by Jim Tanenbaum, CAS
NOTE: At the bottom of this online article, you will find the Appendix that is referenced here and in the print edition of the 695 Quarterly.
If you haven’t read David Waelder’s excellent articles in the last two issues of this magazine, please do so at once. In my 44 years of mixing, I’ve watched radio mikes evolve from almost unusable to amazingly reliable, but they still require a knowledgeable sound person to perform properly. David’s information is exactly what is needed, and I would like to add a few more points. Also, the other end of the system (transmitter and mike) needs some explaining too.
The directional characteristics of log-periodic (sometimes erroneously called “Yagi”) antennas are different in the vertical and horizontal planes. (Log-periodic antennas are wideband; Yagis are fixed frequency – see Sections 3.1 and 3.2 in the online Appendix.) They are more directional in the plane of the elements, thus, when the antenna is mounted with the elements vertical (as it usually is), the gain falls off more rapidly at about 30 degrees to 45 degrees above and below the horizontal. This is desirable because the actors are not often located high above the ground. The horizontal pattern is much broader, sometimes down only 5-6 dB at ± 90 degrees. As a result, it is not necessary to “track” the actors with the antenna if they move slightly, as I have seen some people do. (Note that TV antennas are oriented horizontally, because of the need to precisely aim them at the TV station’s transmitting antenna, and to reduce reflected signals from other directions – “ghosting”, although that is no longer so much of a problem with digital TV.)
If you have an interfering signal, you can swing the receiver’s antenna and try to null it out. Chances are, the actor will still be within the front lobe of the antenna’s pattern. If not, you can relocate the antenna to get the actor ‘in front’ of it while keeping the interference in the lowest gain direction. This works better than reorienting the antenna horizontally because the null is no deeper, and now the actor may have to be tracked. Important: the greatest null direction is not directly to the sides or rear of the antenna’s the pattern is more like a hyper-cardioid or short shotgun mike’s, at about 135 degrees rearward to the left and right. When you have some free time, set up a transmitter in a fixed position and then rotate the receiver antenna while watching the receiver’s signal strength meter. This will give you a feeling for your particular antenna’s pattern. Be sure to do this outside in an open area, so reflections won’t confuse the results. And, if you have more time, move the transmitter to another location and repeat the procedure. Check for the front acceptance angle as well as the location of the rearward nulls on both sides.
Circularly-polarized antennas are indeed good at receiving signals that have had their polarization angle changed by reflection(s), but there is a low-cost alternative. If you are using two 1/4-wave whip antennas, simply orient one 45 degrees to the left and the other 45 degrees to the right, instead of both vertically. Right-angle BNC or SMA adapters are the easiest way to do this if the antennas do not have right-angle connectors themselves. For a pair of sharkfins, modify their mounting brackets to angle their upper edges outward by the same amount. This puts the antennas at a right angle to each other, so at least one will pick up the signal strongly no matter what its polarization angle.
Regardless of what type of antenna you use, keep the cable connecting it to the receiver as short as possible because most coaxial cable has a greater loss than sending the radio signal an equal distance through the air. See Section 4.2 in the Appendix.
As to transmitters, there are a number of things you can do to improve the signal that arrives at the receiver antenna:
1. Most intervening objects block the direct signal path, and, since UHF waves are small (about one foot), it doesn’t take a very large object. This includes people, especially the actor wearing the mike. If the actor will be facing you throughout the scene (i.e. facing the receiver antenna on your sound cart), mount the transmitter or at least the antenna (see 3. below) on the front of the actor’s body.
2. Another improvement comes from spacing the bodypack’s antenna as far from the actor’s body as possible. In addition to mounting the transmitter under the outer layer of wardrobe if possible, slipping a length of rubber or plastic tubing over its antenna will increase the radiated power considerably. Automotive supply stores sell tubing for windshield washer fluid that is the correct size: about 1/4-inch O.D. x 1/8-inch I.D.
3. As David mentioned, raising the receiver’s antenna helps. This is also true of the transmitter’s antenna. If you have to mount the transmitter on the actor’s ankle, use an extension to get the antenna higher on the body.
A simple extension antenna can be made from a length of miniature coaxial cable: RG-174 type, with a braided shield and a stranded center conductor.
