EEVblog #569 - Tour of an Analog TV Transmission Facility

[noisebox]

Awesome video, two fair dinkum guys, it's great when you've got some company in your videos, especially like the videos with Doug Ford aswell. Is everyone in Oz so nice and friendly?  The other Dave really seems to know his stuff here, shame to be losing this part of the job!

[hikariuk]

The ERP will be much, much more than 10kW.

Check out Melbourne's channel 10 transmitter...

More modern, but has NEC stuff in there, too.

Quiet, isn't it?  You can kind-of see why they've gone over to water cooling.

[SeanB]

Easy to keep the dust out. You start with a great big inlet air filter that __WILL__ trap all dust and particles larger than 0.2 microns, then use an electrostatic precipitator and another filter to get the fine stuff stuck together and dumped into the second filter. Then you chill it or heat as appropriate to get the right temperature and humidity and feed it into the facility so as always to maintain positive pressure in the area. A controlled vent to manage the air flow and pressure and there you have an area that remains dust free. You can recirculate the air to reduce the need for conditioning, but you still basically change the inside air x times per hour while keeping it clean, dry and dust free. Visitors have a big sign to keep the doors closed and a big set of mats on the way in to get dirt off shoes.

[rolandpenplotter]

Don't forget your spanner...

[kg4arn]

Don't forget your spanner...

Look out below if he drops that crescent wrench....

[mikeselectricstuff]

..and don't forget this buttock-clenching classic :

[mixt]

I've freeclimbed tall powerlines before which was a rush for more reasons than just height.. but that video above makes me unbelievably uneasy. I think there is one factor in play though - the helmet cam is using a very wide angle lens. Your field of view in real life wouldn't be as wide or distorted, and it would be a little less scary. A little. My main concern would honestly be the structural integrity of small parts that he's grabbing. Rust and all that.

[rolandpenplotter]

You couldn't pay enough money for me to do that, even if you emptied the world banks into my account, NO WAY.

[Zad]

For those people complaining about a lack of detail - look how long this video is already. What would you take out to add something else? You could have a 10 hour video and still miss something. The 2 Daves did perfectly fine! Talking of which, Dave - when you do a tear-down of the gear, might it be possible to invite the other Dave along to the lab? With the best will in the world, you know a lot about a lot, but you aren't a radio bloke, and an experienced eye could add quite a bit. 

I think this is one of those videos that people will watch over and over as it gains awareness through the ham radio communities. It isn't often that people get to see big infrastructure installations like that. I would have loved to have seen inside my local transmitter (Emley Moor) when they dropped the analogue 5* 870kW ERP - no idea what the TX antenna gain is, but that's a lot of welly (technical expression).

I know it is all filtered positive pressure air, but wow, that really is incredibly clean. Normally HV really does pull all the minute particles out of the air and deposit it in obscure places. Aesthetically that hardware looks fantastic. To an RF-head like me, it is comparable to high Victorian engineering like Papplewick.

Incidentally, here in the UK, main transmitters tend to be vertically polarised, with repeaters being horizontally polarised. Obviously for channel separation, and so the consumer has better rejection of distant co-channel transmitters.

[IanB]

Some may have seen these videos by following YouTube suggestions, but I am watching with my jaw resting on the floor:





When I reached the point in the second video where there was a single turn air cored variable transformer transferring 250 kW from primary to secondary my head exploded. At some point I think RF engineering becomes indistinguishable from magic...

[IanB]

"This is an audio pre-amplifier. We want to get a voltage swing of about 1000 V p/p on the output of this amplifier..."

[Zad]

This is pretty much what I imagine the engine room of the TARDIS to look like 

That transmitter is approximately the right age for Dr Who too!

[IanB]

That's not a power supply. This is a power supply...



"We're looking to get 11,000 V DC at about 50 or 60 A, so about 600 kW of h.t. supply..."

[vk6zgo]

How long do those big-ass transmitter valves last?

20000 hours plus!

