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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Tim Steer on November 18, 2015, 04:50:43 am

Title: Helical Antennae: RHCP/LHCP
Post by: Tim Steer on November 18, 2015, 04:50:43 am
When setting up a wireless link using a transmitter and receiver with two helicals facing each other, should both antennas be RHCP or should I be using a LHCP/RHCP pair?

I'm sure this is a simple question to someone who knows about these things, but I'm afraid what I know about RF engineering could be written on the back of a napkin. From what I've gleaned from model aeroplane forums it sounds like both antennas should be identical, and choosing the wrong combination can result in a 30dB loss in received signal, but I wanted to get the opinion of some people in the industry, as one company I work for uses a RHCP/LHCP pair for wireless audio links (to my mind this intuitively 'seems wrong' as the 'threads' won't 'wind on' to each-other, if you catch my drift).

This begs the questions, what is the practical difference between RHCP/LHCP, and why might a company have equal numbers of both types in their hire-stock?
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Scott Helmke on November 18, 2015, 09:57:14 am
You want to use the same handedness / "sense" on both. Think of it like nuts on a threaded rod.

There is attenuation from the polarity mismatch when mixing the two types. A practical application might be putting IEM transmitters on LH and wireless mics on RH to reduce interference between the two.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Pete Erskine on November 18, 2015, 12:12:10 pm
When setting up a wireless link using a transmitter and receiver with two helicals facing each other, should both antennas be RHCP or should I be using a LHCP/RHCP pair?

This brings up an interesting question.  Maybe Henry Cohen can answer.

If the 2 Helicals are facing each other should one be RH and the other LH for aligning the polarity?  Is the polarity mirror image?
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Keith Broughton on November 18, 2015, 12:17:28 pm
This brings up an interesting question.  Maybe Henry Cohen can answer.

If the 2 Helicals are facing each other should one be RH and the other LH for aligning the polarity?  Is the polarity mirror image?
Good question.
As for the OP, in a wireless link situation where antenna polarity can be fixed, is it worth using  helicals or would LP (fins) be fine?
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Mac Kerr on November 18, 2015, 12:23:14 pm
This brings up an interesting question.  Maybe Henry Cohen can answer.

If the 2 Helicals are facing each other should one be RH and the other LH for aligning the polarity?  Is the polarity mirror image?

Which way they are facing does not affect the polarization. As a point to point link they should have the same polarization (twist). To minimize cross interference between IEM transmitters and mic receivers they should have opposite polarization (twist).

Mac
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Tim Steer on November 18, 2015, 12:36:59 pm
You want to use the same handedness / "sense" on both. Think of it like nuts on a threaded rod.

There is attenuation from the polarity mismatch when mixing the two types. A practical application might be putting IEM transmitters on LH and wireless mics on RH to reduce interference between the two.

Thanks, that's how I imagined it to be.

Good question.
As for the OP, in a wireless link situation where antenna polarity can be fixed, is it worth using  helicals or would LP (fins) be fine?

I take your point, but it's a somewhat hypothetical situation that just interested me.

Which way they are facing does not affect the polarization. As a point to point link they should have the same polarization (twist). To minimize cross interference between IEM transmitters and mic receivers they should have opposite polarization (twist).

Mac

Makes total sense. Thanks everyone.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: brian maddox on November 18, 2015, 02:48:07 pm
Which way they are facing does not affect the polarization. As a point to point link they should have the same polarization (twist). To minimize cross interference between IEM transmitters and mic receivers they should have opposite polarization (twist).

Mac

The idea of thinking of it like a bolt really helped me visualize this.  But thanks for confirming that.

Oh, and how much fun is it to answer an RF question posed by Peter Erskine?  :)
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Henry Cohen on November 19, 2015, 04:05:51 pm
Oh, and how much fun is it to answer an RF question posed by Peter Erskine?  :)

Not nearly as much fun as when he asks at which BBQ place we should eat  ;D
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Mac Kerr on November 19, 2015, 04:16:17 pm
Not nearly as much fun as when he asks at which BBQ place we should eat  ;D

Big W's Roadside BBQ (http://www.chronogram.com/hudsonvalley/unorthodox-how-warren-norstein-became-big-w/Content?oid=2129248) in Wingdale NY. I've already dragged Pete there, he cam away a believer.

Mac
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Steve M Smith on November 19, 2015, 04:21:25 pm
The idea of thinking of it like a bolt really helped me visualize this.  But thanks for confirming that.

I was going to suggest a length of threaded rod as an analogy. Rotate it 180 degrees and it is exactly the same.


Steve.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Henry Cohen on November 19, 2015, 04:28:59 pm
Big W's Roadside BBQ (http://www.chronogram.com/hudsonvalley/unorthodox-how-warren-norstein-became-big-w/Content?oid=2129248) in Wingdale NY. I've already dragged Pete there, he cam away a believer.

