Frequently Asked Questions - Wireless

FAQs - Wireless

The gain and performance of the two antennas are the same.The ALP 600 is a PC board version of the ALP 700 and uses the same design parameters and the same number of elements. The ALP 600 uses a 4 layer PC board with strip-line matching and balun sections. We eliminated the external coax matching section that you will see on competing \"shark fins\" since we feel the coax solder joint can be broken if the antenna is really mistreated. The ALP 700 conventional antenna is not as rugged as the shark fin. We have, however, modified the ALP 700 to improve the ruggedness, by using a tapered nut for the jam nut on the antenna elements. The old elements would break at the first thread by the nut if the element was bent over. The new nut covers the threads completely and you can bend the element severely and then straighten it without breaking the element. The ALP 600 shark fin is still much more rugged, though. Aside from price, the only time not to use the shark fin, is if you are working in high winds or have the antenna mounted on a moving car or van.

The shark fin looks like a miniature sail. The shark fin is more expensive because the 1/8" thick, four layer board costs us more than all the metal and machining that goes into the ALP 700. I still find it hard to believe

Active antennas sound good to customers but they have many shortcomings:  

  1. If the antenna cable is less than 25', an antenna amplifier is not necessary and is actually detrimental to the operation of system.  
  2. The antenna amplifier must be relatively wide band since it must handle all the frequencies to which the receiver may tune. If the amplifier is wide band, it will pick up many interfering frequencies.  
  3. It is difficult to design a high powered amplifier that is also low noise. The high powered amplifier is necessary to handle all the garbage that can come into a wide band amplifier.

To put it into a few words, antenna amplifiers are never as good as the front end of a well designed receiver. They are a necessary evil and should only be used when necessary. They are necessary under the following conditions:  

  1. When the antenna has to be more than 35' feet away from the receiver and the signal to the antenna is also weak such as antenna to performer distances of 100' or more. (If the signal to the antenna is strong then you will have enough signal to compensate for the cable losses.) Keep in mind that the antenna amplifier is NOT solving the weak signal to the antenna problem but just compensating for the cable losses after the amplifier. Again, the amplifier can't compensate for losses before the signal gets to the amplifier; only losses after the amplifier.  
  2. If there is a passive splitter after the antenna that introduces loss. This is equivalent to cable loss and the same rule applies; put the amplifier ahead of the splitter. A 2 way splitter has 3 dB of loss, a 4 way has 6 dB of loss and so forth.  
  3. When the receiver is a poor design with a noisy front end and the antenna amplifier can boost the signal enough to overcome the receiver self noise.  

Our "active" antenna setup consists of a passive antenna plus an external amplifier, the UHF50. That way you can use the amplifier only when necessary. We use a high power amplifier that is pretty quiet and we also put in a pre-filter that is two blocks wide (50 MHz) in front of the amplifier. It isn't as good a filter as those in our receivers, but it is better than the universal wide open designs. In addition, we make the gain of the amplifier adjustable so that you can match the gain to the losses in the cable or splitter system.  

What is confusing about the whole antenna amplifier issue is that cable loss degrades the sensitivity of the receiver but more gain doesn't improve it. In fact the additional gain leads to overload and intermodulation problems. This is Mother Nature saying not only can you not win, it's hard to break even. Another way to think about it is that our receivers are already about as sensitive as can be. If an additional amplifier could improve sensitivity, then we would have built it in. 

You can tilt the antennas so that they are at 90 degree angles to one another. That is to say, bend one 45 degrees to the left and the other 45 degrees to the right. The tilted antennas are a reasonable way to operate and the best way if the antennas are fairly close together since they couple together much less than if they are both pointed in the same direction (parallel).

The antenna diversity used in our receivers does not select one antenna or the other; it sums the two antennas together and corrects the phase of one antenna so that the antenna signals do not cancel each other out as they might do if they were 180 degrees out of phase. So it does not make too much difference which way the antennas point since the receiver will correct the phase.

