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Old June 7th, 2013, 09:29 PM   #16
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Re: Sennheiser lavaliere interruption

Quote:
Originally Posted by Richard Crowley
the alignment of a monopole transmitting antenna doesn't really affect the "plane" of the transmitted radio waves.
I'd like to see a reference for that statement, as that's quite different from what I consider to be true.

Everything I've learned about antennas -- going back over 40 years as an FCC-licensed broadcast engineer, and over 35 years as a licensed radio amateur -- tells me that if the antenna is anywhere near being a tuned monopole (or dipole, for that matter) then the polarization of the radiated field corresponds to the physical orientation of the antenna wire.

(The only time the transmitted wave has no polarization is when the antenna length is very much shorter than 1/4 wavelength at the frequency under consideration -- essentially a "point source" antenna. In that case, there is no polarization and the radiation pattern is a sphere. But wireless antennas are not nearly small enough to be a "point source" and in fact they're pretty well tuned to the transmit frequency, to achieve reasonably good coupling efficiency with the transmitter electronics.)

The waves from a vertical antenna aren't radiated in a plane. (Or perhaps when you said "plane" you meant "polarization.") As the antenna length increases from the theoretical point source to a vertical wire of tuned length, two things happen: (1.) the signal polarization becomes more and more strongly vertical, and (2.) the radiation pattern becomes more and more flattened or "pushed in" at the top and bottom, changing from a sphere to a torus (roughly a donut shape).

So if the antenna is vertical, the polarization will be vertical, and the torus (showing the signal strength) will be horizontal (in other words, as if the "donut" is laying flat on a table). If you walk around 360º in a horizontal circle, staying an equal distance from the antenna (which is located in the "donut hole"), the signal strength should be equal at all points. OTOH, if you move in a vertical circle, the signal strength will drop as you get away from the plane of the torus, and will theoretically drop to zero when you are located along the axis of the antenna.

Now, indeed, polarization is "mirrored" if the signal bounces off a good conductor (e.g. a large metal structure, etc.). That is important when circular polarization is used: right-hand circular will reflect as left-hand circular, and vice versa. But a resonant vertical transmit antenna will produce a vertically polarized signal at short range, and that will be reflected as a vertically polarized signal as well.

Last edited by Greg Miller; June 8th, 2013 at 09:22 AM.
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Old June 8th, 2013, 01:47 AM   #17
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Re: Sennheiser lavaliere interruption

What can I say Greg? 100% agree. It's a very curious statement, and seems to go against everything I was also taught.
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Old June 8th, 2013, 08:16 AM   #18
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Re: Sennheiser lavaliere interruption

Quote:
Originally Posted by Richard Crowley View Post
A common practice when one has a pair of close~spaced "whip antennas" is to put them into a "V" formation, rather like the old VHF TV "rabbit ears" from the 1950s. The theory being that having two antennas 90 degrees apart in alignment gives the best possible (in such a limited constraint) coverage of radio waves coming in direct or reflected. It sounds goofy and is counter-intuitive when the transmitting antenna is most likely vertical. But the alignment of a monopole transmitting antenna doesn't really affect the "plane" of the transmitted radio waves.
Agreed - Audio Limited do (used to do) a rt.angled antenna so that one was vertical and the other horizontal.

On a mains receiver, having the two antennas as a "V" has the same effect.
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Old June 8th, 2013, 10:16 AM   #19
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Re: Sennheiser lavaliere interruption

Wish this thread had a "Like" button..... anyway, how does all this work with plugon transmitters? I use one from my sound guy's mixer to my camera. Is there a preferred orientation for the antenna on my receiver for a plugon?
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Old June 8th, 2013, 10:19 AM   #20
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Re: Sennheiser lavaliere interruption

Vertical still works best because the aerial pattern (like a donut, explained above - see torus) has width, but not much up or down, so makes a better match with people moving in front of the camera. The plug in transmitters, or handhelds are always up in the clear, not hidden behind, or in pockets - so the path loss is much better - and dropouts fewer.
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Old June 8th, 2013, 12:24 PM   #21
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Re: Sennheiser lavaliere interruption

Unless you have a rigid antenna fastened to some radio-transparent spacer which is attached to the body of the person who remains upright at all times, you don't really have anything near the equivalent of traditional propagation factors of broadcast antennas. More likely, you have a piece of flexible wire hanging at some random angle (or stuffed into a pocket,etc.) and in close proximity to a large RF-absorbing body. And more often than not, that RF-absorbing body is BETWEEN the transmitting antenna and the receiver.
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Old June 8th, 2013, 04:23 PM   #22
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Re: Sennheiser lavaliere interruption

Nice bit of fudging - but that's not really been the case since we were working VHF with a screwed up piece of wire dangling down - most UHF aerials are now quite stiff. Whichever way you think about it, a horizontal aerial placement will also have rotational effects, which a vertical doesn't have. Sure - body effect has an impact, but body effect with a potential end on aerial path top the receiver IS worse.

