Simple (I hope) question re wireless systems

[Chris Harding]

Hi Greg

Broadcast studios will clip a transmitter on a talent's belt so that antenna is again only 1/2" from their body BUT the antenna is in "free air" ... the only difference inside a groom's inside jacket pocket is one external layer of fabric between the antenna and free air ..both are still in close proximity to the talent's body yet a belt mounted transmitter gives a far better signal. With weddings the groom often turns to face the altar so a rear mounted transmitter is not a good idea asthetically.

I'm certainly no audio engineer but I have noticed the I get better performance if the antennas are in free air .. then again, inside a jacket pocket they also have a bit of space around them as the transmitter is dropped in vertically and only one side of the antenna is subject to body attenuation so wouldn't be the same as cupping your hand over the antennas. You mention VHF is attenuated so would higher frequencies be worse or better ...my old Azden VHF units (I use them to send instructions to a 2nd camera person) are around 170mhz ..my UHF bands run up to 800khz and the unit Jim is testing is not only digital but also runs at 2.4GHz

Chris

[Greg Miller]

Hi Chris,

I think there might be a few things at play here.

VHF antennas are longer. A VHF antenna hanging from a belt might be fairly straight so reasonably efficient. A VHF antenna coiled in a coat pocket, and bunched up against the bodypack, might be much less efficient.

UHF antennas are shorter (and frequently stiffer) so the "bunching" problem isn't as much of an issue. OTOH UHF would probably be attenuated more than VHF by proximity to a human body.

I would think 2.4GHz would be attenuated most of all. I have recently been playing with some digital systems marketed as WiAudioLink and made by Jangus Music. These work in the 2.4GHz band. Transmission is in digital mode and audio is very clean. There is NO external antenna; it's inside the transmitter. If I have the transmitter (about 1" x 3/4" x 3.5") in my shirt pocket (essentially 1/4" from my body) and I walk to a distance where dropouts start to occur, and then take the transmitter out of my pocket and dangle it by the mic cable at arm's length, I can get another 20 to 25 feet of range. That seems to confirm my theory that 2.4GHz absorption is worse than at lower frequencies (after all, microwave ovens work by RF absorption, and they operate at 2.4GHz!).

Of course there are many variables so no two systems will be the same, and may seem to defy theory. Hence the common disclaimer YMMV.

[Jim Andrada]

Maybe I can get the cat to hold it! There's an old expression in Japanese to say that you're really really busy - roughly translated it's "(We) want to borrow even the cat's hand"

I can do another test - start the recording in the other room and put the transmitter in my pocket and wrap myself in the bath towel. But as Greg said, fabric is pretty transparent at the frequencies involved. I should have mentioned that there were three walls and two doors and a second wifi access point in the path last night as well.

[Jim Andrada]

OK - I clipped on the lav and wandered around inside (and outside) up to around 80 feet.

The transmitter was in my pocket and I pretty well kept my hand around the antenna and otherwise tried to behave "badly".

Here's the result

https://dl-web.dropbox.com/get/publi...MeZo8uPc93uRbg

Have to say I always get nervous wearing a body pack - it reminds me of the time I had a major presentation to about 600 folks in Sydney - got all wired up and decided to make a pit stop beforehand. Walked out on the stage to thunderous applause - of course the mic had been live the whole time.

That's the difference between Aussies and Japanese - in Japan (where I worked for six years) the audience wouldn't have made a sound. Oh well, livened up their day.

Anyhow, sounds damned good to me

[Paul R Johnson]

There is basic RF theory working here.

Fabric is transparent to RF - so not significant in attenuating the signal for any practical purposes.

The free space actually is an important component. Transmitters have fairly low RF output, and the object is to get as much as possible on it's way to the receiver. For optimum power transfer, the transmitter has to be matched to the aerial. A quarter wave antenna is pretty efficient, but should really have a ground plane to work against. The physical size of a transmitter pack means that the ground plane required is simply not be enough for efficient power transfer, so by design, not all the power output of the transmitter makes it out of the antenna. Bringing the antenna close to an absorber - as in the human body, impedes the efficiency even more. The side away from the body will radiate, but the side near to the body will propagate poorly because most of the energy is absorbed by the body - which is mostly water, and very often sweaty - and the salt is of course a conductor. It's amazing that an antenna in a pocket in an armpit emits anything at all.