Will have to put that on my itinerary. Have you been to Round Up (http://www.rounduptxbbq.com/Home_Page_1HN9.php) in Cold Spring on Rt 9? Very good.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Pete Erskine on November 19, 2015, 10:20:25 pm
Will have to put that on my itinerary. Have you been to Round Up (http://www.rounduptxbbq.com/Home_Page_1HN9.php) in Cold Spring on Rt 9? Very good.


Lets have a RF get together everyone this weekend!  I've often wondered if LH Brisket is as good as RH and when you should eat each.
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Mac Kerr on November 19, 2015, 10:45:50 pm
Lets have a RF get together everyone this weekend!  I've often wondered if LH Brisket is as good as RH and when you should eat each.

I'm up for it any time. Big W's is my favorite, but I'll try a new joint. Round Up is not far from Riverview in Cold Spring.

Mac
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Henry Cohen on November 20, 2015, 01:08:59 pm
I'm up for it any time. Big W's is my favorite, but I'll try a new joint. Round Up is not far from Riverview in Cold Spring.

Ditto. Of course Big W is the new place for me, so you guys pick it.

LH vs. RH brisket is moot: The mouth is linearly polarized  :)
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Lyle Williams on November 20, 2015, 01:36:56 pm
Generally on a fixed link linear polarity is fine.  The main benefit of circular polarity is that a linear polarity antenna at the far end of the circuit can move around at different angles without causing deep fades due to crossed polarity.  In audio I generally see the same objective persued through diversity reception on two linear polarity antennas.

A bit of trivia - one application for crossed-handedness in circular polarisation is when your signal is to be reflected off an object.  The transmitted signal might start as LHCP but after reflection it becomes RHCP.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Henry Cohen on November 20, 2015, 02:45:38 pm
A bit of trivia - one application for crossed-handedness in circular polarisation is when your signal is to be reflected off an object.  The transmitted signal might start as LHCP but after reflection it becomes RHCP.

A common misconception. Whereas a linear polarized electromagnetic wave is subject to negligible shifts in polarization when reflected, circular (actually elliptical) polarizations are not. In both cases it's the phase of the signal that has changed significantly.

Think of light: If a light with a polarizing filter is shined at an optically neutral mirror, the reflected light remains the same color(s), it's just getting to your eye a few attoseconds later.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Lyle Williams on November 20, 2015, 10:48:09 pm
https://en.m.wikipedia.org/wiki/Circular_polarization#Reversal_of_Handedness_by_Reflection

Granted, the effect depends on the angle on incidence.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Henry Cohen on November 21, 2015, 04:46:06 pm
https://en.m.wikipedia.org/wiki/Circular_polarization#Reversal_of_Handedness_by_Reflection

Granted, the effect depends on the angle on incidence.

If you read the description carefully;
"Upon such reflection, the rotation of the plane of polarization of the reflected light is identical to that of the incident field. However with propagation now in the opposite direction, the same rotation direction that would be described as 'right handed' for the incident beam, is 'left-handed' for propagation in the reverse direction, and vice versa",  it says the reflected wave does in fact have the same handedness as the incident wave, it's just that because the observer remains in the same position, the rotation as observed is opposite, and that's true. But that's not how handedness is determined: It's determined by standing behind the wave front as it moves away from the observer. So if the the wave is reflected, the observer must now move and stand behind the wave as it travels away from the reflecting surface.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Lyle Williams on November 22, 2015, 03:04:40 pm
Consider this example.  Viewed from the transmitting source an outbound RHCP has a clockwise spin. 

(Holding your right hand with the thumb in the outbound direction of propagation, with fingers following the clockwise rotation of the plane of polarisation.)

-

Upon reflection, still viewed from the transmitting source the returning signal still has a clockwise spin.  But that's now LHCP due to the direction change.

(Holding your left hand with the thumb in the inbound direction of propagation, with fingers following the clockwise rotation of the plane of polarisation.)
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Mac Kerr on November 22, 2015, 06:02:32 pm
Ditto. Of course Big W is the new place for me, so you guys pick it.

LH vs. RH brisket is moot: The mouth is linearly polarized  :)

All that food in front of Henry is what he ordered. That would be the combo for 2, Pete and I were 1 tray each. Love Big W's.

Mac

Title: Re: Helical BBQ: RHCP/LHCP
Post by: Henry Cohen on November 22, 2015, 06:14:33 pm
All that food in front of Henry is what he ordered. That would be the combo for 2, Pete and I were 1 tray each. Love Big W's.

Before anyone thinks I'm that much of a glutton, I was under orders to bring home more than just mere leftovers. Though that was quite an amount of food "for 2".
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Pete Erskine on November 22, 2015, 07:31:31 pm
Before anyone thinks I'm that much of a glutton, I was under orders to bring home more than just mere leftovers. Though that was quite an amount of food "for 2".

