TO MUCH SIGNAL

 

 

Editor's note: Contributor Tim Alderman is, without exaggeration, one of the top 'field hands' in the TV reception world in the USA. And that includes both satellite and terrestrial reception. Calling in Tim is akin to asking Red Adair to show up at your oil well fire after you have exhausted all possible techniques to put the blaze out. When Tim finds it difficult, you can but imagine how lesser skilled individuals are faring with the "Great American Switchover" to DVB-T.

 

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"Take it out" cried the customer. "If you cannot get it to work properly, I don't want it in my house." His "house" was one of those upper crust San Francisco Victorian mansions,

the kind you see portrayed on television when they want to show you how "the other half" lives. In fact, the other "1/10th of 1%" would be a more apt description. Think of luxury, square it, and then take it to the tenth power. There I was in my grubby work clothes at the foot of a 50 foot long marbled staircase pondering, "How do we fix THIS one?"

 

The problems were many-fold. The client, if that is the descriptive phrase, of my sometimes employer Myron, was faced with integrating the output of a DirecTV Ku-band dish system with a plurality of off-air DVB-T channels such that the viewer would find it all 'seamless' - simply push the appropriate button(s) on the IR and on the 42 or 60" Plasma screen appeared the channel of choice. People "this wealthy" have a very low tolerance level for being asked to either "think" or do anything that involves more than one sequential step (for example, 'push 34 and then push AV' is asking them to do more than they are willing to do). There are Videophiles (who love to push buttons in some complex sequential stream) and there are average folks - rich 'average' folks who simply cannot be bothered. When you are brought up, as this client had been, with servants providing their every whim at a snap of the fingers, a TV set that requires more than one action to select a channel is unacceptable.

 

Problem number one: San Francisco's primary TV tower is called ‘Sutro Tower’ and it is located such that signals travel far (100-150 miles), strongly, as far as Nevada (get out your Atlas; it is akin to watching Sydney in Queensland). And the client's "Victorian mansion was not more than a mile (1.62 km) from Sutro. Take a normal TV antenna, connect a #47 light bulb to the driven element, and read by the energy generated! So every inch of the Victorian was "hot", so much so that quad shield cable was barely - if in fact - adequate to prevent direct pickup (never mind that the F connector on the rear of the Plasma screen TV sets had 6 inches of RG59 miniature connected to it in the run to the tuner - and you can be sure nobody in Japan in designing this set ever thought about making that six inch chunk 'quad shield'!)

 

Myron had been the 'third choice' after two previous installers had clung for their life on 40 degree pitched slate roofing installed in 1880 or so. This was one of those jobs where the customer says, "I don't care what it costs - just make it work!" and the installer responds, "I don't care how much money you 'throw' at this situation, it ain't ever gonna work to your satisfaction". Of course being a mile from Sutro was a big part of the problem, but working for a man who passed Billionaire status two or three generations ago was the much bigger problem. "Nobody says no to me", he advised Myron. Right one.

 

Pushing my luck (I am past mid-50s and gave up clinging to slate roofs pitched at 40 degrees ten years ago), finger nails screaming for a grip that never came, somehow the peak of the multiple story roof was ascented. I felt like New Zealander Sir Edmund Hillary on Everest as I sat there pondering, "Now that I am here - how in the blue blazes will I get down without dropping 60 feet to the street below?".

 

Two prior guys had been here. One installed an amplified antenna (mast head at the antenna) showing his total lack of skill levels; I measured 46 dBmV on the weakest channel coming out of the antenna with my precious FSM propped up between my legs straddling the extremely sharp slate roof peak line (you can imagine what this could do to your private parts on your own - at 55+ I did not give it a second thought). For reference, a reasonable quality MATV modulator puts out +50 dBmV so +46 is akin to connecting a 110V AC light bulb to a 220V AC line. Poof.

 

Yes, when several dozen analogue and DVB-T signals 'mixed' inside the 'deep fringe model' antenna preamplifier, what came out was total trash. It got better (worse!).

 

As close as Sutro was, directly between my crotch slashing roof peak and the 'TV Mountain' was a 12 story building, surrounded by others of similar height. So the Sutro-to-crotch signals were being bounced rebounded, and skipped from vertical wall surface to reflector plate several times before they arrived in my FSM. Tugging on a rope, I raised the Avcom PSA65 spectrum analyser to my not very comfortable 'sitting' position and proceeded to look at the signals. Very educational. Not one was still horizontal, unbolting the consumer grade antenna, dismantling the amplifier and going 'direct' to the PSA65, I could easily see that by rotating the antenna through a 360 circle, not much changed. Trash in, trash out. It got worse. I tried flipping the antenna from the normal and required horizontal polarity to vertical and then at various steps between the two. As many as a dozen discrete 'point source' signals on each and every channel, and not one of them worth a damn. It was clear to me that either I was going to die and be buried (a good trick, I think) 60 feet above ground on a slate lined roof or somehow lower myself and my equipment back to a safer elevation. Nothing previously installed on the roof (at great expense, I am sure!) was going to be of any use to me.