Start by stripping off several inches of the outer jacket at the end of the coax, being careful not to cut or even nick any of the shield braid wire strands. The length removed should be about an inch and a half more than the length of the whip antenna for the frequency block you are using. Don’t include the length of the connector’s metal shell. (Or you can use the antenna-length Table in the online Appendix. Pick the center frequency of your block.)
Next, carefully push the cut end of the braided shield back to expand it, and continue pushing the shield until it inverts over the remaining outer jacket. Smooth the inverted shield braid out – it should now be the correct length (or slightly longer, in which case trim it back). Cut the now-exposed insulated inner conductor to the correct length, then cover the shield braid and inner conductor with a length of shrink tubing.
After you have successfully completed these steps, cut the coax to a length of five to six feet (to reach from an ankle-mounted transmitter to the shoulder-mounted antenna), and attach the appropriate transmitter-antenna connector to the other end.
4. It also helps to raise the boom operator’s transmitter antenna if using a wireless link. Butt plugs are one solution. If a bodypack transmitter is being used, the extension antenna described above can be mounted on the boom operator’s headphones. I use this method and often get a solid 1,000-foot range. (Zaxcom makes a filtered remote antenna for specific blocks, which also helps to reduce interference with receivers used in a bag.) It is also possible to mount the transmitter as well as its whip antenna to the headphones, although this adds more weight and bulk.
5. One more caution: recently, large (12′ x 12′) metalized cloth scrims (silver or gold) have come into widespread use. Although coated with metal, they absorb radio signals rather than reflect them. Not only will they completely block the signal from an actor behind them, but actors standing in front of one (with transmitters mounted on their backs) will have almost all of the radiated signals absorbed, with resultant R.F. dropouts. This caused me no end of trouble until I figured things out. (For the technically inclined, the characteristic impedance of the metalized fabric is about 50 ohms – see Section 4.3 in the Appendix.)
Once the transmission and reception of the radio signal has been optimized, there are also techniques to improve the quality of the audio:
1. Mike mounting position: Basically there are two choices: torso or head.
Torso: Usually, the lavalier is mounted on the chest, located over the sternum (breastbone). This position is a good compromise – any lower and there is too much ambient sound; any higher and the upper voice frequencies are reduced by the “chin shadow,” and there is also an excessive drop in level if the head is turned to the side.
Head: Extra-small lavs like the Countryman B-6 can be hidden in the hair above the forehead. This keeps them “on mike” regardless of any head turns. If the actor wears glasses, concealing the tiny mike at the hinge point is another possibility. If a baseball cap is part of the actor’s wardrobe, the mike can be mounted under the visor. A plastic hard hat is even better because the transmitter can be secured inside the hat, just above the suspension. With both hats, the mike can be concealed under a sheet of felt (see 5. next page) that is glued under the visor or brim. If the bump from the mike is visible (be sure to remove any EQ sleeves from the B-6), use two layers of felt, with the inner layer cut out to accommodate the mike and cable.
2. Cable strain relief: A taut cable can pull on the lav and cause it to rub against the clothing. Even if it doesn’t, mechanical noise introduced anywhere along the stretched cable will travel to the mike where it will be heard. A full 360-degree loop in the cable, secured with strips of tape both below and above it will break this transmission path. Sennheiser makes a line of lavaliers, such as the MKE-2, that use stainless steel wire instead of copper in the cable. While this construction is extremely rugged and reliable, the stiff steel conductors can carry mechanical noise down the entire length of the cable. Even two loops sometimes does not prevent it from reaching the mike. Using these mikes on studio news anchors usually presents no problems, since they speak up and are relatively motionless. Actors in a dramatic scene, with lowered voices and extensive body motion, often cannot be recorded successfully with these lavs.
