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Are your TV signals weak or noisy? This masthead amplifier could mean the difference between a lousy picture and good reception. Portable masthead amplifier for TV & FM By BRANCO JUSTIC T HIS MASTHEAD AMPLIFIER was originally designed for use with caravans and recreational vehicles. It’s portable, comes with its own inbuilt telescopic (rabbit ears) antennae and runs off a power supply ranging from 7-20V DC or 6-15V AC. This means that you can either power the unit from a 12V car battery or from the mains via a suitable plugpack supply. The “rabbit ears” telescopic antennae feed directly into the amplifier circuit. This circuit typically provides from 16-20dB of gain at frequencies up to 1GHz, which should be plenty 54  Silicon Chip for beefing up an otherwise marginal signal to a portable TV set. If you’re fed up with constantly adjusting the antenna on your portable TV set or if the reception varies when you change channels, this “active” antenna system is the way to go. It not only amplifies the incoming signal but, just as importantly, provides correct impedance matching between the antenna and your TV set. Of course, there’s nothing to stop you from using this design in fixed installations or as a distribution amplifier. All you have to do is ditch the rabbit ears antennae and feed a signal in directly from a fixed antenna or a distribution cable. The unit is easy to install and is suitable for amplifying both VHF and UHF signals, as well as FM signals. As with most masthead amplifiers, the DC supply rails are delivered via the downlead; ie, the TV signal and the supply rails share the same cable. This means that you don’t have to run separate supply leads up the mast, which greatly simplifies the installation. Generally, the best approach is to mount the amplifier as close to the antenna terminals as possible. That’s Fig.1: the circuit is based on a MAR6 broadband RF amplifier (IC1) which provides around 20dB of gain. D1 and D2 protect the input of IC1 by clipping any high voltage transients, while REG1 provides a 5V supply rail. This supply rail is fed via the signal cable to the output terminal of IC1 and is isolated from the TV set using C3. Fig.2: install the parts on the two PC boards as shown here. The MAR6 (IC1) is mounted from the copper side of the board (see photo). really just another way of saying that it should go on the mast. This is done to avoid signal degradation due to cable losses. Quite often, a good signal is available at the antenna terminals but cable losses can result in a severely degraded signal by the time it reaches the TV set. The basic idea is to amplify the good signal that’s coming from the antenna, rather than a noisy signal at the TV set it­self. Well, that’s what the theory says. In practice, you can sometimes get a good result by placing the masthead amplifier at the TV if you don’t want to go to the trouble of mounting it on the mast. This only applies to borderline situations, where the signal is just too weak for the AGC (automatic gain control) circuit to limit the front-end gain of the receiver. In this situation, you get a “snowy” picture because the front-end operates at high gain which results in a poor signal-to-noise ratio. By amplifying the signal before it is fed into the receiver’s front end, the AGC circuit limits the gain and this drastically cuts the noise to give a clear picture. Distribution amplifier Another area where this circuit should prove popular is as a distribution amplifier. Quite often, a signal that’s adequate for one TV set will no longer be adequate when fed through a splitter for distribution to several outlets. That’s because the splitter itself introduces signal losses, typically around 3.5dB or more. The amplifier board (left) is installed inside a length of 100 x 43mm OD conduit. Above is a close-up view of the MAR6 IC, which is mounted on the copper side of the board. August 1996  55 The power supply board is installed inside a small plastic utility case, as shown here. Take care to ensure that the 7805 regulator is oriented correctly and check that the completed unit delivers +5V to the centre conductor of the lead that runs to the amplifier board. The answer is to amplify the signal before feeding it to the splitter. Doing this will ensure a sufficient level at each outlet for a noise-free picture, despite losses in the splitter circuit and the distribution cable. Circuit details Fig.1 shows the circuit details. It’s based on a MAR6 mon­olithic broad­ band amplifier (IC1) made by Mini-Circuits (USA). This device has a rated bandwidth from DC to 2GHz, 20dB of gain at 100MHz and a low noise figure of around 2.8dB. This noise figure is far superior to the noise figure for the OM350 mono­ lithic amplifier used in many older masthead amplifier designs. Apart from the MAR6, there’s just Fig.4: a