[SeanB]

Reminds me of when i was an apprentice and GI Joe comes in and says he has to change the signage 30m up on the sides of the hanger. I asked how, and he replied that "we" would be using the mobile crane. I asked what harness and got the reply back - cargo sling. Asked about safety harness and - none available. I said that I would bring one in the following day, helps having a dad who was a Mechanical and Civil engineer, and who did a fair amount of climbing up structures to inspect, so had the right kit for himself. Next day I was swinging at the end of a crane, with an air drill and a long air line on one shoulder, a rivet gun in pocket, a lot of spare drill bits and a box of rivets in the other. Plus a bag with a dozen signs as well. Safety harness clipped to the hook as well, when I was on the top of the framework i clipped to the steel structure and started with the sign work. GI Joe joked that he would go to lunch and leave me there, I replied I know where he parks his car and would go and attach the signage to it when I climbed down the structure. He had a late lunch......

[vk6zgo]

It really must have felt weird - after decades of doing everything humanly possible to keep those machines transmitting, to stand by and watch them just get turned off. And over the next few months, seeing all that equipment unceremoniously loaded into recycling bins. Knowing that you could let Dave just reef out a few boxes and load them into his car must have felt strange enough!

I must admit I was surprised - I expected that the signals would be combined at signal level and amplified together - I guess that amplifiers of this size just can't be made to be wide bandwidth!

Yes,this can be done,& was by some manufacturers.
The problem isn't bandwidth---it's intermodulation!
It can be overcome,but is messy enough with high power Transmitters,that the approach used by NEC is just as cheap,& eliminates the problem entirely.

[TheWelly888]

Another one here who appreciates the tour given by David Kilpatrick - thanks David for showing Our Dave around!  I admit that high power RF engineering went a bit over my head but the 5kW valve was actually smaller than I thought it would be!! High Power RF was one of the few areas where valves were still superior to silicon transistors back in the 1980s.

[SeanB]

Linear amplification is difficult if you want high power and a wide bandwidth along with power efficiency. Thus they are all amplified separately, filtered then combined in a separate mixer with notch filters on the inputs to attenuate the signal from the other chain. This is done in the massive cavity resonators which are tuned to attenuate the signal other than the transmitted band and then they are all mixed together. This also allows for expansion, like adding the NICAM stereo signal ( still sent here though the number of sets with the decoders is vanishingly small) to the transmitted signal, or to have a ZWEITONE stereo pilot and the associated second audio difference channel as well as the mono audio. Also allows use of different audio carrier frequencies and offsets, along with also allowing for different modulation methods.

High power amplifiers are typically not going to be more than about 1% broad with any efficiency, though they often can handle the digital signal quite well with only a reduction of power and broadening of the tuned circuits slightly to allow this. The only issue is that a class C amplifier will happily mangle higher than QAM signals quite well, though many are capable of being tuned to handle QAM very well. High power gain but not exactly monotonic, but pretty good at converting AC power from the mains into RF power fed up a coaxial cable to the antenna.

BTW high power valves are still cheaper than silicon, imagine doing a changeout of 500 power MOSFETS in a blown transmitter versus changing the faulty component in the tuning side that just caused the valve to glow cherry red until the thermal cutout tripped the unit.

[vk6zgo]

When I was trainee Technical Officer with Telecom Australia , ( Telstra now).  Back then they maintained the transmitters for the ABC.

They took us on  tour of  3AR/3LO AM transmitter out at Sydenham and the ABC TV transmitter on Mt Dandenong. 

They had two radio transmitter that were fed to combiner that was in a hut at the base of antenna. The feeds were open air, and the birds used to land on the feeds. All you could see of the bird, were their feet.

Similar at 6WF/WN.which had a 55 kW (WF) & a 10 kW fed to a combining unit at the bottom of the "Dual Mast".
They were both fed by "6 wire lines",which consisted of 4 outer conductors which were at Earth potential,& 2 parallel "centre conductors"(not sure what the idea was,as both the inners were at the same potential all the time)
It was effectively a "pretend" coax!


The birds would perch on the outer wires,get curious,& peck at the centre conductors.
The Transmitter would "grunt" & recycle.
Next time we looked at the feeder we would see a couple of bird feet!

They were real Transmitters---you could walk around inside them!
Their replacements just look like TV Transmitters from the outside,with big Coaxes coming out of the back!