3. Mounting lavs directly on the actor’s skin: Individually-packed alcohol swabs are useful in removing skin oils before taping down the mike. There are three types of medical tape available that work well for different situations. The one I use most often is “3M Micropore,” a plastic tape perforated with many tiny holes. These serve to allow perspiration to escape rather than lift the tape by hydraulic pressure. They also make the tape easy to tear cleanly. While all three types are hypoallergenic, for actors who express a concern about their “sensitive skin,” a version of tape made from paper with a less aggressive adhesive may be used, but will require a greater area of contact to remain in place. It is porous but not as much as Micropore. For applications involving abrupt and vigorous body motions, or where the transmitter must be taped to the body, there is a cloth tape that has a much greater tensile strength and a much stronger adhesive. (Avoid body hair if at all possible with this tape.)
Most men have a depression in the center of their chest that is a good spot for the lav. For women, between the breasts (unless they’re pushed together) is ideal, possibly attaching the lav to the center of the bra. If the clothing rubs against the mike, there are two choices: double-sided tape between the cloth and the skin, or attaching one or more “bumpers” to the skin near the mike to keep the cloth away from it. A piece of makeup foam works well for this purpose. Trim the foam to a smoothly-rounded contour on the side where the fabric will contact it and use “TopStick” double-sided adhesive toupee tape on the flat side to attach the bumper to the skin.
4. Chest hair: Some men have a thick mat of chest hair with the consistency of steel wool that rubs on the back of the lav. (Robert Urich was extremely cooperative and shaved a patch of his pelt down to the bare skin every day for me, but you are unlikely to encounter such generosity.) The best solution is to have the actor wear a cotton T-shirt or tank top, but if that is not possible, tape a 6″ square of felt (see 5. below) to the body hair behind the mike, using the paper tape mentioned (see 3. previous page.) You will need lots of tape and use the alcohol swabs liberally. If the actor won”t go along with this, taping two or more layers of felt to the wardrobe so that they cover the back of the mike will help to a certain degree.
5. Windscreens: Foam windscreen material is not very effective when used in thin layers next to the mike. The mesh “ball” windscreen provided with some lavaliers (e.g. Sanken COS-11) is better, but is too large to hide under most wardrobe. I have found that a layer of wool felt provides considerable protection without attenuating high frequencies excessively. Important: you must use 100% wool felt; wool-polyester blends or 100% polyester felt is very noisy. (See Illustration on page 22.)
For most installations, cut the felt into strips about 3/8″x 1″ for B-6s and 5/8″ x 1-1/2″ for Sankens and Trams/Sonotrims.
Next, cover exactly one-half of the strip with a piece of TopStick double-sided tape, notched to clear the business end of the lav. Place the mike on top of the tape, with its end just shy of the middle of the felt strip and the cable running down the center of the strip.
Finally, fold the strip over the mike and press the edges together along its length. This will space the fold in the felt slightly away from the end of the mike to improve the windscreen’s performance.
Buy as many different colors of felt as you can – this will help in concealing the mike, especially when a leather jacket (or other sound muffling material) is involved. If you can match the color of the jacket’s lining, it is often possible to position the mike very near the opening. The various shades of felt are also useful for windscreening and/or concealing planted mikes.
Tram, Sonotrim, and other flat lavs that mount with “vampire clips,” have a grill on one side that can be mounted facing the clip so the solid back of the mike faces forward and helps block the wind. The gap between the grill and the clip can be filled with a thin sheet of foam windscreen material, or felt for even more protection.
6. Clothing noises: If you have any input in preproduction as to wardrobe materials, natural fibers such as cotton, linen, wool, and even silk, are preferable to synthetics like polyester. These plastic fibers are much more rigid and will carry sound through the fabric much more readily. Unfortunately, wardrobe people like synthetics because they are wrinkle-resistant and easier to clean. If you encounter this problem on the set, isolating the lav with a piece of makeup foam will help. Latex works best but has recently been replaced by a synthetic to avoid allergic reactions. There are also commercially-produced cylindrical mike sleeves available in black or white foam.
TopStick works well to tack rubbing layers of clothing together. A supply of various sizes of safety pins is also useful. Neckties have multiple layers that can rub together and be picked up by a lav mounted underneath. To complicate matters, the backs of most ties are sewn shut, so you cannot get inside to tape the layers together. You can use a safety pin to immobilize all but the front layer, and sometimes the tie’s pattern will allow you to snag the front layer as well. There is a “silk” safety pin available from dressmakers’ supply stores that is very small and has a flat-black coating, which is ideal for this purpose. (White, pink, and other painted colors are also available for use with sheer wardrobe.)