I commercial comms systems, vertical polarisation is the standard for all things mobile. Why? Because it is the most efficient. Horizontal aerials offer directivity.Obstacles can be in the way in any polarisation. Typical video use has movement mainly in the horizontal plane, which makes vertical polarisation the most effective. Just how it is.
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Old June 8th, 2013, 05:54 PM   #23
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Re: Sennheiser lavaliere interruption

Perhaps we're picking nits here.

If the wireless mic is used optimally, the antenna wire should end up being much closer to vertical than to horizontal. And therefore the original transmitted signal should be pretty much vertically polarized.

But yes, absorption and reflection can change the polarization from vertical (as it was transmitted) to something else that is not predictable. This problem will be worst when shooting indoors, where there are lots of metal conductors of various dimensions and various proximity to the transmit antenna. If a true vertical signal is reflected by a vertical conductor, the polarization should be unchanged. But if a vertical signal is reflected by an oblique conductor (e.g. the arm of a metal chair) then there may be a change in polarization.

From my past experience, watching the signal indicator lights on diversity receivers, I got the impression that a horizontal antenna got a lot less useful signal than a vertical antenna. (This was with a "mostly vertical" dangling transmit antenna.) But yes, using a bit of a "V" pattern, when one antenna tilts perhaps 15º to the right, and the other perhaps 15º to the left, seemed to work quite well. I've seen this used extensively. The polarization undoubtedly does not stay precisely vertical.

Still, as Mr. Johnson points out, vertical is certainly preferable because of the pattern. A horizontal wire antenna will be dead along the axis of the wire. If you seemingly "aim" the wire toward the transmitter (by sighting along the wire as if it were a pointer) you will be aiming the antenna in the worst possible direction. The horizontal torus pattern of a vertical (or nearly vertical) antenna will obtain the best coverage around the room. And since the receive antenna(s) is(are) (nearly) vertical, you should try to keep the transmit antenna as close to vertical as possible.
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Old June 8th, 2013, 06:43 PM   #24
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Re: Sennheiser lavaliere interruption

Here's a summation from my point of view.

Engineers and experienced pros will argue this stuff forever, which is all well and good.

For the practical shooter in the field just trying to record good audio here's the drill.

Wireless is always prone to more potential sound affecting audio issues than wired mics. If you truly need a wireless rig, it's important to make sure ALL the components are in proper operating order. This includes the antenna systems AND a dependable source of power.

In order to get quality results from any wireless system you need one thing WITHOUT FAIL. And that's a monitoring system that lets you hear your audio AT THE LAST STAGE OF THE CHAIN in real time.

That means dependable headphones and a place to plug them in that's not too far down the audio chain to hear problems in later stages.

With such a system in place, if you do experience signal issues - it's pretty easy to test to see where they are coming from. In the case of wireless units, for example, you can simply remove the transmittter from wherever it isn't working and put the transmitter and receiver in closer line of sight proximity. If the problem persists, it's NOT distance or antenna masking. Similarly if you re-orient either the receiving or transmitting antenna and the problem significantly changes, then and ONLY then is it reasonable to assume that the problem is an antenna problem.

Theory is wonderful. Nothing would ever get created without it. But in the field with people waiting for you to deliver results theory is at best, instructive, and at worst, misleading.

I've had to do stuff that people who know the theory behind radio mics would say were butt head dumb (like gaffer taping a body pack transmitter to a handheld transmitter when I needed two signals and the performer costume made it totally impossible to hang a body pack on that person ANYWHERE! ) but at the time it was the ONLY way to solve my problems and the gods smiled and it worked great. Would I recommend it? Hell no. But my job isn't to get the theory right, it's to get the performance capture right. Period.

For my money, the real thing that matters in this thread is the practical figuring out where the problem is coming from in real time - and fixing it RIGHT THEN. It sounds like on your set someone let the problem get baked into your signal out in the field rather than stopping to fix it at the time.

That's the big lesson. And the most valuable one you can get here. Much more valuable than all the antenna theory in the world.

My 2 cents, anyway.
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Old June 9th, 2013, 08:49 AM   #25
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Re: Sennheiser lavaliere interruption

Mr. Davis:

I agree that, in the field, a quick solution, and sometimes trial-and-error, are needed, obviously.

