IMPROVING RADIO MIC PERFORMANCE: THE ANTENNA FACTOR
by Edwin J. Somers, CAS
Let's talk about ways to improve radio mic performance. Let's start by talking about antennas. You already know that high gain antennas increase range. Well, how much better do they work? Let's examine the facts. It would be difficult to put a high gain antenna on the transmitter in most cases, so we will look at receiving antennas. The whip antenna that comes with the receiver has a gain of 1 dBi (dBi stands for isotropic, and is used for the measurement of antenna gain). Ground plane and di-pole antennas have a gain of 3 dBi.
When you increase the received signal by 3 dBm, it is the same as doubling your transmitter power (dBm is used for the measurement of power). When you increase the received signal by 6 dBm, it is the same as quadrupling your transmitter power. Log Periodic antennas have 6 dBi or more gain.
Due to the losses involved in transmitting radio signals, you will get approximately a 20 to 25% increase in range when you double the power, or increase the antenna gain by 3 dBi. In physics, it is called the Inverse Square Law. You don't need to understand the laws of physics; you just want to know how the performance will improve. The use of gain antennas is a very inexpensive way to increase range. In the case of beam antennas (Yagi and Log Periodic), you will have the added advantage of a directional antenna that can be positioned to ignore identifiable interference.
There is one instance where you can increase the gain of a transmitting antenna, and that is when using a VHF plug-on transmitter first developed by Lectrosonics. We have found significant improvement in range by adding a counter-poise antenna to the H series transmitters. This is nothing more than a thin 17" stainless steel wire attached to a copper clad, one sided piece of circuit board material, strapped to the side of the transmitter (there is no direct connection, it is capacitively coupled).
Another very important rule is antenna polarization. If the transmitting antenna is vertical, then the receiving antenna should also be vertical. This is called vertical polarization. If the antenna alignment is off by 90°, the loss is infinite. You will only be receiving reflected signals. Obviously, antenna alignment will always be a compromise, so you should pay close attention to it. Receiving antenna height is almost always an advantage; the higher the better.
IMPROVING RADIO MIC PERFORMANCE: THE CABLE FACTOR
by Edwin J. Somers, CAS
As promised, the next few articles will discuss ways to improve radio mic performance. One factor and the focus of the first article, was on using a better antenna to increase range. Now we will be discussing how to get the radio signal from the antenna to the receiver.
The importance of using the proper transmission line cannot be stressed strongly enough. The term "transmission line" refers to any cable system used to transfer an electrical signal between pieces of equipment, a good example of which is an antenna cable. It is possible to use several different kinds of cable to deliver the signal from the antenna to the receiver; however, for radio mic applications, coaxial cable is the most effective. Coaxial refers to a design where there is a center conductor surrounded with an insulating spacer, which in turn, is surrounded with a shield. The main advantages of this design are convenience, and even more important, immunity from interference.
One of several issues with coax is characteristic impedance. For maximum transfer of energy, the impedance of the coax must match the antenna and the receiver. Almost all communications equipment is designed to be 50 W (Ohms) with the exception of television, which is either 300 W or 75 W. Video cable is not suitable because it is 75 W, and using it for radio mics is one of the more common mistakes.
What would be the effect of using the wrong impedance coaxial cable? Unfortunately, wherever there is a change in impedance, there will be a reflection of the signal at that point. Reflections cause loss of signal strength and ringing. A good example is with television; reflections show up as ghosts in the image.
Another issue is design loss. Different models of coaxial cable exhibit different degrees of signal loss, and it is frequency dependent. The higher the frequency, the greater the loss. With VHF wireless, coax was not a major issue; however, with UHF it is.
Let's look at some specific examples.
Manufacturers normally rate their cable by so many dBm of signal loss per unit length at specific frequencies. The most popular coax is RG-58. It is less than 1/4 inch in diameter, light, flexible, and inexpensive. Unfortunately, it is very lossy. A 100' length of RG-58 coax at 400 MHz will attenuate the signal 11.5 dBm. At 700 MHz, the loss for 100' is 17 dBm. You should realize that for every 3 dBm of loss, you cut your signal in half.
That's a rather large amount of loss, isn't it? Let's look at the types of coax suitable for our application.
Type RG-58 RG-8 Belden
9913F IW&C
9096
O.D. Inch .193 .405 .405 .405
Impedance 50 ohms 50 ohms 50 ohms 50 ohms
Attenuation
dB/100 ft.
@ 400 MHz
11.5 dBm 4.1 dBm 2.7 dBm 2.7 dBm
Attenuation
dB/100 ft.
@ 700 MHz 17 dBm 6.5 dBm 3.6 dBm 3.6 dBm
The final issue is signal loss per unit length. All coaxial cable attenuates radio signals - the longer the cable the greater the loss. Since the loss is linear, if you double the length, you've doubled the loss.
So, what can we deduce from all this glark? You should use the shortest cable you can get away with, and you should use the lowest loss cable you are willing to put up with. You will notice from the chart that the low loss coax is thicker and, of course, stiffer than the small stuff. The low loss coax is available with BNC, PL-259 and "N" connectors. Location Sound stocks International Wire & Cable 9096 coax, and we can make cables in any desired length.
IMPROVING RADIO MIC PERFORMANCE: THE RANGE FACTOR
by Edwin J. Somers, CAS
This is the third article in a series of ways to improve your radio mic performance. In previous issues, we discussed how using a high gain antenna with the proper polarization in conjunction with low loss transmission line (coaxial cable) will have a dramatic improvement on range. But what else can you do to maximize range?
The first thing you should think about is keeping a fresh battery in the transmitter. Manufacturers tell us that their transmitters have a 6 to 8 hour battery life, which is very true; however, in most systems, as the battery voltage drops, so does the transmitter power. The range will definitely drop off as the battery gets weak. When you want to maximize the range, change the battery after about an hour. Trying to stretch battery life is false economy, especially if you lose an important scene because of a weak battery.
Some manufacturers make high power transmitters. Typical power for VHF transmitters is 50 milliwatts. In fact, that is the maximum allowed by law. Typical power for UHF transmitters is 100 milliwatts. The maximum allowable power for UHF is 250 milliwatts. Some manufacturers do make 250-milliwatt UHF transmitters that will give you about a
4-dBm boost in signal.
UHF is slightly more efficient, so you will realize an increase in range just by switching away from VHF.
Switching to a diversity receiver will have an 'apparent' increase in range. I say apparent because the primary advantage of diversity receivers is to reduce drop outs; however, the net effect is an increase in usable range (which is the subject of another article).
I have been able to demonstrate a drop in signal (and range) when the transmitter is on one side of the performer and the receiver is on the other, at least with using UHF. This means that the radio signal must pass through the performer's body and is partially absorbed by it. If the transmitter is placed on the performer's back, then place the receiver behind them.
The final suggestion is to keep the transmitter gain adjusted properly. The mic gain adjustment controls the percentage of modulation of the FM radio signal. You want it as high as possible. By law, all transmitters have a limiter circuit to prevent over-deviation and co-channel interference. If you set the mic gain so high that it triggers the limiter, it will sound terrible. It is not designed to sound good. The trick is to adjust the gain so that it just triggers the limiter, and then back off a bit. Under normal conditions, you will not have to adjust it very often, only when you encounter a very loud or very soft-spoken performer. The important thing is to think about it each time you use the transmitter. When you maximize the transmitter gain, you get the best signal-to-noise ratio, which translates into greater range.
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