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Passive rebroadcasting for TV signals Do you have a problem with weak TV reception & no pos­sibility of “line of sight” to the TV transmitter. If so, then this article on delivering a TV signal over a distance of up to 1km will be of interest. By MIKE PINFOLD A letter featured on page 93 of the August 1993 issue prompted me to put pen to paper. It referred to the possibility of passively re-broadcasting TV signals picked up in a high signal strength area by beaming them down into a low signal area. This was to be done by coupling two television antennas back to back with a length of low loss coaxial cable. At first thought, the idea seems a good one but with simple propagation theory and antenna maths it can be demonstrated that it has only limited potential. The letter also mentioned the use of a masthead amplifier and the matching of a long feedline from the high signal area down some distance to the TV set. But first, let’s address the problem of passive re-transmission of TV signals. There are a number of mathematical formulas that enable one to calculate field strength at a distance from a transmitter. The first of these is used to calculate the power density “P” at a point “r” metres from an isotropic radiator: P = Pt/4πr2 where P = received power in watts/square metre; Pt = transmitted power in watts; and r = distance in metres between transmitter and the reference point. This formula shows the in14  Silicon Chip This photo shows the author’s original open wire feeder system in use with a vertically polarised VHF antenna in a remote part of New Zealand. verse-square nature of radio waves. The energy level reduces in proportion to the square of the distance. Note that the frequency of the signal does not enter this equation. The electric field intensity “E” of a radio signal “r” metres from a point of “P” watts is given by the equation: E = √(30Pt)/r where E = the intensity in volts per metre; Pt = the transmit power in watts; and r = the distance in metres. The power density of a signal and the electric field in­tensity are related by the equation: Pr = E2/120π where Pr = received power density in watts per metre squared; E = intensity in volts per metre; and 120π = the resistance of free space. The above formulas are for theoretical signal strengths between isotropic sources in free space. However, there are other external influences that may times 20dB is 100 times. Thus, the formula to include gain arrays is: HIGH SIGNAL AREA Pr = 1.64Pt/4πr2 ANY POLARISATION Antennas have a performance factor known as “antenna aper­ ture” and it determines how 60dB PATH LOSS much of that potential signal the PASSIVE antenna extracts from free space. RE-BROADCAST LOW SIGNAL AREA The larger the receiving area, SYSTEM the more power is intercepted. Aperture is determined by the following equation: A = λ/4π For a gain array with a gain of “G” times over isotropic, the equation is: Fig.1: this diagram shows the general concept of pas­sive rebroadcasting as outlined in the article. The hilltop antenna picks up a strong signal which is re-radiated A = Gλ/4π downhill by another antenna to the receiving antenna at the bottom of where the hill. λ = wavelength of the signal in metres; G = gain of the antenna (not in VERTICAL POLARISATION +30dB dB format) over isotropic; and +30dB A = Aperture, as a decimal HORIZONTAL POLARISATION fraction. Remember that an isotropic antenna has a gain of unity and 60dB PATH AMPLIFIED LOSS RE-BROADCAST a dipole has a gain 1.64 times LOW SIGNAL AREA SYSTEM more. Thus, the amount of power available is derived by: ISOLATION: GEOGRAPHICAL Pa = PiA AND POLARISATION where Pi = power density in watts/ metre2; Pa = power available; and Fig.2: this is a variant of the passive rebroadcasting system with better isolation A = Antenna aperture as a decbetween antennas and a masthead amplifier interposed between the hilltop imal fraction antennas. By combining the above equations, one arrives at an equa­tion have a bearing on the outcome and of a halfwave dipole, broadside to the that can determine the received power signal strengths can be assumed to dipole, is: in an antenna of known gain: be slightly less than those calculated. Pr = 1.64Pt/4πr Pr = Pt.Gt.Gr.