The reason I thought it would be helpful to interject some antenna theory is that the OP stated:
Quote:
Originally Posted by Patrik Vale
normally the lavaliere on speaker is put verticaly and the one on camera is horizontally. Never thought that can be the problem.
The OP had the problem repeatedly, and did try to solve it in the field (e.g. he tried several different interconnect cables to isolate the problem). But since he did not know antenna orientation was a variable, he never tried that. So, clearly, the OP needs some information.

I suppose one could just say "try moving the antennas" but I don't think that's really enough. For example, he might change from a horizontal antenna perpendicular to the transmitter, to a horizontal antenna pointed toward the transmitter. Neither one would be the ideal solution. IMHO it's better to give a little extra information, and let the OP (or any reader) make an informed decision in any given situation.

That's my 2¢ worth...

P.S.: Don't worry... I've never seen any theory that says gaffer tape is bad. ;-)
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Old June 9th, 2013, 01:13 PM   #26
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Re: Sennheiser lavaliere interruption

Man, this is just amazing information!!!!! Some of it I'm actually able to follow;-)

You guys are the best.

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Old June 9th, 2013, 01:51 PM   #27
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Re: Sennheiser lavaliere interruption

I was looking for a smaller wireless system as currently I use a older azden 500 udr (http://www.azdencorp.com/pdf/TV_Tech...UDR_review.pdf) which I got second hand a few years back but in combination with my smaller camera's it's too bulky. Thing is I heard from colleagues that work with smaller systems, similar to the sennheiser to which is refered to by the op, that they do experience occasional dropouts in sound. This is something I never have encountered with my azden and it is essential to me that this doesn't happen during a live shoot. Beside the direction of the antennas, is there something a wireless receiver must have to perform with a much higher reliability of not loosing the signal? Was I just lucky with my Azden's to never have experienced any issues?
Btw, great info in this thread, only most of the stuff is way over my head :)
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Old June 9th, 2013, 02:07 PM   #28
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Re: Sennheiser lavaliere interruption

All joking aside, the most useful thing to remember is that the most expensive radio system is nearly as good as a $10 cable! In theatre we spend an awful lot of our time listening to the radio channels, attempting to predict which channel is going to be a problem, praying that the talent doesn't take a step rightwards into the RF hole you know is there. When you hear a 'phutt', were you quick enough to spot which channel it was - or not?

You stick the pack onto somebody, with the aerial sticking up, but you can see the bulge that tells you it's gone horizontal when they sat down, and you are using directional paddles, and the 90 degrees is going to give you problems at any moment.

Despite all the theory we've explained, RF is still a black art, and there is always magic involved.

You can do all the planning you like, but unexpected events always mess up. When I do sports, I often stick a shotgun on a short stand, in clear line-of-sight to my camera up in the stadium - works fine, but last time - the nasty phut in the headphones made me cast a glance towards the mic and there was a big fat bloke sitting down right next to the mic, directly in the way - and RF doesn't like it. All the places he could have sat, he picked there. Yet, I've had complaints a handheld mic was coming through somebody else's PA system over 500m away. Quite funny - they had an outside event that had finished, but they had left their PA on, but in a locked up building - and my microphone was relaying everything to people walking past!
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Old June 9th, 2013, 03:21 PM   #29
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Re: Sennheiser lavaliere interruption

Quote:
Originally Posted by Noa Put View Post
I was looking for a smaller wireless system as currently I use a older azden 500 udr (http://www.azdencorp.com/pdf/TV_Tech...UDR_review.pdf) which I got second hand a few years back but in combination with my smaller camera's it's too bulky. Thing is I heard from colleagues that work with smaller systems, similar to the sennheiser to which is refered to by the op, that they do experience occasional dropouts in sound. This is something I never have encountered with my azden and it is essential to me that this doesn't happen during a live shoot. Beside the direction of the antennas, is there something a wireless receiver must have to perform with a much higher reliability of not loosing the signal? Was I just lucky with my Azden's to never have experienced any issues?
Btw, great info in this thread, only most of the stuff is way over my head :)
Noa, I think you are just lucky. I had that same Azden a few years back and had some similar issues. I loved the professional look of that thing though.

I also have 2 sets of the sennys, the older one and the later ones. The older ones never suffer that problem. The newer one occasionally does.

There is another thread on this board that addresses that issue.
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Old June 9th, 2013, 03:45 PM   #30
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Re: Sennheiser lavaliere interruption

If you monitor the RF level at the receiver on ANY system, you will see that for the majority of the time, the level is virtually full, but at the point they go 'phutt' the signal strength drops to virtually nothing. With 50mW or so that's not really very much, so finding nulls is very easy. Diversity reception is by far the best solution, but it's still not perfect. Apart from dropouts, we also have co-channel interference, intermodulation interference, and of course receiver desensitisation. I've got many different makes of kit, and all of them, at one time or another have been compromised.
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