"That's one small step for man... one giant leap for mankind."
When Armstrong uttered his immortal words, few would have appreciated that Tranquility Base’s radio power was a mere four watts or, in our jargon, 4,000 milliwatts (mW).
Voyager probes are still sending telemetry signals back ten billion miles to earth on just five watts.
When you ask most sound mixers what is the most important determining factor for wireless mike range, invariably you will hear “milliwatt” in their reply. You may further learn that power is related to range; doubling the power doubles your range, some will say.
Before I debunk that myth, consider this fact:
In a typical shoot scenario, less than one percent of your transmitter (TX) power, i.e. less than ONE milliwatt, even radiates in the general direction of your Portabrace bag or sound trolley.
Meanwhile, back at launch pad 37B… This is why the power/range theory exploded before clearing the tower.
If you have 1/40th of Tranquility Base’s power then, by interpolation, you might expect to get 1/40th of 250,000 miles; say 6,000 miles? Is that right? Unfortunately not; a few pesky details get in the way. NASA has this sneaky trick where they use high gain (50dB+) receiving dishes on the ground. These amplify the received signal to the equivalent level of a 400,000 watt transmitter.
The dishes are frightfully big and your neighbours are sure to complain. Alternatively, 400,000 watt transmitters are heavy on batteries, hard to hide and price-prohibitive (to borrow a term from Marketing). There is also the small problem of the talent cooking, like a microwave chicken, after a few seconds. Not sure if Tom Cruise’s management will go for it.
The ‘Best Case’ Wireless Mike Scenario.
Imagine this… Your talent is doing an impression of the Statue of Liberty. Standing in an open area, she holds a 100 mW transmitter above her head. While the DP whinges about the TX being ‘in shot’, I hasten to point out this is a ‘Best Case’ scenario, not a ‘Most Likely’ scenario. With our talent holding this ridiculous pose, consider the situation we have.
The transmitter (TX) aerial is away from the body and unobstructed. Ten, out of ten, points for placement.
What can be achieved, under these conditions, is about the best possible. Why?
The TX is operating in a close approximation of ‘free space’ conditions.
Try this on a slow day. If you can’t enlist anyone to do their Lady of Liberty impression, clamp the TX on a C-Stand with the aerial vertical and prominently clear of any surroundings. Go for a walk around the TX. Be sure to take a cut lunch and plenty of water as the range will be greater than you expect.
Worse Case Scenario.
This is quick and easy; simply clip the TX onto the waist of the talent right in the middle of their back. This has two affects:
· It keeps the kidneys warm on cold days; and
· It replicates the Worst Case performance conditions;
We have now introduced two factors that greatly diminish the effective radiated power that we had in the Free Space scenario. Yet this is exactly how many people use them daily. Fine for studio work, or pieces to camera, over a few yards, but if you want reliable range over demanding distances you need to appreciate why this is the worst possible case.
First off, we placed the TX next to the body. The aerial detuning, in combination with the absorption of the torso, reduces the effective power by around 16dB; i.e. down to 1/40th of the power you started with.
Secondly, by placing the TX on the middle of the back, we have probably interposed the torso between the TX aerial and the receiver (RX) aerial. The attenuation of the human torso is 20dB minimum, at 500 MHz, ranging up to 35dB at 900 MHz. Both losses, in dB, are additive, i.e. >36dB in total.
The effective radiated power, arriving at the RX, is now the equivalent of 100 mW minus 36 dB, or 0.025 mW.
How would you react to a glossy brochure at the NAB extolling the virtues of a fully digital, spectrum analyzing, all singing/dancing wireless mike with a mind-boggling output of 25 microwatts? Leaves you a bit flat, does it?
There is only one less effective place for a TX, but I’ve promised to refrain from proctology jokes.
Having looked at the problem, and quantifying the ballpark impact, what options do we have to minimize the damage?
TX Placement.
Wherever possible, hide the TX on the front of the body. Yes, that can be difficult but there are some very unobtrusive transmitters on the market that are easy to hide. Don’t be deterred by low power specs, we are talking of improvements in the order of 20dB or more. A drop of 3dB in TX power is insignificant. The inside of lady’s thighs is one possibility and definitely more fun than some alternatives. Some use elasticized bandages, available at pharmacies for thigh injuries. Tucking the TX inside the bandage is an effective way of holding the device in place.
Aerial Placement.
If you have to fit the TX to the back, locate the aerial on the front of the body. Most TXers have SMA or MXC sockets. Use a length of miniature coaxial cable with a connector on one end and the shield removed for a ¼ of a wavelength at the other. Place the far end of the cable on the front of the torso. The loss of the coax cable is negligible and at least the TX will perform as if it was mounted on the front of the torso. It is the aerial position that counts, not the TX location; thin cables are pliable and easy to conceal.
In any of the above scenarios, it helps greatly if the aerial can be kept away from the body. Any gap will assist as the proximity loss follows an inverse square law function.
If you double the spacing, the proximity loss drops by a factor of four. Increase the gap by a factor of three, and the proximity loss drops to one ninth. If circumstances permit, adding polystyrene foam beads (as used for protective packaging) is a cheap and convenient way of creating a spacing which might otherwise not exist. The improvement is well worth it.
RX Aerials.