For completely intractable clothing noise, it is sometimes possible to stick a B-6 out through a button hole and support it on its cable, half an inch away from the fabric. This technique works especially well if you have B-6s in all the available colors. You can also use colored markers on a white mike to match various colors. “Dry-erase” markers are the easiest to remove, but be careful that the color does not rub off before the shot is over.
Two often-neglected sources of noise are flapping zipper tags and the circular springs inside the female part of snaps that rattle when the snap is unfastened. These can be amazingly loud when the lav is nearby. A small piece of double-sided tape will secure the zipper tag to the body of the zipper, and another piece can be wadded up and stuck inside the snap opening. Warning: be sure to remove all the tape from wardrobe items when the shot is over.
7. “Soundproof” wardrobe: Zipped-up leather jackets (when under the collar is not an option) and down-filled parkas are two of the most difficult items to deal with. It is sometimes possible to locate the lav behind the zipper, so the sound can reach it through the gaps in the zipper teeth. If the teeth rub against each other audibly, asmall amount of Krazy Glue applied to the teeth immediately in front of the mike will stop that. Another possibility, if the wardrobe person will permit it, is to cut a short section of the stitching that fastens the zipper to the jacket and bring a B-6 out through the gap, leaving the end of the mike flush with the edge of the leather bordering the zipper. Down-filled parkas (or other insulation) are almost impossible to mike successfully, especially nylon ones. The audible noise made by the sleeves rubbing against the torso is so loud that even using a boom mike it is often impossible to get an acceptable track. The muffling effect of the insulation adds to the problem because any part of it that gets between the mike and the actor’s mouth will absorb most of the high-and-upper-midrange frequencies. The only saving grace is that most scenes involving such heavily-insulated clothes usually have the actor also wearing some kind of headgear, with the possibility of hiding the mike there.
1. If metal objects in the vicinity of the transmitter antenna happen to rub against each other, they can produce static in the audio signal. This occurs because they act like antennas and pick up some of the RF energy from the transmitter. This produces microscopic sparks between them where they touch, and this in turn produces a static radio signal over a wide range of frequencies, including the audio band. This signal can enter the transmitter’s audio circuits where it will be combined with the audio from the mike. Lavs and transmitters with plastic cases are particularly susceptible to this problem. Either separate the offending objects or insulate them where the meet with a piece of tape. (You could also solder or clamp them firmly together.) Some car seats have internal metal springs that rub together. Moving the transmitter from the actor’s back to the front of the body usually solves the problem. A bag transmitter can cause this problem too, unless its antenna is located far away from the other items in the bag, such as on your headphones.
2. Modern automobiles and trucks are equipped with special resistive spark plug wire to suppress ignition interference. But many hot rodders replace it with solid copper ignition wire to improve performance, and this causes the vehicle to radiate a considerable amount of radio interference. Unfortunately, I have encountered this on some camera cars. Motorcycles with magneto ignition systems also produce this type of interference, unless they’re upscale models with a built-in radio. Auto stores sell plug-in suppressor resistors that you can temporarily install between the spark plugs and the cables that attach to them. (Unfortunately, some recent vehicles have the spark plugs hidden under plastic shrouds, or worse, buried under miles of smog control or other plumbing.)
3. A single AC– or battery-power supply can transfer interference between multiple units connected to it unless the individual outputs are isolated with EMI filters. Most commercial power distribution systems incorporate filters but not all. The audio input cable to a transmitter used for a camera hop can carry RF energy down its length to whatever is feeding it. (So can Comtek transmitters.) A cylindrical ferrite RF choke snapped over the cable will block most of this, and should be located as close to the transmitter end as possible. Keep it in place with a nylon cable tie, and cover it with shrink tubing.
4. Be sure that the mounting hardware for all transmitter mike input connectors is tightened securely. A loose collet nut on the mike plug can also cause problems. Broken shield wires anywhere along the cable are another point of entry for interference. Periodically check your lavs by listening as you wiggle the cables down their entire length, from mike to plug, while they are connected to the transmitter.