λ2/(4πr)2 An isotropic radiator is not exactly a or The situation of passive rebroad­ useful concept in the real world, alE = √(49.2 x Pt)/r casting (receiving signals on one though it is a base on which to place where antenna and feeding them down to firm theory. Pr = power density in w/m2; another for rebroadcast) can be shown There are different correction factors Pt = power transmitted in watts; to be something of a hopeless case and that are added to the equations to take r = distance in metres; and will only work if the received signal account of antenna performance and E = field intensity in volts/metre. strength is exceptionally strong and other configurations. For a half­wave The above equations give the pow- the rebroadcast distance is relatively dipole oriented for maximum radia­ er density at a point “r” metres from short; ie, a couple of hundred metres. tion, there is a correction factor of 1.64. the source. If you have a transmitting By using the above formulas, the This factor when converted to dB gives antenna with a gain of 10dBd (over received power level at the hilltop the apparent gain difference between a dipole), then this factor must be receiving site can be approximated if anten­nas referenced to a dipole and incorpo­rated into the equation. 10dB you know several important factors: those to the isotropic source. When is a power increase of 10 times, so the the radiated power of the transmitter, looking at manufacturers’ antenna gain input power in watts must be multi- the frequency of the signal, the gain figures, check to see if they are refer- plied by the apparent increase over the of your receive antenna, and the disenced to isotropic (dBi) or to a dipole original antenna (with its compensa- tance between the transmitter and the (dBd). Those referenced to isotropic tion factor present if required). receiving anten­na. appear to have 2.15dB more gain but This multiplication factor is its Let us put a few figures into the their real gains are the same. power gain not in dB form but linear equation and see how our theoretical The formula for the field intensity form; eg, 6dB is 4 times, 10dB is 10 system is going to perform. To get a VERTICAL POLARISATION May 1994  15 antenna is 6.16nW. Let’s assume the transmit antenna has a gain of 10dBi and that the receive antenna has a gain of 10dBi. The power received by the home TV antenna is found by uti­lising the same equation and a transmit power of 6.16nW (the original received signal). This results in a signal level of 5µV/m, a totally useless signal for any TV. 75mm 600mm BLACK POLYTHENE SPREADER Calculating the path loss 8-12 GAUGE FEEDLINE SPACER TAPERED MATCHING SECTION APPROX. 2m LONG 10mm SUPPORT ROPE CHOCOLATE BOX CONNECTORS PVC STRAINER BLOCK 300  RI BB ON Fig.3: this is the author’s open line feeder system which gives very low signal loss over a long path. The open line is matched to 300Ω ribbon with a 2-metre long tapering section and terminat­ed as shown. good quality signal at the TV set, you need a minimum of 250µV (assuming a modern sensitive TV set). For the purposes of this exercise, we’ll assume the following: • 100 watts of transmitter radiated power (Pt x Gt); • UHF channel 42; 640MHz approximately; wavelength = 0.468 me­tres; • Distance from transmitter to receiver = 30km; • Antenna receive gain = 6dBi (4 times relative to isotropic antenna). Pr = Pt.Gt.Gr.λ2/(4πr)2 = 1000 x 4(0.468)2/(4 x π x 30,000)2 16  Silicon Chip = 876.096/(1.42 x 1011) = 6.16 nanowatts This is a respectable received signal strength and can be converted into volts/metre by the following formula: E = √(Pr.R) where R is the impedance of the antenna in ohms. The result is a signal of 679µV into a 75Ω antenna, a good signal indeed. In our setup, the receive antenna is connected via a short length of low loss coax to the “transmit” antenna as shown in the diagram of Fig.1. The power delivered to the transmit In order to produce the required signal level at the home TV, a level of amplification equal to the path loss between the rebroadcasting antenna and home receive antenna has to be insert­ ed at the re-transmitting site. This is easily calculated. It is the difference between the re-transmitted power level of 6.16nW and the home television received signal: 6.16 x 10-9 - 3.4 x 10-15 = 60dB This is about 60dB of path loss and therefore the gain required is 60dB. This amplification should be provided between the hilltop receiving antenna and the rebroadcasting antenna. While it is relatively easy to provide 60dB of gain into the rebroad­ casting system connecting coax, one must maintain adequate RF isolation between the receive and transmit antennas. This is to prevent feedback and thus stop the system becoming an RF oscillator at the frequency of maximum feedback. The two antennas must not “see” each other. One antenna could be placed on one side of the hill and the other placed on the other side, “hidden” from view of its mate. Even more isolation can be obtained by having one antenna with vertical and the other with horizontal polarisation. This can amount to as much as 20dB. One also has to make sure that the coax between them is well and truly decoupled to prevent RF coupling along the outside of the coax. The other factor