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high frequencies. However, it’s very expensive, it’s very heavy, it’s labourintensive to fit and install and it doesn’t handle temperature cycling very well (this is an issue for outdoor use, where a rigid line might go several hundred feet up a tower. Expansion and contraction of the line with temperature causes friction and rubbing—and wear—of the inner conductor at each flange). Finally, the uniform length of each section of a long line (typically 20 feet or so) causes a small discontinuity that repeats uniformly. This degrades VSWR performance of the line, a little or a lot depending upon the care of assembly. Once bigger air lines for this application came along they were welcomed with open arms. In today’s world, perhaps 90% of the transmission lines made are consumed by the wireless/cellular radio industry. This has already had an effect on which products are available to broadcasters. Wireless repeaters use foam lines. Nowadays broadcasters often find that these are the only lines they can get; certainly the only lines that are stocked. Rigid transmission lines aren’t used much by any group except broadcasters and, as a consequence, the larger companies no longer manufacture them. They are still being made by a few smaller manufacturers, thankfully. Again, as technology progresses, we see that the wireless industry is rapidly moving away from the use of transmission line products except as short jumpers and increasingly using fibre optic cable on towers. What this will mean about the availability of familiar transmission line products to broadcasters remains to be seen. AVCCAM AG-AC90 Camcorder LEARN MORE Click the button for more information. Dan Roach works at S.W. Davis <strong>Broadcast</strong> Technical Services Ltd., a contract engineering firm based in Vancouver. If you have a question or comment, contact him at dan@broadcasttechnical.com. BROADCAST DIALOGUE WEEKLY BRIEFING — Essential Reading • February 7, 2013
Estimating maximum power Trying to work out a reasonable estimate of maximum power-handling characteristics for RF connectors and transmission lines for a given project can sometimes be a real pain. Today we’ll discuss some of the factors that you’ll want to take into account. When working out safety margins, it always comes down to the available dollars otherwise we’d always just overspecify everything and we’d sleep well! Transmission line catalogues will generally specify a peak power capability and an average power capability. Peak power limit is related to the breakdown voltage of the insulation between the inner and outer conductor, and is a static value independent of frequency of operation. Average power limits are caused by heating of the line and, so, you’re generally presented with a table of frequencies and power-handling capability. Because of the skin effect, higher frequencies heat the inner conductor more and once it reaches a certain temperature the average power limit has been reached. ENG INE ERI NG by Dan Roach Other Factors Well, that just looks too easy, doesn’t it? Look up two values for our line and we’re done! In the real world, we also must consider, well, the real world: • Peak power rating assumes a VSWR of 1, standard atmospheric pressure and low humidity. Peak power rating decreases with altitude unless the line is pressurized. Foam dielectric cables often have higher peak power ratings than air lines but in actual practice the ends of the line will have air spacing where the connectors are installed so, generally, these cables will have the same breakdown voltage as air lines. Allowance for non-ideal VSWR eats up your safety margin in a hurry. And if it’s an AM installation, you obviously have to take the positive peaks into account as, at 100% mod you’re dealing with 4x the unmodulated power—but nowadays who stops at 100%? Another safety margin gobbler! • Average power rating assumes calm air at 40 degrees C, no direct heating from the sun, dry air or nitrogen for air lines and standard atmospheric pressure and, of course, a VSWR of 1. If the sun’s rays can hit and heat the line, if the line is buried in soil instead of surrounded by cooling air or if the load is non-ideal, derating is necessary to some degree. There are charts available from the transmission line makers that help with these calculations. Just as an aside at this point, it’s interesting to note that a couple of BROADCAST DIALOGUE—The Voice of <strong>Broadcast</strong>ing in Canada • December 2012/January 2013 61
Page 1 and 2: Engineering 101: Dan Roach’s 101
Page 3 and 4: Table of contents 27. 28. 29. 30. 3
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Page 7 and 8: headgear and clothing. Take a look
Page 9 and 10: As broadcasters increasingly embrac
Page 11: Transmission lines down through the
Page 15 and 16: Two months in, do you feel CALMer?
Page 17 and 18: This’n’that In my last column I
Page 19 and 20: MDCL - Worth a second look? ENG INE
Page 21 and 22: occasionally—even a few minutes u
Page 23 and 24: these units have gone out of busine
Page 25 and 26: chips and it results in a figure th
Page 27 and 28: Over in radio, you hear an awful lo
Page 29 and 30: The more subtle and insidious probl
Page 31 and 32: that pain will likely be involved.
Page 33 and 34: the popular E1 standard (their vers
Page 35 and 36: go away, by moving the nulls around
Page 37 and 38: television the convenience of an an
Page 39 and 40: These quacks don’t even have to b
Page 41: The pressure inside typically build
Page 45 and 46: gain, compensating for the reductio
Page 47 and 48: When you pick up a few belts of one
Page 49 and 50: fashioned but I always like to have
Page 51 and 52: to overlook the PI code. Some of th
Page 53 and 54: ENGINEERING …and thus the whirlig
Page 55 and 56: ENGINEERING Reflections on standing
Page 57 and 58: ENGINEERING I remember the CAB tech
Page 59 and 60: ENGINEERING Monitoring surround sou
Page 61 and 62: ENGINEERING Imay have seen the futu
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ENGINEERING A cure for voltaic pile
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ENGINEERING Ruminating on the DTV r
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ENGINEERING Circuit breakers, power
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ENGINEERING Confessions of a serial
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ENGINEERING Bring me your lemons BY
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ENGINEERING It’s giant leap of fa
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ENGINEERING I, Bach returns! BY DAN
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ENGINEERING Audio monitoring in the
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ENGINEERING Acoustics and monitorin
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ENGINEERING It’s AES/EBU for you!
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ENGINEERING Bulletproofing your sit
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ENGINEERING Loads of fun with quart
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ENGINEERING Daring Dolby tackles TV
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ENGINEERING Batten down the hatches
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ENGINEERING Remote controls we have
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ENGINEERING When is new not better?
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ENGINEERING Searching for the right
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ENGINEERING Stop this paradigm shif
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ENGINEERING Alphabet soup for break
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ENGINEERING String, tacks and seali
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ENGINEERING Gibbled audio in the di
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ENGINEERING Nazis sank my crystals!
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ENGINEERING Switch-hitting your pow
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ENGINEERING Rogers to the rescue BY
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ENGINEERING The story of Conelrad B
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ENGINEERING The marginal path: FM r
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ENGINEERING Yagi, Yada Yada Yada BY
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ENGINEERING Lightning, grounds and
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ENGINEERING Safety Code One or diat
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ENGINEERING Many flavours of dog bi
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ENGINEERING The history of broadcas
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ENGINEERING Further adventures with
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ENGINEERING How stuff breaks BY DAN
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ENGINEERING A safety primer for tra
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