Additionally, in any usual environment, the signals coming to the receiver from the transmitter are not in any well defined phase relationship or direction. The signals are reflected from cars, the ground, metal studs, wire in walls, camera equipment and even people, so that the signal that gets to the receiver is pretty well scrambled and impossible to predict. The problem with reception occurs when all the signals from all the reflectors get to the antenna and cancel out. If you use two antennas, then the signals probably will not cancel out at both antennas simultaneously. There is a new problem, though, if you simply add the two signals together. When the signals at each antenna are equal and exactly out of phase they cancel out at the receiver. The phase diversity system that we use on our small receivers detects this condition and simply inverts the phase of one of the antennas. Now the antennas add the signals together for a 3 dB pickup in power. For a good explanation of this, that is more comprehensive than what I can do here, go to this link to our web site.

Dropouts and Noise-ups 1

It is part of our wireless guide. In fact you might want to down load the entire wireless guide because it is pretty good and pretty neutral in its treatment of wireless microphones.

It is surprisingly hard to do. The big problem is the battery terminal voltage is heavily influenced by how the battery has been discharged in the past. If it has been run down slowly with power gradually pulled out say over a 24 hour period, the relationship between remaining battery capacity and terminal voltage is fairly well defined. If the battery has been discharged heavily, say by a Lectro UM250, the relationship is not so clear. Basically the battery bounces back to a high voltage and can look like it is still pretty fresh. Under either a light or heavy load it will run down quite rapidly. The problem is, the battery tester has no way of knowing the past history of that particular battery. 

As a demonstration, if a fresh 9 Volt is accidentally shorted out with a piece of metal for 1 minute, you will get very odd results. The battery will get moderately warm. If the battery then sits unused for 8 hours, the terminal voltage will then measure pretty close to a new battery even under a brief load, but it will only run a transmitter for 5 minutes or so and then die almost instantly. In fact, if you then let it sit for a while again, the voltage will come back up again and die again in a transmitter in just a few minutes. I agree that this is an extreme case but it does demonstrate the problem of prior history.

Even when you know the history you can get bit. Recently we have been running battery tests on different brand batteries and we have found that some alkaline batteries tend to die very rapidly at the end of their life but other brands continue to run with lots of warning before they finally die. What's worse, different batteries from the same manufacturer may act differently. The reason we did the test is that we were getting complaints that the UCR201 was not giving sufficient warning with batteries made by XXXX brand, a major manufacturer. We tested the batteries and found that Evereadys gave 34 minutes of operation after the battery indicator started flashing its warning and the XXXX brand were giving about 3 (!) minutes of warning. We found this to be consistent with XXXX from 3 different parts of the country. Since so many of our dealers sell XXXX, we aren't sure what to do other than recommend Eveready as the standard. The XXXX brand is a perfectly good battery but it has a slightly different chemistry that is optimised for things other than high current drain. 

The safest answer is that a low voltage reading will always indicate that a battery is weak but a normal or high reading may not necessarily mean that a battery is good. This is why so many pros that absolutely depend on their equipment, put in a fresh battery at the beginning of a job or whenever there is the slightest doubt that the battery will "last long enough"

This information was gathered for a question on the RAMPS newsgroup for a UM200 transmitter but should be proportionally the same for other transmitters.

(See also FAQ #009-WIRELESS)

For newer tests on the iPower LiPoly rechargeable 9 Volt See (FAQ #086-WIRELESS)

Here's some more battery information as I promised a few days ago. It took a while to run all the batteries down. Here is what we did: we used the same transmitter, a Lectro UM200 for all the testing. This is a 100 mW UHF belt pack transmitter. This particular unit pulled 75 mA. We ran four different kinds of batteries to a final voltage of both 7.0 and 6.6 Volts. 7.0 Volts is where the LED is pretty dim and where two of our receivers with battery readouts start indicating low battery and 6.6 Volts is the very low battery indication. The transmitter is getting close to completely dying at 6.6 Volts but will usually run to 6.4 Volts or less. The LED goes out totally at 6.8 Volts. I'd put all this in a table but I don't think it would survive the news readers' formatting. So I'll list the type of battery and then the very dim LED point (7.0 Volts) and then the maximum use (6.6 Volts). Your mileage may vary.

  • Ultralife Lithium 16.0 hours and 17.2 hours
  • Duracell Ultra Alkaline 6.5 hours and 8.25 hours
  • Eveready Alkaline 4.75 hours and 6.75 hours
  • Varta NiMh rechargeable 2.5 hours and 2.5 (!) hours
  • Varta after 2 months of sitting around is the same as above, 2.5 hours.