 

Back in the shop, I pondered the problems (plural, there were many). The signals were strong - so strong that before we got done, attenuators not amplifiers would be required. Next, the direct path to Sutro was blocked and while I might be able to locate a suitable 12 or 20 story 'off-path' reflector to 'bounce' some of the channels into the rich man's two TV sets (imagine that - a house the size of a modern hotel and only two TV sets!), the reality was that any antenna design and any antenna heading I found that might work for 'some' channels would not work for others. Or so I thought. So how many custom design antennas could I 'stack' on this crotch splitting rooftop before I ran out of space, or crotch?

 

The "answer" to this dilemma was far easier said than accomplished. Because the receive site was so bloody close to the maximum power transmitters (some to 5,000,000 watts ERP!), and, the reception site is shielded by a much taller building only a few hundred feet away, the house was basically in 'RF Burn Heaven'. The 'point source' (Sutro Tower transmitters) was in fact hundreds of discrete point sources; every building,  sign board, freeway ramp for miles around me was acting as a re-radiator of super strong signals (some of the reflected signals peaked in the area of +20 dBmV).

 

Fortunately, all but one DVB-T channel appears in the UHF band (470-806 MHz) so perhaps a common solution for all (but one) could be found. Normally, when you attempt to achieve a very sharp reception pattern (an antenna with a 'narrow focus' angle), the physical size of the antenna must be enlarged; a 7 foot UHF TV dish, for example, has a much more precise 'aim-it' signal pickup pattern than a simplistic dipole. Unfortunately, the client had some rules: "No part of the TV antenna shall be visible from any place on the property" came right after, "It must work perfectly on all channels". So a large antenna, which possibly would have produced far too much gain anyhow (time to use an attenuator in line!) was not doable simply because it could be / would be "seen".

 

Some years prior while attempting to produce satisfactory off-air ATSC DVB-T (SatFACTS #117, p. 18) in a badly shielded area where the problem was the opposite of the present situation (too little signal), I had resurrected a twenty year old Channel Master 7 foot UHF dish and carefully rebuilt just the feed portion to improve the wideband (470-806 MHz) response. The American ATSC digital system depends primarily on slotting in DVB-T transmitters onto channels which were not in use by analogue transmitters. As in Australia where 7 Network uses channel 8 for digital, this means the dial goes from half full to totally full in a geographic region as large as the San Francisco Bay area; almost literally, either an analogue or a digital transmitter on every TV channel. Limited to a single UHF antenna, and a small one at that to 'avoid visual detection', I fell back on just the feed portion of the old Channel Master 7 foot dish.

 

The original design used a pair of wideband ('bat wing') dipoles, fed in parallel from a common point between the two antennas. The batwings were a part of a large sandwich - in one direction the 7 foot 'sort-of parabolic' signal reflector, and on the opposite side a screen grid reflector which turns the bat wing stacked dipole into a two-element broadband pickup antenna. The reflector in this case acted to prevent direct signal pickup by the bat wing dipoles, restricting it to capturing signal that landed on the 7 foot reflector surface and then focused to the dipole array.

 

There would be room for the double bat wing plus the grid reflector to be mounted clandestinely partially hidden by a roof skylight, on a 3 foot stubby pole.

 

 

The long ago abandoned CM 7 footer was no longer available so cannibalising it for the feed was not an option. However, I had carefully created a clone of the original substituting larger diameter brass for the bat wing stacking lines, and more brass in place of the original CM aluminium and steel segments found in their bat wing dipoles and the reflector backplate.

 

            The Channel Master original also provided twin loops to allow connection of a 300 ohm feed line (now - those were the days!), which I eliminated in my clone by structuring direct solder-to points for a standard CM 0089 balun (producing a 75 ohm output at the antenna stacking line connection).

 

What this new antenna would do is allow me to not only carefully point the 'array' for maximum signal but of greater importance, rotate the full backplate plus twin bat wing structure looking for the cleanest signals.