If you must place the TX on the back, one sure winner is to place a receiver aerial behind the talent where the signal is highest. Again, don’t get paranoid about coaxial loss. The benefit greatly outweighs cable losses. Obviously, if you face this problem often, it would help to use serious aerials and low-loss cable.
RX Aerial height, etc.
Obviously, any receiver aerial elevated on a C-Stand, or other support, will dramatically improve range; as will any passive aerial with gain, such as the humble Yagi. Masthead amplifiers, whether inbuilt or in-line, can introduce more problems than they overcome in the wrong hands. Unless you fully comprehend the consequences of pre-mixer RF gain, the perils of broadband amplifiers should not be underestimated. Even narrow-band amplifiers can be (and are) misused. I see industry ‘gurus’ adding them into systems in much the same way as annoying TV salesmen like to throw in free sets of steak knives.
But wait… there’s more!
In my four decades as an RF professional, I have resolved ten times as many problems by removing amplifiers, than by adding them.
Doubt this? Well ask your local TV aerial installer about clients who install unnecessary masthead pre-amps, then watch his eyes roll back into his head.
As Professor Julius Sumner Miller would say "Why is it So?"
Log Periodic Dipole Arrays (LPDA) a.k.a. shark fins.
Could I please ask everyone to refrain from mentioning these, especially those with inbuilt amplifiers? Why? Well… I go into severe apoplexy and uncontrollable vomiting and the only effective cure is a long and tedious treatise on how inappropriate they are in virtually every wireless mike scenario.
I once had the honor of meeting Dr. Guertler (one of the team that developed the LPDA), at an I.R.E.E. show in the seventies. He might be amused by the claims made by those who put wireless mike marketing ahead of reality or fact. Don’t get me wrong, the Log-Periodic is a remarkable invention; it’s just that they were designed for a different application. The UHF ones provide modest gain over one octave (500-1000MHz). Yagis and collinears offer much higher gain over just 50MHz.
If your wireless system operates over a spread of just 50MHz, what benefit is there in having an aerial which works over 500MHz? The answer is simple. None.
In the real world, bandwidth is more of a curse than a benefit. A long Yagi, or collinear will perform better and are much less susceptible to interference from walkie-talkies, or cell ‘phones, due to their narrow bandwidth.
Check out my further comments on Log Periodic Dipole Arrays
Using 'short' TX Aerials.
There is another trick in my sorcery bag that can be used with good results; use a ‘shorter' length TX aerial. You may have noticed that the aerial length varies with frequency. Manufacturers often colour code aerials so that you don’t get them mixed up. This is fine in theory, but when the aerial is close to the body, theoretical length is as relevant as an ashtray on a Harley.
All conductors have a property known as inductance which is proportional to length. Along this length the conductor also experiences a phenomenon known as ‘self capacitance’. At certain predictable lengths, the inductance and capacitance compliment each other in a state known as resonance. Under this condition, the aerial will effectively couple the TX with the outside world (in Marconi’s era, we erroneously referred to this medium as the ‘ether’).
Even the human body (a fairly good conductor) has resonance; in Free Space, around 70MHz. I have this theory, that if I had a wireless mike on 70MHz, using the human body as an aerial, I could communicate with Voyager probes in deep space. I’ll work on that and get back to you…
When the TX aerial is in close proximity to any part of the body the stray capacitance is greatly increased. So much so, the resonant frequency can drop by over 100MHz and the aerial will cease to function efficiently. You can cancel out much of this loss by shortening the aerial, thus lessening the inductance.
If you only ever use TXers fitted against the torso, the cure is simple. Get the pruning shears out! Alternatively, you may choose to keep a spare aerial cut deliberately short for these occasions.
During a reality shoot on a remote Pacific island, I experimented with systems and found that the aerials designed for 750MHz were ideal for operation on 550MHz TXers fitted to the waist. That is a useful yardstick, although you really need instrumentation and patience to find the optimum solution. However, even a crude shortening of the aerial will help by 6dB. The attached chart will put you in the ballpark. Aim for a frequency increase of 25%; i.e. for 600MHz operation, cut the aerial to 750MHz.
The improvement is around 6dB. The equivalent of quadrupling your TX power!
NB: The above rule does not apply to receiver aerials (unless you wear them on your waist ;-)
Some may argue that shortening the aerials from their theoretical length will lower aerial efficiency when the TX is away from the body. This is true… but when the TX is not against the torso, you are eliminating losses in the order of 16 to 36dB. The loss of aerial efficiency is inconsequential if you remove the torso from the equation. The Bible states this logicality quite succinctly:
"Why do you look at the speck of sawdust in your brother's eye and pay no attention to the plank in your own eye?
Matthew 7:3-5
‘Kicking’ the High Power Habit.
Supposedly, used car salesmen love to see prospects kicking tyres. To them, it’s a sure sign the prospect is a ‘wood duck’ with no concept of what they are doing. Similar clues can be gleaned from those who insist upon 250 mW transmitters. Aside from the OH&S litigation risk, it demonstrates as much finesse as tyre-kicking.
It is the technical equivalent of wearing a neon sign on your head which reads "I have no idea how this stuff works... so I rented really powerful transmitters".
Choosing the right aerials and placement, is more rewarding than simply hiking up the RF power level.
“All generalizations are false, including this one.” – Mark Twain
© 2009 Microphone Magic