Digital Private Cable

Microwave Engineer combines latest technology and years of experience to

provide free to air signals in heart of “cable country”

By Tim Alderman

 

I had been called out to do a “TVRO site survey” in the Oakland Hills by a customer who wanted me to locate a spot for a satellite dish he had moved from another location. It was a mess, a literal pile of rubble that I saw behind a grove of trees.  An 8’ dish, still assembled, and a “superjack” pulled apart.  Obviously taken down at the old location by someone who had no experience.  The feed had been removed, and in the process, the Ku waveguide had been broken. I found his old one-run cable in knots.  Amine Nebri, the customer, was a computer programmer who decided he wanted to keep his C band dish rather than subscribe to cable.

 

            Walking around the yard with my Gourmet-Entertaining Sat-Site tool, I located two spots where the arc was fully visible, if only a few branches were trimmed.  The steep hillside meant the dish would be far down the hill, and away from potential conflicts with neighbors.  We quickly settled on a spot and left a marker where the pole would go.  I also looked at his house, and decided that the best way to distribute was to re-use his existing cable, installed by previous owners.  The whole neighborhood had been burned out during the Oakland Hills “firestorm” of 1991, which had taken 2500 homes and 30 lives.  The retaining wall, next to where the pole would go, was a blackened reminder of what remained.  Everything else, including the house on 4 levels, was less than a decade old.  The house was cabled to a central location in a sub area near the top with 6 home runs of RG6.

 

            While walking the property and making decisions about equipment placement, Amine raised the question about disconnecting cable and using it for internet service only.  I agreed to provide a quote for both re-installation of his dish and bringing in local off-air TV, and combining both with local modulators to create a classic “private cable” for his family, and get him off the monopoly.  He was to do all the “grunt work”, installing conduit, running wire and erecting the antenna on his roof. I provided an estimate based upon this scope of work request, 15 hours for the dish at $750 labor, and another 15 for the antenna, at $450 labor, plus parts, and a complete block diagram upon job completion.

 

            We also agreed that this was to be a “digital only” system for off-air reception, after explaining recent developments in off-air television where all the local broadcast channels were duplicated on UHF of the traditional VHF analog assignments coming from San Francisco’s combined transmitting location called “Sutro Tower”.  I also knew that one channel, KCSM, a PBS affiliate, was still broadcasting from the older tower farm at “San Bruno Mountain” which is about 10 degrees of antenna orientation different than Sutro, from his location.  I went to a local electronics store that still carried outdoor antennae and purchased a UHF double-row-bow-tie antenna suitable to pick up both.  As I already had a Blonder Tongue VHF yagi antenna for cut channel eleven, brand new in a box, for the single San Jose channel, I didn’t need to buy another.  I also knew that that San Jose channel, KNTV, had taken over as local affiliate for NBC, and that they used VHF channel 12 for their digital signal.  As he wanted all the principle networks to choose from, we agreed my quote would be for two antennae on his roof.

 

 

            Before construction could begin, I insisted a “signal study” be done to determine the exact channels possible.  I brought my own ATSC tuner and monitor out and together we set it up on his roof near a chimney.  I brought a ten foot pole and the two antennae and connected them.  I was surprised to find that, half way down a steep canyon, his UHF reception was spotty at best.  Digital signals either work or they don’t. There is no “ghosting”.  What I found was that it took me half an hour, using my spectrum analyzer and the ATSC tuner, to peak the Sutro/San Bruno Mountain signals so both were available.  The strength of the signals was such that it took a long time to “sync in”, or acquire the digital channels as I tuned from one to the next.  Then I found “tiling” which is much more objectionable as a marginal signal than ghosting is on analog.  I also noticed the signals on my spectrum analyzer indicating that knife-edge diffraction was present, as there was a “comb filter” effect as I slightly rotated the antenna,

causing some channels to vary in strength by up to15dB.   My local knowledge of these same signals is based upon my own rooftop, which is in the flatlands about a mile west of the Caldecott Canyon, where Amine wanted reception. My signals average a +25dBMV, which is enough to roast a turkey, and require directional coupler attenuation before presenting to my two televisions.

