As I strategy my three hundredth RF Technology column for TV Tech, I assumed it could be helpful to explain a number of the issues I’ve discovered in additional than 50 years of working with broadcast transmitters. Throughout my profession, I labored with ideas and programs that had been obscure, however by expertise and assist from consultants I used to be lucky to satisfy I used to be capable of perceive them higher.You’ve in all probability had related experiences, from issues as primary as studying to drive to creating spreadsheets on a pc or configuring an IP community. One ingredient of that is having the ability to perceive how these programs or duties work on an nearly intuitive stage. That supplies the premise for added studying and experience even throughout completely different fields. I’ve discovered many individuals who’ve constructed a very good understanding of IT are additionally fast learners on the subject of RF programs. Over the years, I’ve observed extra of my readers will not be engineers with RF backgrounds, however folks, typically with expertise in different fields, who’re fascinated about RF. This month’s column is for them, as I’ll be masking some primary rules. If you’ve had expertise with RF, I welcome your feedback on different RF matters non-engineers would discover helpful.One factor that helps in gaining an understanding of how RF works is discovering methods to see it work. Radio frequency electromagnetic fields are related in some ways to a lot larger frequency power, comparable to gentle. Just as buildings block some gentle and create shadows, additionally they block RF. However, the shadows will not be utterly darkish, as some gentle finds its means in by reflection from different objects and scattering within the environment. RF behaves the identical means, though the quantity of reflection and scattering within the environment will fluctuate with frequency and wavelength, which is the frequency divided by the pace of sunshine. Comparing RF to LightLike gentle, RF power could be centered. In transmit antennas, this concentrates the facility on the bottom and can be utilized to focus on particular areas whereas avoiding others the place protection isn’t wanted or interference must be decreased. In obtain antennas, the focusing supplies acquire, which, like a telescope, will increase the depth of the sign coming from one route and reduces alerts from different instructions that will trigger interference. (Image credit score: NASA) (opens in new tab)As RF frequencies improve and wavelength decreases to a millimeter or much less, antennas may even begin wanting like optical gadgets. At satellite tv for pc and microwave frequencies, parabolic reflectors are generally used. These additionally develop into fairly efficient at gentle and infrared wavelengths, as anybody who has had an LNB cowl soften when the solar moved behind the satellite tv for pc the dish was is aware of.Antennas and WavelengthKeeping in thoughts the connection between frequency and wavelength may also assist in evaluating antennas. Antennas that need to work on decrease frequencies, like low-band VHF-TV (54–88 MHz), FM radio (88–108 MHz), need to be bigger than these used for high-VHF TV (174–216 MHz) or UHF TV (470–608 MHz) to work effectively. This doesn’t imply small antennas gained’t work on the decrease frequencies for reception, simply that the antenna itself might be much less environment friendly. (Image credit score: Antennas Direct)One answer is so as to add an amplifier. However, the amplifier will add its personal noise, decreasing sensitivity, and since the small antenna might be much less directional (centered) it should choose up extra surrounding noise and interference. For greatest outcomes, a low-noise amplifier must be positioned on the antenna the place it could offset the loss within the line to the TV and supply a very good match to the road. On the transmit aspect, matching the resonant frequency and impedance of the antenna to the transmitter is extra vital. Without moving into the maths, matching the impedance is like connecting two pipes of the identical diameter along with the tap on one finish supplying water on the optimum charge for the pipe and the gadget (say, a turbine on this analogy) on the different finish. The water flows easily with the least quantity of loss. The similar analogy applies in a system with a transmitter, transmission line and antenna. However, if the impedance of the parts isn’t matched, it should result in extreme present (inflicting heating) and voltage (doubtlessly inflicting arcing) at completely different factors within the system, relying on wavelength. Most broadcast programs are well-matched, until the antenna is broken so issues usually tend to happen when a connector begins to lose contact, rising loss and warmth resulting in contamination within the line, maybe resulting from carbon created by overheating from a nasty contact. In most circumstances failures will create a mismatch within the line, which could be positioned on the base of the tower both by sending a really quick pulse up the road and in search of the time it takes for the return reflection, or sweeping the frequency throughout a band of frequencies (and completely different wavelengths) and searching on the time area response throughout the frequencies. Because these measurements contain “sweeping” between frequencies, that is typically known as “sweeping the road.”TV Antenna Specs Debunked One of the issues that bugs me when studying opinions for TV antennas or advertisements is comparisons primarily based on vary in miles or antenna good points that embody a built-in amplifier. More accountable producers will embody the acquire of the antenna at completely different channels. Most are primarily based on acquire above an isotropic antenna (with out going into particulars, a “excellent” antenna) as “dBi” slightly than acquire above a dipole or “dBd” (like a set of rabbit ears with whole size of half a wavelength). Gain in dBi might be 2.15 dB larger than acquire in dBd. Ideally the specs will specify whether or not the acquire is in dBi or dBd. (Image credit score: Future)The vary numbers in antenna advertisements must be ignored, as they get rid of too many elements, comparable to the peak of the transmit antenna above floor. Just as gentle diminishes shortly after the solar units, when the transmitter’s antenna is under the radio horizon the sign will drop off shortly. Read my TV Tech column “Estimating Coverage: Quick Analysis for Facility Mods,” for extra info. For a transmit antenna 2,000 ft above common terrain, the radio horizon is 63.3 miles away. As the sign drops off shortly past this distance (as with gentle previous sundown) ranges of greater than 70 miles solely would apply from mountaintop to mountaintop or for very excessive transmitter websites. Experimenting With RFThe greatest option to get snug with RF is to experiment with it. While troublesome to do on the transmitter aspect (until you’re a licensed novice radio operator) there’s a lot that may be achieved on the obtain aspect. Check out my article “Inexpensive Tools for RF Field Measurements” for extra info. The Airspy Software Defined Radio (SDR) is an effective way to discover the RF spectrum. It can not demodulate broadcast TV alerts, however it should present the TV sign’s spectrum and sign energy. A handheld spectrum analyzer just like the TinySA (learn my column “tinySA: Finding Interference and Aiming Antennas”) is an effective way to discover RF spectrum. I obtained an electronic mail from a reader who was seeing some odd conduct choosing up distant stations in Chicago. He was fascinated about attempting out completely different antennas and areas, so I instructed he get a tinySA. He did and is now capable of see how antenna kind, orientation and placement impression the sign. He observed the ripple (“spikes”) and I defined these had been resulting from reflections. With a little bit of time and tinySA, he now understands extra about TV reception than many individuals as we speak who work in broadcasting.A clarification: In my article “RF on the NAB Show—ATSC 3.0, Part 1” I mentioned Saankhya Labs developed their multi-standard ATSC 3.0 tuner in cooperation with Coherent Logic. Vasanth Shreesha from Saankyha mentioned, “The Saankhya ATSC 3.0 receivers use our personal chipset (SL3000 or SL4000)” and so they weren’t developed by Coherent Logic.” Email me at [email protected]. I attempt to reply all emails promptly, but when I’m busy and the e-mail will get buried, I would miss it. If you don’t get a response inside per week or so, electronic mail me once more.
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