5. Interference from other transmitters (taxicabs, local paging systems, walkie-talkies, etc.) can cause several types of problems. Audible noise, either whistles or the actual program material, affects analog radio mikes. Muting (audio dropouts) occurs in digital systems, both hybrid (Lectrosonics) and full digital (Zaxcom). Both analog and digital systems can suffer R.F. dropouts if the interfering signal is powerful enough to swamp your receiver’s front end, and analog radio mikes can also have distortion introduced in their audio if they don’t lose your transmitter’s signal entirely. I have found it very useful to carry a small handheld analog scanner receiver to help identify the source of the interference when using
In closing, let me tell you a secret: radio mikes work partially by magic, and I have found that a few drops of goat blood applied to the receiver antennas at midnight under a full moon improves their performance by at least 20%. The color, sex, and age of the goat don’t seem to matter, but the animal must be alive when you obtain the blood.
Text and photos © 2011 Jim Tanenbaum, all rights reserved.
BASIC RADIO ANTENNA TECHNOLOGY
1.0 Radio waves are a form of electro-magnetic energy, like light or gamma radiation. They consist of rapidly varying transverse electric and magnetic fields, oriented at right angles to each other. In a vacuum, they travel at the speed of light, about 300,000 Km/sec or 186,000 miles/second, denoted by the letter “c”. In air or other substances, they travel slightly slower. The length of a radio wave is given by λ= c/f, where “λ” (the Greek letter lambda) is the wavelength, and “f” is the frequency. Frequency is measured in Hertz (cycles/sec) and multiples of 1,000: Kilohertz (KHz), Megahertz (MHz), Gigahertz (GHz), etc.
For example, a 300 MHz signal has a wavelength of 300,000 Km/sec / 300 MHz, or 300,000,000/300,000,000. The dimensions are m/sec / cycles/sec, or m/cycle, so λ = 1 meter, or about 40 inches.
1.1 In addition to frequency/wavelength, radio waves have another parameter known as “polarization”. This refers to the orientation of the axis of the electric field to some reference such as the surface of the earth. A vertical whip transmitter antenna produces radio waves with a vertical polarization, and this signal will be most effectively received by another vertically-oriented antenna. However, the polarization of a signal can be changed by reflection. Refection off a horizontal surface can rotate the polarization axis up to 90 degrees from the vertical. Reflection off a vertical surface can also rotate the polarization axis, depending on the angles involved.
It is also possible to generate a circularly-polarized radio wave, whose polarization axis rotates (either CW or CCW) as the wave travels. Because the required antenna is large, it cannot be used with concealed bodypack transmitters, but it is sometimes used with receivers – see Section 3.3 below.
1.2 The “Inverse-Square Law”, describes energy that falls off as the inverse square of the distance or E = 1/d2. (E.g. 1/4 the power at twice the distance; 1/9 the power at three times the distance, 1/16 the power at four times the distance, and so on.) Theoretically, this law applies to a radio signal transmitted by any antenna, whether omni- or highly-directional, but in the real world, the ground and other nearby objects (including people) can absorb some of the signal and reduce its level even faster. At best, the Inverse-Square Law can be used for an estimate of the minimumloss.
The signals transmitted by both omni- and directional antennas obey the 1/d2 rule, but the signal level will be higher at any given point from the directional transmitter antenna.
1.3 The power level of the transmitter also has an effect on range, but not as great as some mixers believe. To double the range (using the I-S Law) requires four times the power, or going from 50 mW to 200 mW, with the resultant reduction in battery life. (Doubling the power is only a 3 dB increase, because of the nature of decibel arithmetic.) Moving the receiving antenna closer is preferable, if you can move the receiver along with it to avoid a long run of antenna cable – see Section 4.2 below.
2.0 Radio transmitters and receivers couple to radio waves by means of antennas. There are two types of antennas: electric and magnetic.