Testing in open spaces is a pointless exercise if the real working conditions have reflectors and absorbers that conspire against you. The usual problems are not caused by gradual drops in RF level at the receiver, they are sudden HUGE drops where the TX or RX is in a null. Lifting an arm, or rotating 5 degrees can suddenly restore operation, and the RF meter goes from 0 to full scale.

There's very little you can do at the transmit end, so the only real thing you can do is at the receive end. Many years ago, when we were using VHF systems, with much longer aerials, we had this problem on a shoot where we were working at longer distance than were ideal. One of the guys was also the rigger, and always wore a hard hat - we pinched it, lined the inside with aluminium foil, and drilled a hole in the top and attached the receiver antenna to that - the cameraman was 6' 3" and with this crazy hat on we got solid reception, simply by having the antenna a bit higher, working with a better ground plane, and a greater capture area, compared to the helically wound rubber duck on the receiver.

When I first qualified, I spent a lot of time up towers setting up radio links with much higher powers, but our business moved to radio microphones where the theory is exactly the same, but subject to all kinds of weird folklore - most of which is grounded in fact, but some is just silly.

Receivers have sensitivity differences, and selectivity differences - but the killer is aerial systems. Diversity systems on a battery operated small unit are never that good because the two aerials (and I include those who use the connecting cable as one of the aerials) are just too close together for the diversity system to work efficiently.

[Jim Andrada]

Actually I did have it in an armpit.

Anyhow, I'm reasonably happy with the unit - think it will be more than good enough for what I need.

[Paul R Johnson]

If it sounds good, and your distances are modest - that's all that's needed. It just gets harder when you have more channels on the go. For one system, it's rare to find an unusable one. I found a couple of my old ones the other day and they must now be 25 years old, and were not the most expensive when new - they still work and sound fine.

[Chris Harding]

Thanks again Jim

I'm sure that manufacturers also take this into account when designing a system so make sure that the system will actually work when the transmitter is on a person. I usually always try to use the left jacket pocket if it exists as the groom at a wedding is on the right of the bride (from the camera POV) so at least the transmitter doesn't need to punch thru a heap of body fat as it's on the side of his jacket closest to the camera. I'm also 99% of the time only less from 20' from them as any further away means I am highly likely to get blocked.

The bottom line here is that it works fine even in armpit mode at reasonable distances which is good enough for me!!

Yeah, I also had a groom already miced up at a wedding and he decided to visit the bathroom and of course I could hear the whole procedure on the camera but wasn't recording yet ... it does show that transmitters work pretty well even in unusual conditions

Chris

[Jim Andrada]

Hi Chris

I know it wasn't a very scientific test but it gave me a chance to play with the unit a bit more. I think my conclusion is that it should work well in just about any orientation - maybe even clipped to the belt in back to avoid bulging pocket syndrome. At the distances you'll be working I don't think I'd worry much about the wearer's position relative to the receiver or where on the wearer the transmitter was placed.

I'm rather favorably impressed with the unit and it seems to be a good price performer as well. I started out thinking it would be hard to find something under $500 that would be good enough for what I need, but I was wrong.

[Paul R Johnson]

Chris - you can't design a system to be better when worn on a person - as this is fundamental in the majority of use. As I said, the aerial system on the pack is always a compromise, and there is no international standard for a 'person'. Some people do seem to be radio nightmares - there is very often, in a multi-system install one cast member who just has radio issues. They are not necessarily big, or more fatty - which you would kind of expect to soak up more RF, leaving gaps in the radiation pattern. Sometimes they are just the same size the ones who's systems work? Sweat does seem to play a big factor.

