UHF Construction Precautions

Any time a circuit or system operates at very high or very low frequencies, voltages, or currents, impedances, power levels, or other quantities, certain design, test, and assembly precautions become necessary. Obviously, a power system handling kilowatts of AC must be assembled with large components, and a very sensitive microphone preamp must be shielded to prevent hum and noise or even radio signal pickup. While these examples are obvious, others are less obvious or even obscure and mysterious. UHF and microwaves are a good example. If you are building a kit that operates at VHF (over 30 MHz) or UHF (>300 MHz) you should be made aware of proper wiring and construction practices. This is largely experience combined with a good theoretical understanding. Remember that in most cases, only a small portion of the circuit operates at the high- est frequencies, the remainder at lower frequencies. A good example is our 2 Watt ATV Transmitter. Much of the circuitry handles video, audio and lower RF frequencies, and only 3 of the 12 transistor stages operate at over 400 MHz. The following suggestions should be followed so as to stay out of trouble 1) Do not rely on audio, computer or other such low frequency experience. Even the fastest computer clocks operate at frequencies a UHF experimenter would consider low. If you have no UHF experience, just carefully follow the instructions and do not embellish or get creative until the circuit works and you have a known reference point, even if you were "taught not to do that" by someone in the past, or "it looks sloppy and messy", or "it looks flimsy",etc. Actually, well constructed UHF circuits may look odd and unconventional but they are mostly very neat and clean. It may prove useful to examine how comm- ercially built UHF equipment is assembled, or to dissect some junked TV tuners and IF strips, or old 2 way radio gear ,etc. to see this and learn from it. 2) The best component lead length is no leads at all. Any excess lead length acts as an unwanted inductor or capacitor not intended as part of the circuit. Straighten out any preformed component leads. In some circuits, a scrap of wire a few tenths of an inch may be needed to make an inductance for a filter or matching network. In this case, the component lead could serve. Above 500 MHz, chip components having no leads are often used. While small and a little more difficult to work with, they are generally much better in UHF performance. 3) Cables should be terminated properly with proper connectors to best provide continuity of impedance. Where connectors are not used, the shield braid should be soldered directly to ground and not twisted into a separate lead, and exposed center conductors are as close to zero as possible in length. This avoids an impedance "bump" or mismatch. This mismatch can ser- iously degrade the output power of a transmitter or cause a loss of received signal in receiving systems. In any of our projects, NEVER leave shields unconnected at one end of a cable (often done in audio work to control ground loops, this is not applicable at UHF) unless specifically instructed to do so. BOTH ends are to be used. 4) Use only recommended connectors. They are designed mechanically so as to be reliable and preserve the impedance of the system. Audio or DC connectors are worthless at high frequencies except under very special conditions and uses not encountered in our kits. Improper connections and cables may actually ruin performance and even help to destroy RF output transistors. NEVER leave shields unconnected at one end of a cable unless specifically instructed to do so. BOTH ends are to be used. 5) Use metal cases for mounting RF assemblies as plastic or wood does not provide shielding and grounding. Remember that PC boards should be at least 1/4 inch (6.3mm) from metal surfaces. It is normal to have to repeak some tuning adjustments after a PC board is mounted in a case due to extra stray capacitances and small changes in apparent coupling between coils, etc. PC boards with microstrip or other large area RF components will be more notice- ably affected than lower frequency boards. Make sure to provide for heat sink- ing and ventilation if needed. 6) By their nature, UHF circuits are mainly analog and require adjustmant and setup due to unavoidable component tolerances, variations, and even, in the case of antennas, environmental factors not possible to predict accurately as well as unique situations. So do not expect "Plug and Play" operation if you want optimum performance. Chances are at first try, things will not be anywhere near optimum unless you are lucky. On the other hand, it is possible to get some things working the first time if you take care to do a good assembly job, and have some basic knowledge and test equipment. You do not need a lab but a VOM/DVM, power supply, dummy load, and an inexpensive frequency counter realistically should be available. It is possible to find frequency counters good to 1300 MHz for $100 USD, about the price of a decent DVM. Actually, if you are going to do work much at UHF you will need this equipment for many other things so it will be an eventual necessity anyway. 7) Be aware that even good quality ham and CB test gear is rarely accurate at 400 MHz and almost never above 1000 MHz so do not rely on your CB wattmeter to check a UHF transmitter or antenna. If the project behaves properly and appears to work well, and does what it should, ignore "readings" from any but professional grade, well calibrated test equipment. You can waste a lot of time and energy looking for nonexistent "problems", and you may even actually create some really nasty ones to waste lots of time and test your frustration tolerance in the process. In short, if it isnt broke, dont fix it.