 

To the roof

 

The concept is that by carefully 'balancing' the physical elements of the new antenna, it would have an exceptionally clean pattern with a very high rejection of non-desired polarity signals. The best Sutro signals were found to be off path when the back plate plus bat wings were rotated 20 degrees off of horizontal. I tightened the bolts and climbed down reasonably confident that +20 dBmV on the weakest signal was going to be more than enough to do the job. Well, it did; almost. As domineering as the Sutro location is, there are other locations where TV (FM, two-way radio, cellular phones) fill the available ground space. And unfortunately CBS affiliate KPIX (channel 5 on analogue, channel 29 on digital) was not on Sutro. I had used their signal as a reference while on the roof with the spectrum analyser, aware that if they were clean for any of the adjustable receive antenna settings, the rest should also be (being closer to us and much stronger). I was around 99% correct but the last 1% would cause our client to insist "You fix that if you wish to be paid!". Right on. A digital signal that burps and "tiles" even if only say once an hour for two seconds was, in this customer's mind, unacceptable. Billionaires can be funny, that way ("if you throw enough money at a technical problem, it can be fixed" - the great American mindset!).

 

And there was one more piece of unfinished business. In the wonderful world that is American broadcasting, the San Francisco NBC channel had been replaced by what we in the USA call "an independent station" which means no network programming, just lots of sports, movies and syndicated programming. This caused quite a furore in San Francisco but as the city has something like 77% cable TV penetration and a companion high take-up of the Ku-band DISH and DirecTV services, NBC was "missing" not even for minutes; cable and Ku quickly found a substitute NBC channel to deliver to customers.

 

It happened to be in San Jose, 60 miles south of San Francisco, which meant it is at best a 'near-fringe-area' signal into 'The City' proper. Yes, the client wished it as well, but we had to remember the number two rule - "No antennas visible from the property". Now, a UHF antenna is easily nestled in the palm of your hand, and becomes invisible if held behind your back. A channel 12 VHF aerial, slightly taller than the average person when created for fringe area reception, is a much more difficult object to 'hide'. With some very precision planning, a 6 foot stub mast could replace the original 3 footer, the Blonder Tongue BTY series single channel yagi could be mounted just to clear the roof and point towards San Jose. The UHF antenna I created would nestle on the same mast. Alas, I was relearning the lesson so many DVB-T installers have also learned - signal 'level' is not the name of the game.

 

I admire the skills of people such as Unaohm who have created those marvellous satellite plus terrestrial meters which provide the wide variety of measurement numbers which a successful installation now depends upon. But such instruments, unlike in Australia, are very rare in the USA so I had to improvise. With each trip up onto the estate roofline my concerns about ending up in a heap on the ground were diminishing so the quick solution, under the now mounting customer pressure to "Get it right, now!", was to haul a Motorola DSR550 IRD plus a 13" TV monitor up there to the peak. This Motorola IRD has a built-in ATSC (which is the USA version of DVB-T) tuner and if you can navigate your way through as many as ten layers of menu, some very useful information appears on the screen. One is signal level in dBmV and another is "noise" which of course is the opposite of signal, but in a very clever way. With this IRD, anything that is not 'ATSC digital TV' is considered 'noise' whether it comes from analogue sources, manmade interference or instrument overload.

 

Finally, I had the tool required. For failing KPIX digital, I discovered that while my spectrum analyser had not 'lied' when it directed me to twist the polarity of the modified bat wing antenna 20 degrees, it was measuring more than the KPIX signal. By watching the DSR550 noise readout, I saw where by further rotating the bat wing antenna from 20 to 45 degrees, I picked up 14 dB of signal to noise ratio with only modest compromises in the already over powering Sutro site transmitters. KPIX would never 'tile' again.

 

For NBC digital on VHF channel 12, the Blonder Tongue yagi's sharp narrow front beam was a wonder. Just a few degrees either side of the dead-on heading for the distant transmitter and Sutro's powerful VHF signals wiped out NBC. But when carefully adjusted, the Sutro 'noise floor' went away magically allowing 20+ dBmV to sneak in on the desired signal.

 

What we learned

 

A VHF cut to channel yagi, a carefully honed broadband UHF antenna - both precision adjusted for peak performance even if the UHF antenna 'looked' like it would be skewed - married in a VHF - UHF combiner and run through quad shielded RG6 completed the job. As prior attempts by other installers had revealed, while this is no longer an analogue world, there are still basic laws of physics which will not change.

 

Too much signal, especially when it seems to arrive from multiple directions simultaneously, is a major challenge when installing a receiver system nearby to the transmitter 'antenna farm'. Signal polarity, which may begin as carefully engineered horizontal, will vary greatly at even close-in receive sites. And, digital STBs or receivers are very much at a disadvantage when faced with a multiplicity of strong signals (especially when half or more are the one-day-to-be-retired analogue variety).

 

In this client's case I batted 98% in the end. The missing two percent? Before leaving I roamed through the grounds of the estate, peering at the roof point where I knew the antennas had been 'hidden'. I found one location, about as large as you would create by standing still and raising your arms straight out, where if you held your head 'just so' you could see one element for the larger than life VHF antenna.  

 

Of course this 'antenna view hotspot' requires you to be at least 5'10" tall, and be standing off the manicured pathway between some prickly plantings. I doubt many people will ever notice it!