spirals or launches the wave away from your eye: Clockwise = right hand sense, counter-clockwise = left hand sense. The PWS antennas are easy to determine owing to the clear plexiglass. The Sennheisers in general ship right hand but I believe left hand can be ordered.

Justice - The ARRL Antenna Handbook is an excellent reference aimed at those with less than a PhD.
 
Re: Helical Antenna "Spin" Determination?

Henry Cohen said:
Justice - The ARRL Antenna Handbook is an excellent reference aimed at those with less than a PhD.
Click to expand...

A very good suggestion - even an older copy is fine though it probably won't have much on circular polarization. I don't really know much at all about real antenna theory, but I slogged through the basic antenna and transmission line chapters in the ARRL book and it put me ahead of 99%+ of the other wireless mic guys out there.
 
Re: Helical Antenna "Spin" Determination?

Henry Cohen said:
Scott is correct: Same sense (right hand versus left hand) for a point to point application. Note that it's not left hand or right hand polarization; it's circular polarization with either left or right hand sense.
Click to expand...

And for those would prefer to be even more pedantic, circular polarization is simply a special case of elliptical polarization, just as a circle is a special case of an ellipse.
 
Re: Helical Antenna "Spin" Determination?

Henry Cohen said:
The accepted method used by most commercial antenna designers and manufacturers for determining sense (left or right hand) is to view the antenna from the rear and note the direction of the antenna element as it spirals or launches the wave away from your eye: Clockwise = right hand sense, counter-clockwise = left hand sense. The PWS antennas are easy to determine owing to the clear plexiglass. The Sennheisers in general ship right hand but I believe left hand can be ordered.
Click to expand...

Henry, thanks so much for jumping in. Is this the method used by Professional Wireless? That's what I think you're saying but I wanted to be sure, otherwise I'll give them a call.
 
Re: Helical Antenna "Spin" Determination?

I did, but the box was un-marked. Another box was marked, but it had been opened before and therefore I couldn't be sure that a different antenna hadn't been substituted. Out of the 6 of these things this company owns I couldn't be absolutely sure of the polarity of any.
 
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