masthead amplifier is useful for boosting the signal before it is fed to a splitter for distribution to multiple TVs. Fig.3 a balun is necessary if you intend using the twin telescopic antenna. It is wound using lightduty single core wire. a 7805 3-terminal regula­ tor, three diodes and a few minor components. All the required gain is provided by the MAR6, so there’s no need to make things complicated. Let’s take a closer look at how it works. The signal from the antenna is coupled to the input of IC1 via capacitors C1 and C2 which provide DC isolation. Diodes D1 and D2 are there to protect IC1 from excessive input voltages, as could be induced by nearby RF transmitters, lightning strikes or static build-up. Note that BAW62 diodes are specified here, as these are a high-speed switching type with very low capacitance. As a result, they provide good protection for IC1 with very little signal loss. In operation, they clip any high voltage spikes to ±0.6V. The amplified signal appears at the output of IC1 and is coupled directly to the centre conductor of the coaxial cable downlead. It is then subsequently fed to the antenna terminal of the TV set via C3. Power supply Fig.5: here’s how to include a VCR in a distribution system. The combiner is just a 2-way splitter wired back-to-front. 56  Silicon Chip Power for the circuit is derived from an external AC or DC plugpack supply. D3 either rectifies the AC supply or, in the case of a DC supply, provides Do You Need A Masthead Amplifier? “Will a masthead amplifier solve my TV reception problems?” That’s a question that’s often asked and the answer is “it depends”. A masthead amplifier is not a universal panacea for crook TV pictures and there are many situations where it will offer little or no improvement. It will not eliminate most ghosting problems, for example, as the ghosts just get amplified along with everything else. Nor can a masthead amplifier clean up interference problems or give you a good picture if there is little or no signal in the first place. reverse polarity protection. The resulting DC rail is then filtered by C5 and drives 3-terminal regulator REG1. The 5V output from REG1 is then filtered and applied to the output terminal of IC1 via R1, L1 and the centre conductor of the downlead. Inductor L1 presents a high impedance at signal frequencies and thus ensures that IC1’s output is not loaded by the supply rail. It also serves to keep signal frequencies out of the regu­lator output circuitry. Construction The assembly of the masthead amplifier is straightforward, with all the parts mounted on two small PC boards. The MAR6 RF amplifier and its associated parts go on the smallest board and the completed assembly installed inside a length of 100 x 43mm OD plastic conduit. This That said, there are many situations where a masthead amplifier can dramatically improve picture quality, particularly in fringe areas. Basically, you should use a masthead amplifier under the following circumstances: (1) You live in a fringe area and one or more channels is noisy; (2) Reception is poor due to losses in the downlead; (3) The signal strength is inadequate because of splitter and cable losses in a distribution system; (4) The antenna system is only very modest. is fitted with end caps for weather­ proofing – an important consideration if the unit is to be mount­ed outdoors on an antenna mast. The power supply parts are accommodated on the second board. This board fits inside a small plastic utility case which would normally be hidden somewhere behind the TV set. Fig.2 shows the parts layout on the two PC boards. Begin by installing the parts on the amplifier board, taking care to ensure that diodes D1 and D2 are oriented in opposite directions. The two capacitors are non-polarised and can be installed either way around. The MAR6 amplifier IC is a surface mount device and is installed from the copper side of the PC board. The accompanying photographs show how this is done. Make sure that it is correctly oriented. Its type number Where To Buy The Parts Parts for this masthead amplifier design are available from Oatley Electronics, 5 Lansdowne Parade, Oatley, NSW 2223. Phone (02) 579 4985 or fax (02) 570 7910. Prices are as follows: Basic kit (incl. PC boards, MAR6 IC, all on-board parts & balun core) ....$15.00 Twin telescopic antenna .............................................................................$5.00 Plastic case for power supply .....................................................................$2.50 Plugpack supply .......................................................................................