2.1 The simplest electric antennas are constructed of a straight length of wire (called a whip) connected at one end to the transmitter or receiver and free on the other end. The length is ¼ of the wavelength of the desired frequency, usually written asλ/4. If you look at a single cycle of a sinewave, you will see that it starts at zero, reaches its maximum positive value at λ/4, returns to zero at 2λ/4, reaches its maximum negative value at 3λ/4, and finally ends back at zero at λ. Thus a λ/4 whip antenna will produce the maximum voltage at its end, and have the largest current induced in it. A shorter or longer antenna will produce less voltage and current at the same frequency because it intercepts the wave at a point before or after the maximum. A vertical a λ/4 antenna is omni-directional in the horizontal plane, with a null at the top and reduced output below the horizontal. If space is limited, the full length of wire can be coiled up in a helical configuration to shorten the overall length (the typical “rubber ducky”). It will appear to be the same length (and frequency) to the radio signal, but the output will be somewhat less that an equivalent straight antenna because its smaller size doesn’t capture as much of the signal’s energy.
The performance of a simple λ/4 whip can be improved by the addition of a secondλ/4 whip underneath, pointing downward. This configuration is known as a “dipole” (and now is a half-wave antenna, or λ/2), and has an omni-directional pattern in the horizontal plane and a figure-8 pattern in the vertical plane, with the nulls at the top and bottom. (This dipole element is at the heart of many advanced designs, to be discussed below.)
If a λ/4 whip antenna is mounted on top of a metallic surface (called a “ground plane”) whose dimensions are at least as large as the antenna’s, it will appear to be “reflected”, and act similar to a physical dipole. It is also possible to have several ground wires sticking out radially from the base of the whip instead of a solid sheet of metal for a ground plane.
2.2 The following table gives the length for λ/4 antenna elements. Look in the row designated by the first digit of the desired frequency, and then in the column headed by the remaining digits. (E.g. for the cell in the third column of the fifth row, 7.0 inches, the frequency is 400 + 20, or 420 MHz.) For exact lower frequencies between the table’s 10 MHz intervals (e.g. 14 MHz), or frequencies outside the table’s 10 – 990 MHz range, divide 2,946 inches by the desired frequency in MHz. (E.g. for 1,000 MHz: 2,946/1,000 = 2.9 inches.)
λ/4-ANTENNA LENGTH IN INCHES
2.3 For low-frequency applications (<50 MHz), magnetic antennas are preferred because they are much smaller than a λ/4 electric antenna, which would be 60 inches or longer at those frequencies. Magnetic antennas are coils of wire, wound on a plastic or ferrite core. They are directional with a 3-dimensional figure-8 pattern, whose nulls are aligned with the winding axis. These antennas are most commonly encountered in portable A.M. radios (about 0.5 – 1.5 MHz), and will not be discussed further here.
3.0 More complicated (electric) antenna designs are said to have “gain”, but this is not the same as the gain produced by an amplifier. Instead, it refers to comparing their performance to a simple λ/2 dipole antenna. Two of the most common types are Yagi and Log-periodic. These two antenna types are different in both construction and functioning.
3.1 Yagis are tuned to a specific frequency and have very little performance above or below it. This type of antenna has a “driven element”, a λ/2 dipole with one or both arms insulated from the supporting strut (unbalanced or balanced circuit) and at right angles to it. Behind them is a “reflector” element: two arms of slightly greater (about 5-10%) length that are both grounded to the strut. In front of the driven element are “directors”: pairs of slightly shorter (about 5-10%) arms also grounded to the strut. All the directors are the same (shorter) length, and the more of them there are and the longer the strut, the more directional and higher gain the antenna is. The spacing of the reflector, driven element, and directors is constant, but can vary greatly, from λ/10 to λ/2 in different designs. Gain ranges from 4 dB for shorter antennas to 12 dB for ones 10 λ long.
Here is a very basic (and over-simplified) explanation of how a Yagi antenna functions. The reflectors are longer (lower frequency) than the driven elements, thus the incoming radio wave is too small to go around them, and is reflected back toward the driven element. Technically speaking, the reflector reacts “inductively”, causing the phase of the current induced in it by the radio wave to lag behind that of the voltage. The directors are shorter (higher frequency), so the radio waves coming in at a small angle are “snagged” as they pass around them, and are bent toward the driven elements. Waves coming in at progressively greater angles are deflected away. Waves arriving directly on axis are not affected. The directors react “capacitively”, causing the phase of the induced current to lead that of the voltage. Like the microphones mentioned in the article, there is some sensitivity to waves arriving from the rear.