 

            Fortunately, the Blonder Tongue VHF eleven antenna worked looking southwest towards a ridge line just blocking Loma Prieta, site of the 1989 earthquake, but more importantly, the transmitter location, some 40 miles away.  The channel 12 digital signal immediately “popped in”, despite being on the band edge of a channel eleven antenna.  Such is the nature of VHF reception.  Later on, I discovered that he also picked up a second NBC affiliate, KSBW in Salinas, off this antenna, from a transmitter on Fremont Peak south of Gilroy, an additional 50 miles further south.  But that signal was analog, and a bonus free channel, as my commitment was strictly for only digital.

 

            So, we had obtained spectacular digital from distant VHF and lousy local UHF from the signal survey.  In addition to the variations I saw on the local UHF, due to knife edge diffraction, the overall signals were in the –15dBmv region from both antennae.  Yet the KNTV-12 digital “popped right in” while the San Francisco/San Bruno UHF signals waited for several seconds while the ATSC tuner struggled with error correcting the signals.

 

            The antenna from the local store was returned and while there I talked with the staff and other customers who knew of Caldecott Canyon.  They told me I was crazy, that canyon was a “white area” for rooftop antenna reception.  I next went to another local retailer looking for a fringe UHF antenna from Winegard, a company with a history of fighting cable to deliver free signals to American households.  When the owner, who had 40 years experience in the business, told me I was flat out crazy, Charring Cross, the street Nebri lived on, was, he said, a street he wouldn’t touch.  A quick eyeball of the rooftops in that canyon confirmed this was indeed cable’s home turf and territory.

 

           

 

I had built a reputation on difficult reception issues, having brought the first legal reception of CNN and MTV into what was then the Soviet Union.  On the roof of the Communist Party Central Committee’s Moscow hotel, a decade before, I fought not only being 1500 miles outside the Ku satellite footprint, but had to contend with interference from a local in-band terrestrial microwave signal

some 50dB stronger.  I managed to use my childhood knowledge of how signals worked around knife edges by using the copper roof to catch MTV while avoiding the same frequency coming in locally, by moving the non-penetrating roof mount a fraction of an inch at a time.   To me, this local “white area” reception hole made the design a challenge.

 

            I needed at least 25dB gain from a UHF antenna, and beamwidth well under the 30 degrees quoted for “deepest fringe digital” reception currently available.  I knew the answer was a dish, a UHF dish. Only a parabolic or spherical, from the Oliver Swan era, could do the job.  Yet this dish also had to be mounted in a “normal” fashion, on a pole, so weight and wind loading were constraints, not to mention appearance to the neighbors of some gargantuan device in their neighborhood.

 

            Back in ancient times, Channel Master made model 4251, a seven foot parabolic reflector that met all of these conditions. It was light weight, broadband, and had relatively narrow beam and at least a 20dB front-to-back ratio.  It was also discontinued.   Having one on my roof for 20 years, I knew it was a real deal. But where could I find one?   Well, a decade before, I had sold one to an old friend in Nice, CA, in a true white area.  But he never installed it, off air not being his interest.  I also gave him a Paraclipse 12 dish for C band, which is all he wanted.  At some point, he threw away the feed, but kept the reflector and button hook, as they had not rusted away.  I managed to convince him to give me the worthless reflector, and I took the feed off mine and took it to a local welder to see if he could duplicate the backplane out of wire.  He did a magnificent job for a mere $75, even the bent spokes that held the double stacked V dipoles.

 

 I then went to a local hobby store and bought Teflon plastic in 3/8” thick block for $10.  Carefully tracing the outline from my feed, I duplicated this insulator, and followed that with aluminum sheet I cut to match the pickup elements.

 

            Next was to test this new feed on my existing rooftop to see if it matched the performance of my existing feed.  Not only did it meet, but it beat it by 1.2dB in signal strength.  I checked out the cross-pole isolation by rotating the feed while watching the spectrum analyzer on the ladder at my feet, some 40 feet up.  I was surprised to find that the cross pole null was at least as good as a C band polorotor feed, surprised to find the in phase peak was rather sharply defined, and that the TV stations transmitting digital all were horizontal polarity.

Next, I hung the feed in free space to find the feed’s true front-to-back ratio was also equal to 20dB and the cross pole was also a healthy 23dB.  Yum..