Here's my conclusions: Assuming that a sound mixer with good common sense would toss a battery when the LED is very dim (or sooner) and using an standard alkaline as a reference, you'll get 3 times the life with a lithium, about 40% more life with an Ultra alkaline and about one half the life with a top quality nickel-metal hydride (NiMh) battery. (Though the Varta NiMh claims only 150 mAh, they start out at more than 180 mAh.) Also, the NiMh batteries don't self discharge as quickly as NiCad batteries since the battery after sitting for two months was still at close to full capacity. 

Disclaimer section: These were fresh, new batteries at room temperature. This was just one test, performed on just one transmitter.

Anti-disclaimer section: Most brands of alkaline batteries are about the same, alkalines and lithiums have a long shelf life, and our transmitters are pretty consistent. We have found the Eveready batteries to give the longest life for a standard alkaline battery. In any case, the ratios of battery life should be good numbers. You guys know what kind of battery life you are getting now, and the ratios should be informative.

Here's the total and long winded story on Lectro low end frequency response. Once upon a time, all the transmitters were set up to be flat (1 dB down)to 50 Hz. Some UM195 users were having problems with low frequency rumble driving the bass compander and causing rumble modulated breathing in the system. We did 3 different fixes in the system, one of which was to roll off the input at 70 to 90 Hz. We picked this number cause that's what Vega was using and they were the Big Dogs at that time and we figured, correctly, that they knew what they were doing. This improved the operation of the systems and all seemed well except that Jerry Bruck (the Schoeps importer) wanted response down to 50 Hz. So for a while we did a Jerry Bruck modification that was basically back to the original input reponse. Jerry knew what he was doing and could handle the room rumble and wind noise in other ways. Then a lazy engineer (me) decided this was too much paperwork to track all his orders and redesigned the UM series transmitters to have a variable rolloff. That way Jerry could have his cake and we didn't have to build bake a special. You can hear the rolloff on voice if the rolloff is set to maximum (185 Hz). If you set it for 100 Hz or lower, it leaves voice alone.

The UH195 and UH200 plug on series were never rolled off since most pro microphones have a low end roll off anyway or have switchable filters. Or they have extended bass reponse and the user has selected the mic for that bass reponse. In any case, we left the UH's alone and they are flat to below 50 Hz.

Here is a reply given to this question on the RAMPS group:   There was a query about whether to send mic level or line level to a camera that always had a mic preamp in the signal chain and used an attenuator to convert line level down to mic level anyway. The answers ranged from it didn't make much difference to the line level would pick up less interference. Both are reasonable answers. In general, it is better to keep signal levels high from the source and if necessary attenuate them at the input to the "load" (in the case described, the camera audio input). If the signal is attenuated to mic level at the source, then any noise picked up in the cable enters the camera at full noise level. If the signal is at line level at the source and then attenuated at the camera, the noise picked up in the cable is attenuated also. This could improve your noise rejection by 30 dB or more.

A case in point: We made a law enforcement wireless system a few years ago that went in the trunk of a police car. Our system provided audio to their VHS video recorder that was tied to a small video camera mounted behind the windshield. The officer wore a transmitter. The distributor buying the wireless systems from us specified mic level to feed the recorders since the ALC (automatic level control) was for that input. In some installations there was severe alternator whine because of ground loops and other clicks as equipment turned on and off. The grounds to the system included the antenna ground to the car chassis at the car whip antenna, the audio ground to the recorder, the power supply ground to the receiver, and a digital ground from our squelch circuit back to their readout box. Those were just the grounds to our receiver and didn't count the grounds to the recorder, camera, etc. The installers in different states and cities all had their own way of hooking things up and varied from good to horrible. An isolation transformer in the audio line usually fixed the problem but was too much money. We finally convinced the distributor to use an attenuator at the input to the recorder and let us send a line level signal. This reduced the ground loop noise by 30 or 40 dB and "solved" the problem. The distances involved were only a few feet and it wasn't really a cable pickup problem. Later on we convinced the distributor that the ALC wasn't a good idea anyway because it confused juries as to what was going on and made it hard to hear. In addition, the ALC was really upset by gunshots, as were the officers of course. Sending line level out of our receiver to the line level in on the recorder made for a bullet proof installation, so to speak.  

So the rule of thumb is high levels at the output and attenuate as necessary at the input, even though it won't make a difference most of the time.  