The use of Yagis in production is limited to fixed-frequency applications, such a remote feed link, and for this application the elements are usually oriented horizontally.
3.2 Log-Periodic antennas are designed to operate over a range of frequencies, 2:1 or even 3:1. There are a number of pairs of elements, of differing lengths, arranged on the supporting strut with the shortest element at the front and the longest at the rear. Unlike the Yagi, the spacing between the elements decreases toward the front. This type of antenna does not have particular elements with assigned functions. Instead, all the elements are insulated from the strut and connected together, with alternating pairs connected out of phase. It is easy to see this on a “shark fin” printed circuit model. On one side, the upper elements of pairs 1, 3, 5… are connected together, and also connected to the lower elements of pairs 2, 4, 6… The other side has the upper elements of pairs 2, 4, 6… connected together and also connected to the lower elements of pairs 1, 3, 5… This effectively “shorts out” all the antenna elements, so every pair of elements can act like a reflector or director as in the Yagi configuration.
With one exception: the pair of elements whose length most closely matches the desired frequency – they will now become the driven element. In their case, the RF signal they intercept will be so much larger than the out-of-phase signals from all the other pairs that it overwhelms them. Now all the longer elements behind the driven element act like reflectors, and the shorter ones in front act like directors. As in the Yagi, the more directors there are, the greater the gain and directivity, and this occurs at the lower frequency end of the range. The total number of element pairs in the antenna design has a significant effect on the gain for another reason: with more pairs of elements, the length of the pair acting as driven elements will more closely match the wavelength of the desired frequency, and have a voltage closer to the maximum.
The compromise in making the Log-Periodic antenna frequency agile is that it has less gain than a Yagi of equal spar length, typically 3-6 dB for a frequency range of 2:1. Also, the gain is not constant over the frequency range. You should check your particular antenna’s performance (as mounted on your cart) at different frequencies to find if there are any “dead spots” (gain of only 1-2 dB, or sometimes even less).
3.3 “Circularly polarized” and “helical” designs are currently the most expensive types, and offer the ability to handle radio signal with polarizations of any angle. However, simply angling the two conventional antennas of a diversity pair outward about 45 degrees each will accomplish much the same effect at no additional cost.
4.0 “Impedance” is a characteristic of electrical components and/or circuits that contain capacitance and/or inductance in addition to resistance.
4.1 Radio mike antenna systems are designed to have an impedance of 50 ohms (Ω, the Greek letter omega), and use 50 Ω coaxial cables, most commonly type RG-58A/U.
Video systems are designed as 75 Ω systems and use 75 Ω type RG-59 A/U cable. If video cables are used to extend radio mike antennas, there will be a reflection of some of the signal away from the mismatch point with the resultant small loss of power. (Standard RG-59 cable can be recognized because it is slightly larger in diameter than RG-58.) There may also be a mechanical interference between 50 Ω and 75 Ω male and female connectors that can damage one or both of them if they are interconnected.
4.2 In addition to impedance, coax has a characteristic signal loss of so many dB per foot, and this may sometimes exceed the loss caused by transmission of the radio signal through the air. For any given cable type, the loss increases with frequency. For standard RG-58 50 Ω cable types, this loss at 400 MHz ranges from 8 to 12 dB per 100 feet. At 700 MHz, the loss increases to 12 to 15 dB. If you must have a long coax run, use RG-8 low-loss 50 Ω cable. It is much larger in diameter, but has only about 2.5 dB loss per 100 feet at 400 MHz and 3.5 dB at 700MHz.
Instead of extending the antenna cables, moving the receiver with its directly-connected antennas closer to the transmitter and sending the audio back on XLR cable will avoid all the cable losses. The only drawback is that you do not have immediate access to the receiver’s meters and controls.
4.3 Most objects in the real world tend to be either insulators (very high impedance) or conductors (near zero impedance). Insulators will generally allow radio signals to pass through without too much attenuation. Conductors will block radio waves by reflecting them away, but again without too much loss, only a change in direction and sometimes polarization angle. The problem is with certain substances that have an impedance near 50 Ω as they will absorb a large amount of the radio signal. The human body, some vegetation, and the metalized scrims mentioned in the article are prime examples.