 

            Finally, I had something, but not enough to get the signal from Amine’s rooftop and all the way downstairs to the sub area.  So I ordered a Blonder Tongue UHF line-powered amplifier and power supply.  This enabled me to boost the entire UHF band and give some headroom for the inevitable attenuation I knew I’d have to deal with from knife-edge diffraction I had discovered deep in that canyon.

 

            In the meantime, Amine asked me to take over the conduit installation as he was to busy at work, and I was otherwise unemployed.  I kept careful track of these as “change order modifications” to the original quotation, for my bill.

 

            When the equipment arrived, my custom feed was ready.  I mounted the dish and put the VHF yagi right below it on a 20 foot pole I had saved from a second stack I wanted to erect on my own rooftop, but which unemployment had cancelled.  I erected the stack on his roof instead, and the following day came back to find the winds that came up suddenly had almost ruined my carefully laid construction plans.  A quick call and he approved a third change order modification, a “wind kit” that also came out of my boneyard.

 

            Setting up the guy wires, I noticed that when I tilted the pole off plumb, pointing the UHF reflector up the hill, the knife edge hole deepened. But what also happened was the overall gain increased by 2dB.  I measured the look angle with a roof pitch angle pointer and found the roofs of the houses that formed the knife-edge some 1,000 feet away was only 8 degrees.  I took down the heavy galvanized upper section of the mast and substituted a standard EMT pole, which I pre-bent to that 8 degree angle.

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This produced actually a 3dB increase, and I was also able to rotate the entire reflector on the mast, and found that the horizontal polarization from Sutro Tower was off by 2 degrees.  This enabled me to further increase the knife-edge effect, and thus pointing the stack in the right direction, I got an additional 2dB of reduction of the highest level signals.  Thus I sacrificed most of the gain from pointing the stack up the hill to get a more balance of signals to the pre-amplifier. Blonder Tongue said their amp had a high input signal overload capability but I wasn’t taking any chances. I needed to be sure enough strength to be certain the digital ATSC tuner that Amine bought would not start tiling when the winds picked up.

 

 

 

 

 

            Down in the sub area, I built a mini-headend.  During the process, I found that a local FM radio station, located on the ridge line just behind the Blonder Tongue yagi, was pouring a ton of signal into the VHF download.  Left unchecked, it would eat up at least half the power bandwidth of the MATV amplifier I had chosen.  So, I inserted a Blonder Tongue MLHF multiplexor backwards, and terminated the VHF-Lo side with a 75 ohm terminator. This lowered the 94.5Mhz FM signal to 6dB less than channel 57, my highest frequency digital signal.  And, I had an agreement that this was a digital ONLY system, so any analog signals that came through were strictly a bonus.

 

 

            I had done such a superb job in getting UHF gain off the rooftop that

I discovered the unamplified VHF was 20dB lower, as were the local UHF double-sideband consumer modulators coming back from his living room.

I merely used an old trick, a directional coupler run backwards, to solve the problem.

 

            Finally, the whole thing needed some gain, to make up the loss of the directional couplers to distribute signals to the sets.  I chose low loss directional couplers and a low distortion MATV amplifier to drive them.  In analog television, low signal levels mean noise or snow, especially on UHF, due to poor noise figure RF stages.  With digital RF, it’s the bit error rate that counts.  Signal levels of minus10dBmv just mean that the digital tuner’s internal AGC level is increased.  So low distortion is much more important in the digital world than signal strength above zero dBmv.

 

            My calculations proved correct, to the customer, his ATSC tuner delivers picture in under a second, and it translates the digital channels to the older VHF assignment as well. This gives him all the advantages of digital with out having to memorize new channel assignments.  Plus the ATSC tuner runs a channel on his local modulator, where anyone in his house can tune in on analog channel 22, and the TVRO is on channel 15.  

 

            Finally, private cable gives him growth capability.  By adding additional ATSC tuners, individual sets can tune in different channels from the main set, and only cost is for the tuner itself, now down to around $300.  All ATSC tuners also are capable of HDTV, and when he is ready, his private, free flow of information, digital system is already there.

 

            I am proud to have delivered exactly what the customer wanted, and deliver it for modest cost, and he will never have to pay cable again.