Here is an adapter for the UCR100 to attenuate its output down to balanced mic level.

This reply was posted to this question on the RAMPS group:

Most wireless systems, even some "pro" systems do not have a limiter-compressor in the transmitter. This forces you to do exactly what you are describing, which is to attenuate the mic input to prevent the occasional overload. All the Lectro transmitters for the last 15 years have a shunt FET limiter before the input preamp. The nice thing about the shunt limiter is that it is out of the audio circuit until a potential overload comes along, then the excess signal is shunted away. The limiter has a range of 25 to 30 dB. At usual gain settings, the transmitter won't overload until after the typical electret lavaliere microphone is already clipping.  

Interestingly, the Vega microphones from years ago had a very effective limiter using an LED/LDR (Light Dependent Resistor). Vega referred to it as a "soft compressor" and it was. Though it wasn't effective when the transmitter gain was set low, for real world use, it was very nice sounding and, in my opinion, one reason Vega was the number one pro wireless. More interestingly is the fact that most current wireless mics have taken a giant step backwards by leaving limiter-compressors out of the bag of design tricks. Check the specs on the data sheets to see if there is an input limiter-compressor. Chances are there isn't one. The COMPANDER used in all current wireless mic systems has nothing to do with the input limiter by the way. The input limiter is in addition to the compander and additionally increases the usable dynamic range.

Here's the answer I posted on RAMPS about 9 Volt battery life:

Here's the last of the 9 Volt battery tests. This is a similar test to what we did in a previous post but with a high power transmitter. (See also FAQ #005-WIRELESS) For newer tests on the iPower LiPoly rechargeable 9 Volt (See FAQ #086-WIRELESS)   Here is what we did this time: we used a 250mW transmitter, a Lectro UM250 in the testing. This is a 250 mW UHF belt pack transmitter that eats 9 Volts like they were potato chips. This particular unit pulled 105 mA. We ran three different kinds of batteries to a final voltage of both 7.0 and 6.6 Volts. 7.0 Volts is where the LED is pretty dim and where two of our receivers with battery readouts start indicating low battery and 6.6 Volts is the very low battery indication. The transmitter is getting close to completely dying at 6.6 Volts but will usually run to 6.4 Volts or less. The LED goes out totally at 6.8 Volts. I'll list the type of battery and then the very dim LED point (7.0 Volts) and then the maximum use (6.6 Volts). Your mileage may vary.

  • Ultralife Lithium 5.5 hours and 6.6 hours
  • Duracell Ultra Alkaline 2.6 hours and 2.8 hours
  • Eveready Alkaline 1.8 hours and 3.2 hours (!)
  • Varta NiMh rechargeable 1.0 hours and 1.25 hours

These are interesting results. If you saw the earlier post with a similar table, you will notice that the Ultra alkaline has the same 50% advantage to 7 Volts but when run to 6.6 Volts, has instead, a 13% LOSS. This is not the same as for a 100 mW transmitter. There the Ultra was 50% ahead at either end point voltage. The Ultra fell like a rock when the voltage got to 6.6 Volts. In fairness to the battery manufacturer, these 1/4 watt units are very hard on batteries.   Same disclaimer as before: These were fresh, new batteries at room temperature. This was one test, performed on one transmitter.   And same anti-disclaimer: Most brands of alkaline batteries are about the same, alkalines and lithiums have a long shelf life, and our transmitters are pretty consistent. In any case, the ratios of battery life should be good numbers. You guys and gals know what kind of battery life you are getting now, and the ratios should be informative. We have found Eveready to be the most consistent general purpose alkaline.

On to other projects. I've seen enough battery strip charts for a while.

The 185 series has diodes to the power supply to protect the output from 48 Volt phantom power or other high DC voltages on the output jack. If the voltage on the output line exceeds 12 Volts, the diodes turn on and shunt the excess voltage. Since the diodes are now conducting, they also shunt (kill) the audio. The solution is to remove the 48 Volts. Some years after the first 185 design, we ran into mixing boards (poorly designed IMHO), where the 48 Volt was on all the mics or none of the mics. We redesigned the protection to either include non polar capacitors or shunt resistors to ground as well as the diodes. The newer designs prevent the audio from being shunted. This is what you will find on all receivers after the 185 series. It is possible to change the output on your 185, but I recommend just removing the 48 Phantom power when necessary.