that helps in antenna isolation is the front-to-back ratio. An antenna with a very good front-to-back ratio will have little problem ignoring signals coming to it from behind, again improving antenna isolation. This setup is shown in Fig.2. The foregoing should give the reader an insight into anten­na concepts and propagation. While it is possible for passive re­broadcast systems to work, the received signals must be very CAPACITOR 75  75  300  300  GND GND CAPACITOR HERE OR HERE C 300  OR C 75  Fig.4: this diagram shows the modifications needed to a standard 4:1 balun to enable DC to be sent up the ribbon to the masthead amplifier. Two such baluns will be required. strong, the antenna gains high and the rebroadcast distances short. Masthead antenna The other comment in the letter relates to using a masthead amplifier to drive a long feedline to the home TV below a hill in a weak signal area. My experience is that this setup can work extremely well, contrary to the comments from the magazine. I once built such a system for my parents who lived in the country and whose TV reception left a lot to be desired. It used a standard TV antenna on a hilltop and a homebuilt masthead preamplifier (BFY90) with a gain of about 15dB. This preamplifier fed signals down about 1km of balanced open wire feeder. Power was fed to the preamplifier via the open wire feeder. This feeder was made from single-strand copper wire spaced at about 75mm and used spreaders made from 12mm black polythene tubing. Matching into and out of the open wire feedline was by a 2-metre long tapering section that brought the open wire feeder down to the spacing of 300Ω ribbon. The 75Ω coax was matched to a short piece of 300Ω ribbon by the usual 4:1 ferrite balun, modi­fied slightly to enable it to pass DC into the open-wire feedline from the 75Ω coax. Fig.3 shows the details of the polythene spreaders and the tapering match and termination to the 300Ω ribbon. Open wire feedline can also be made from single strand galvanised fencing wire. At UHF, open wire feeder can have a loss of less than 1.5dB/100 metres. Even good quality coax has a much greater loss than this and is much more expensive. That means that you could run a 15dB pre­amp­ lified signal down 1km of open wire feeder and still have the same signal level present that was available at the receiving antenna terminals ahead of the masthead preamp. On the other hand, open wire feeder is more “messy” to use than coax as it has to be supported on poles and must not come too close to metal objects; ie, no closer than 200mm. Matching to 75Ω is somewhat involved and you must use modi­fied baluns to pass DC and RF simultaneously. Power to the masthead amplifier is best fed as AC as this will reduce electrolytic corrosion at connections but you can use DC if you want to. To get DC through the 300Ω-to-75Ω balun re­quires it to be modified slightly. For this, you need a small low-value (eg, 470pF) ceramic capacitor. Look very carefully at the way the balun is wound. The windings you want to investigate are those that appear to crisscross. The capacitor is placed in series with one of those wires. Fig.4 shows how the balun is wired. You will have to dis­connect each wire, leaving the others connected, and test for a loss of continuity between the inner and outer of the 75Ω side. When this is achieved, test for continuity from the 300Ω side to the 75Ω side without any shorts between either side of the re­spective feedpoints. If all is well, solder the capacitor in series between the 300Ω terminal and the “disconnected” end of the winding. It goes without saying that you need to put this assembly into a water-tight container. Two of these baluns are required, one for each end of the open wire feeder. A good way of connecting the 300Ω ribbon to the thin end of the taper is to use the insides from a “chocolate block connector” (ie, the internal metal sections from plastic barrier terminal blocks), as shown in Fig.3. References (1). Hewlett Packard. Spectrum Analy­ zer series Application Note 150-10, 1979. (2). I.T.T Radio Reference Manual, 4th Edition (3). Introductory Topics in Electronics and Communications, Antennas, by F. R. Connor, 2nd edition. ISBN 0-71313680-4. (4). Radio Communication in Tunnels, by K. F. Treen, Wireless World, March SC 1979. CALLING ALL HOBBYISTS We provide the challenge and money for you to design and build as many simple, useful, economical and original kit sets as possible. We will only consider kits using lots of ICs and transistors. If you need assistance in getting samples and technical specifications while building your kits, let us know. YUGA ENTERPRISE 705 SIMS DRIVE #03-09 SHUN LI INDUSTRIAL COMPLEX SINGAPORE 1438 TEL: 65 741 0300    Fax: 65 749 1048 May 1994  17