$10.00 RG59 coaxial cable..............................................................................90c/metre Payment may be made by cheque or credit card. Please add $5 for packaging and postage. Note: copyright of the PC board artworks associated with this design is retained by Oatley Electronics. PARTS LIST 1 amplifier PC board (Oatley Electronics) 1 power supply PC board (Oatley Electronics) 1 twin telescopic antenna (optional) 1 balun core 1 100mm length of 43mm O.D. plastic conduit 2 43mm I.D. end caps 1 plastic case, 84 x 54 x 30mm 4 plastic cable ties 1 15µH inductor (L1) 1 68Ω resistor (0.25W) Semiconductors 1 MAR6 wideband RF amplifier IC (IC1) 1 7805 3-terminal regulator (REG1) 2 BAW62 fast switching silicon diodes (D1,D2) 1 1N4004 silicon diode (D3) Capacitors 1 100µF 25VW PC electrolytic (C5) 1 .0033µF ceramic (C4) 3 .001µF ceramic (C1,C2,C3) Miscellaneous Light-duty single core wire (to wind balun), clamps, silicone sealant, coaxial cable. should be visible from the component (top) side of the PC board, while a small white triangle or dot indicates the input pin. If you are using 75-ohm coaxial downlead from the antenna, this can be soldered directly to the PC board as shown in Fig.2. Alternatively, the rabbit ears antenna comes with 300-ohm ribbon cable and so a balun is necessary to match this (and other standard antennas which don’t already have a balun) to the 75-ohm input impedance of the amplifier. Fig.3 shows the winding details of the balun. It is wound using light duty single core wire. The amplifier side consists of a single turn through the core, while the antenna side consists of two turns wound from the opposite end of the core. On the prototype, the rabbit ears antenna was mounted on one of the end caps (see photo) and secured using machine screws and nuts. Once August 1996  57 can now be installed in its case and the external connections made. You will need to drill holes in one end of the case two accept the two coaxial cables and the power supply leads. As before, attach cable ties to the various leads just inside the case so that they cannot be pulled out. Although not shown on the prototype, we recommend that the power supply leads be run to a suitable jack socket mounted on the end of the case. That way, the plugpack supply can be easily disconnected and used in another application if required. Installation The plastic conduit case makes a neat weatherproof assembly which is easily attached to a mast using a large hose-clamp. the connections have been made, the completed amplifier board is pushed into its plastic conduit housing. The output lead emerges through a hole drilled in the bottom end cap. For a fixed installation, the 75-ohm antenna lead is fed in through a second hole in the bottom end cap. It’s a good idea to fit a couple of cable ties to the cables just inside the end caps to provide strain relief for the soldered connections. The two entry holes can later be sealed with silicone sealant after the assembly has been completed and tested. 58  Silicon Chip The power supply board can now be assembled and tested. Take care to ensure that D3 (1N4004), C5 and REG1 are all cor­rectly oriented. Inductor L1 (15µH) looks like a resistor. It has a light green body and carries brown, green, black and silver colour bands. Once the power supply board has been completed, temporarily apply power and check that the output side of L1 is at +5V with respect to ground. If there’s a problem here, switch off imme­diately and carefully check the circuit around REG1 and D3. Assuming that all is well, the board The way in which the unit is used as an active antenna for portable TV sets is obvious – just unplug the existing antenna and plug this unit in instead. Don’t forget to apply power to the amplifier though. For use with an outdoor antenna, the amplifier unit should go up on the mast as mentioned previously. This arrangement will provide the best signal-to-noise ratio although a short length of high-quality coaxial cable between the antenna terminals and the masthead amplifier shouldn’t make too much difference. If used as a distribution amplifier, the unit can be mount­ ed indoors, provided that input signal from the antenna is noise-free in the first place. The output of the amplifier is connected to the splitter input and the splitter outputs then run to the various TV receivers. Fig.4 shows the basic idea. Finally, if strong signals on one or more channels cause receiver overload (as indicated by an interference pattern), try fitting a tuned attenuator for the offending channel(s). This should be fitted right at the antenna terminals (ie, before the masthead amplifier). A 1/4-wave stub makes a very effective tuned attenuator. This is simply a length of coaxial cable cut to exactly a 1/4-wavelength of the offending channel. If the stub attenuates the signal too severely, try making it slightly shorter until you get the desired result. Another approach is to initially cut the stub slightly shorter than 1/4-wavelength and then tune it towards resonance using a trimmer capacitor across the far end. Just keep on exper­imenting SC until you get it right.