I think Henry had the right idea - Big W's is an hour drive from me and I only took home enough for 1 more meal, not the weeks full of food Henry is going to enjoy.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Lyle Williams on November 23, 2015, 02:42:24 pm
That's an awesome looking feed.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Keith Broughton on November 23, 2015, 04:33:20 pm
That's an awesome looking feed.
I'll drive down from Toronto for food (and company) like that!
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Henry Cohen on November 23, 2015, 09:00:19 pm
I think Henry had the right idea - Big W's is an hour drive from me and I only took home enough for 1 more meal, not the weeks full of food Henry is going to enjoy.

"Weeks"?! There's barely (chomp, chomp) enough (chomp) left (chomp, chomp, chomp) for lunch tomorrow (burp)!  ;D
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Tim McCulloch on November 24, 2015, 03:40:42 pm
"Weeks"?! There's barely (chomp, chomp) enough (chomp) left (chomp, chomp, chomp) for lunch tomorrow (burp)!  ;D

And from this thread I have learned something very important:

Henry's consulting fees are a bargain when compared to buying lunch for this crew... ;)
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Henry Cohen on November 24, 2015, 07:38:34 pm
And from this thread I have learned something very important:

Henry's consulting fees are a bargain when compared to buying lunch for this crew... ;)

I still have to make it down to your neck of the woods. Rumor has it there's some decent BBQ to be found there  ;)
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Tim McCulloch on November 24, 2015, 11:53:19 pm
I still have to make it down to your neck of the woods. Rumor has it there's some decent BBQ to be found there  ;)

Actually some pretty decent BBQ here, considering we're not Kansas City or Memphis.  If you're ever down this way, Henry, the first 2 lbs are on me.

The worst BBQ I every tried to eat was in Texas (Waco).
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Henry Cohen on November 26, 2015, 10:06:04 am
The worst BBQ I every tried to eat was in Texas (Waco).

That's because they cook the wrong animal there (brisket notwithstanding).
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Rob Spence on March 07, 2016, 10:25:01 pm
Deryl & I got to Wingdale last evening.

We got the big rib rack to eat and another to go. We made a start on the second one tonight.

Great BBQ.

Thanks for the pointer guys.


Sent from my iPad using Tapatalk HD
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Mac Kerr on March 07, 2016, 10:38:12 pm
Deryl & I got to Wingdale last evening.

We got the big rib rack to eat and another to go. We made a start on the second one tonight.

Great BBQ.

Thanks for the pointer guys.

I hope you guys enjoyed it as much as I do! Glad you gave it a try.

Mac

Title: Re: Helical Antennae: RHCP/LHCP
Post by: Rob Spence on March 07, 2016, 10:41:33 pm
I hope you guys enjoyed it as much as I do! Glad you gave it a try.

Mac

Definitely worth the drive. We were going home to Boston from NYC. It was not a big detour.

We will do it again.


Sent from my iPad using Tapatalk HD
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Keith Broughton on April 14, 2016, 08:29:15 am
When setting up a wireless link using a transmitter and receiver with two helicals facing each other, should both antennas be RHCP or should I be using a LHCP/RHCP pair?

I'm sure this is a simple question to someone who knows about these things, but I'm afraid what I know about RF engineering could be written on the back of a napkin. From what I've gleaned from model aeroplane forums it sounds like both antennas should be identical, and choosing the wrong combination can result in a 30dB loss in received signal, but I wanted to get the opinion of some people in the industry, as one company I work for uses a RHCP/LHCP pair for wireless audio links (to my mind this intuitively 'seems wrong' as the 'threads' won't 'wind on' to each-other, if you catch my drift).

This begs the questions, what is the practical difference between RHCP/LHCP, and why might a company have equal numbers of both types in their hire-stock?
Asked the Shure guy at an RF seminar and he said "same polarity" like the threaded rod mentioned earlier.
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Pete Erskine on April 14, 2016, 10:49:07 am
Asked the Shure guy at an RF seminar and he said "same polarity" like the threaded rod mentioned earlier.

Except if it is Metric.....
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Keith Broughton on April 14, 2016, 11:09:33 am
Except if it is Metric.....
Then you are really screwed  ;D
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Ike Zimbel on April 14, 2016, 12:22:09 pm
Asked the Shure guy at an RF seminar and he said "same polarity" like the threaded rod mentioned earlier.
That would be Tim Vear...
iz
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Jerome Malsack on April 14, 2016, 01:44:01 pm
Then you are really screwed  ;D

Can we use a Nail or Glue ?
Title: Re: Helical Antennae: RHCP/LHCP
Post by: Russell Ault on April 16, 2016, 03:11:14 pm
Can we use a Nail or Glue ?

Glue will probably have less of an effect on the operating characteristics of the antenna (especially if it's metric glue)...

-Russ
Title: Re: Helical BBQ: RHCP/LHCP
Post by: Doug Fowler on April 16, 2016, 07:11:08 pm
Ditto. Of course Big W is the new place for me, so you guys pick it.

LH vs. RH brisket is moot: The mouth is linearly polarized  :)

Probably true, but a correct brisket slice requires a 90 degree shift on one axis, and about a 30 degree shift on another.