Without some kind of standard, we should place little emphasis on a test that seems to suggest product A is better than product B, or C. The factors essential to making this determination have not been measured in these user reports. Power output, aerial efficiency, and receiver sensitivity. The last one is quite important. Sensitivity can often be increased in a design, but usually at the expense of selectivity, which may not matter in a rural location with just one transmitter running, but in a busy city, could be a killer flaw. In RF testing, performance on strong signals is often pointless, it's how they cope with low RF level, and strong sources on the next channel that matter.

Probably worth also keeping in mind that attenuation gets worse as the frequency rises - so we should be complaining that 2.4GHz kit keeps dropping out, but of course, it doesn't.

By all means test the audio quality, and construction - but don't get sidetracked by RF performance based on observation, not measurement - it's far more complex than it appear to be. The only real rule should be to make the path length as short as it can be. You can talk to the space station perfectly well on RF levels similar to radio mics - because there is nothing in between!

[Chris Harding]

Thanks Guys

As long as it works and we care with placement I see no issues. The only issue I ever had was with the Azden 330 which was the dual receiver and that had multiple dropouts inside a Church. My old Azden L100's have no issues except our Government has re-allocated the frequencies so I have no option but to change and with more re-allocations in the pipeline the 2.4GHz AT units were attractive hence all the questions.

We are into our Winter season in a week's time with no weddings scheduled for 10 weeks as it's cold and wet so I will have a bit of time to test and decide. All the advice is greatly appreciated of course and Jim's practical tests are of far more use to me than a corporate sponsored "review" .. especially kind of you Jim!!

Chris

[Jim Andrada]

Thanks for the kind words. It really wasn't much of a test and it gave me an impetus to familiarize myself with the unit.

One thing I don't like about it (at least on the small camera top receiver) is that the labels on the switches and ports are just molded into the black plastic and they're small - and my eyesight is awful so without a strong light it can be difficult to read. I actually looked at the instruction sheet and just memorized what was where. Probably not a problem for "youngsters" though!

[Chris Harding]

That's also my issue! I normally need reading glasses for small print but have an extra lens on the camera's viewfinder so I don't need them ...that doesn't help situations like yours and I have had that myself and you either need to have a pair of reading glasses hanging around your neck or struggle to read and like you end up memorizing where switches should be set! Old age is a bugger and what especially annoying is that with my still cameras I have dioptre settings on the viewfinder but nothing to magnify the LCD screen so checking what you have shot is tricky if you cannot see it in 20/20 vision

[Jim Andrada]

Yes, size matters!

I preordered a 7" Sound Devices video recorder - I need the big screen. It's absolutely hopeless trying to use the (CRAPPY!!!!!) viewfinder on the C-100. The LCD screen is nice enough - but not outdoors here in the Arizona desert!

Come to think of it maybe it's more a matter of age than size.

[Greg Miller]

Originally Posted by Paul R Johnson

A quarter wave antenna is pretty efficient, but should really have a ground plane to work against.

I've always assumed that the manufacturers hope that the ground plane of the PCB inside the case, capacitively coupled to the wearer's body, would somewhat serve as the ground plane. Of course that doesn't eliminate the numerous variables that have been mentioned.

Originally Posted by Paul R Johnson

Bringing the antenna close to an absorber - as in the human body, impedes the efficiency even more. The side away from the body will radiate, but the side near to the body will propagate poorly because most of the energy is absorbed by the body - which is mostly water

More than that, being coupled to a human body will most surely detune the antenna, which will cause more reflected power back to the transmitter final stage, and less power reaching the antenna. (And of that "less power" much is then absorbed, as stated above, so the loss of total radiated power can be very significant.)

Originally Posted by Paul R Johnson

and very often sweaty - and the salt is of course a conductor. It's amazing that an antenna in a pocket in an armpit emits anything at all.

I've never performed a scientific test of this (thank God) but I suspect that the effect of surface sweat is minuscule compared to the effect of all the salt water contained inside the body. After all, we are not placing two conductive electrodes in electrical contact with the surface sweat. The effect here is one of absorption of the electromagnetic field radiated by the [insulated] antenna.