[Steffen]

That was a nice video Dave & Dave. From my small experience with high power RF equipment like klystrons, waveguides and components, amplifiers and auxilliary stuff like high voltage power supply for andode and some misc. stuff for filaments it was very interesting to know how such kind of plant is constructed.
Any data how rigid coax compares to traditional waveguides? Sure waveguides at that frequency are large and could be more challenging to keep it compact. As I know Size WR650 type flanged waveguides very well (f₀ 1,3 Ghz) it can be quite challenging to match lenght and phase when using parallel lines. But for few hundred KW CW it could be nice to have classic coax connections.
Looking forward to new RF plant videos.

[vk6zgo]

Linear amplification is difficult if you want high power and a wide bandwidth along with power efficiency. Thus they are all amplified separately, filtered then combined in a separate mixer with notch filters on the inputs to attenuate the signal from the other chain. This is done in the massive cavity resonators which are tuned to attenuate the signal other than the transmitted band and then they are all mixed together. This also allows for expansion, like adding the NICAM stereo signal ( still sent here though the number of sets with the decoders is vanishingly small) to the transmitted signal, or to have a ZWEITONE stereo pilot and the associated second audio difference channel as well as the mono audio. Also allows use of different audio carrier frequencies and offsets, along with also allowing for different modulation methods.

High power amplifiers are typically not going to be more than about 1% broad with any efficiency, though they often can handle the digital signal quite well with only a reduction of power and broadening of the tuned circuits slightly to allow this. The only issue is that a class C amplifier will happily mangle higher than QAM signals quite well, though many are capable of being tuned to handle QAM very well. High power gain but not exactly monotonic, but pretty good at converting AC power from the mains into RF power fed up a coaxial cable to the antenna.

CH7's TV Tx is within 1dB up to 7Mhz wide--about 3.8% .
Ch2 is more like 10%.

Remember,TV finals normally have fairly moderate power gains,of around 13dB.

Class C?-------No TV Tx built since the late 1960s use class C output stages.
These older units grid modulated.the Final PA stage,with much of the  bandwidth shaping for VSB achieved using  high level "Filterplexers" which combined the VSB filter with the sound & vision combiner.

Modern Analog Transmitters include the VSB filter in the IF stage.
The PA stages are linear stages ,but that said,may not be "linear enough" for Digital,especially in the phase
domain.

[vk6zgo]

Another one here who appreciates the tour given by David Kilpatrick - thanks David for showing Our Dave around!  I admit that high power RF engineering went a bit over my head but the 5kW valve was actually smaller than I thought it would be!! High Power RF was one of the few areas where valves were still superior to silicon transistors back in the 1980s.

The same tube is used in 10kW & 13kW Transmitters!

[vk6zgo]

Is that just plain old, garden variety composite video that goes in at the front end there?

If so, is that a raw feed of what you got over microwave from the broadcaster? Or did they perhaps send you something like YC/RGBs/Y-Pb-Pr which got combined in a different rack there at the facility?

It always was Composite video in the old days,as it is a more rugged format than the others,needs only one cable/link channel,& everything is designed for it.

I'm suspicious that it may have been sent in digital form & converted on site in more recent years.
About the time I left TVW7,they were rewiring the Studio for Digial video,so it is probably the same in Sydney.
This would make it more convenient to stick with Digital up the link.

Guys,I'm sorry about this great slather of postings,but it is a field I know quite a lot about!

[mj0rgr]

I really enjoyed this EEVBlog episode: particularly David K's contributions. My understanding is that the signal combiner is, in essence, a cavity tuned to the amplified signals. It would be interesting to learn more about cavities and particularly how the input signals are introduced to the cavity, and the output signals derived: are they electromagentically / capacitively coupled with the cavity acting as a form of low loss faraday cage?   

[RetroSwim]

It always was Composite video in the old days,as it is a more rugged format than the others,needs only one cable/link channel,& everything is designed for it.

Wasn't it all Betacam in those days? They stored video as separate luma/chroma.

I'm suspicious that it may have been sent in digital form & converted on site in more recent years.
About the time I left TVW7,they were rewiring the Studio for Digial video,so it is probably the same in Sydney.
This would make it more convenient to stick with Digital up the link.

I wouldn't be surprised.