The Call of the Open Sidewalk

The 12+12 DTV Solution: Implementation

2011-12-14T23:18:48Z

Here are the other articles in this series:

Here is what the antenna looks like:

Doesn't that look great? Pure function...

The support structure is a 10 foot chain link fence top rail bought at the local home improvement store. It is attached to the roof with a tripod sold as an antenna support. The top antenna points at channel 13. The middle antenna points at channel 7. The bottom antenna (low gain UHF band) receives channels 27, 35, 40 and 51 and points between the transmitters. The cables are held to the mast with multiple wraps of black vinyl electrical tape, which seems to be the local custom.

Here is the place where the signals are combined:

Here is the place where I ground the antenna:

The requirements for grounding things like TV antennas are different from place to place. If you can't manage to ground it exactly as required, don't just give up. Ground it as best you can. You should ground the outside conductor of the cable where it comes into the building as well. You can buy a thing called a grounding block to make this convenient.

As the digital TV conversion is finally complete in my area I can provide results:

Channel	Strength	Quality
7	90%	100%
13	87%	100%
27	86%	100%
35	80%	94%
40	94%	80%-100%
51	63%	75%

Strength is proportional to the level of the signal. Quality is a measure of how certain the demodulator can be that a particular bit is a 0 or a 1 (higher is better). Channels 7 and 13, each with a dedicated high gain antenna, are as would be expected quite good. The rest of the channels are fairly good with the exception of channel 51. I have no real explanation for this. The transmitter is less than 3 km away. The quality of channel 40 is fluctuating. That is probably due to either noise or a reflection that is changing faster than the receiver can compensate.

The 12+12 DTV Solution: Design

2011-12-07T21:04:57Z

Creating a usable over the air (OTA) TV antenna system is one of those things that usually involves a significant amount of experimentation ... but you have to start somewhere.

A good place to start is with the worst signal you want to receive. Here is the important bit from my TV Fool report (as discussed in a previous post):

The farthest transmitter in our list of desired channels (7 13 27 35 40 51) is channel 13 at 41km (25.2 miles). Applying a realism filter to the claims of the people that make TV antennas, we come up with a requirement for some sort of medium gain antenna. How about we specify an antenna that does both our bands of interest and just point it at the channel 13 transmitter? This, by the way, is all we have to do in most situations. Deal with the worst and hope for the rest.

We would then have the close 27 40 and 51 directly behind the transmitter. Good. Most antennas work reasonably well to the rear. We unfortunately have 7 and 35 that are not so close and they are pretty much at right angles to the antenna. Most antennas are quite good at rejecting signals to the side. The solution here is to rotate the antenna so it points between 13 and 7/35. Lower gain antennas are better at receiving stations over a wider angle so we might be better off downgrading to a low gain antenna ... an unhappy but common compromise.

If I was in a more rural environment I might be done. I am instead in what might be termed a high density suburban area. My neighbours all have cable and do not have to bother with interference caused by their electronics. I am not blameless with respect to interference either. Low gain antennas are also good at receiving noise over a wider angle. I used to use a single low gain antenna in the analog days and had to live with all the received noise.

We go on ...

Before we unleash the dogs of complexity we might want to think about our available assets. I just happen to have a couple of high gain recovered antennas that are optimized for channel 12 and were expected to work reasonably well on channel 7.

Channel 7 is on our list. So that just works. Channel 13 is only a few percent different from channel 12 in terms of radio frequency. It would have a lot of gain when used on channel 13. Unfortunately the 13 and 7 transmitters are are at right angles so a single high gain antenna is the last thing we want here.

We have two antennas available. I didn't pay anything for the antennas. Why not point each antenna directly at a transmitter?

In the analog days the answer to the proceeding questions would of been; ghosts otherwise known as reflections. The antenna pointing in the wrong direction for the desired channel would receive reflections quite well. Special filters were required to eliminate these reflections. Digital TV is very good at cancelling out these reflections (relevant post). As a result, multiple antennas can work quite well here in the 21st century.

You can combine the signals with a splitter/joiner:

If your antennas are pointing in more or less the same or opposite directions you should make the cables between the antennas and the joiner the same length. That is in the hope that the two signals will add together rather than cancel. In our right angle case it doesn't really matter. If you have matching transformers on one of both of the antennas you should reverse the antenna connections on one of them to see if things get better.

That takes care of channels 7 and 13 leaving 27, 35, 40, and 51. Here we finally get lucky. With 7 and 13 being in the VHF-hi band and 27-51 being in the UHF band we can use a band separator/joiner. In this case we want a UVSJ (UHF, VHF). The band separator/joiner will completely isolate the 7 and 13 antennas which allows us to consider the problem of receiving 27-51 with no consideration of what we have done so far. Channel 35 is the only remaining transmitter that is not right next door so we just use a low gain UHF band antenna with the hope that we can find a magic angle that will work for all 4 channels. This might work because the UHF band is at a higher frequency where there is less woman made noise.

We end up with this:

This was a very specific example. The point here is that this can be a reductionist process. That can make this a lot of fun for those that are into that type of thinking.

We now have a configuration we can build and try out...

Circadian Light: SF

2011-12-01T17:48:54Z

Much of the earlier habits of Earthly society have been given up in the interest of that same economy and efficiency: space, privacy, even much of free will. They are the products of civilization, however, and not more than ten thousand years old.
The adjustment of sleep to night, however, is as old as man: a million years. The habit is not easy to give up. Although the evening is unseen, apartment lights dim as the hours of darkness pass and the City's pulse sinks. Though no one can tell noon from midnight by any cosmic phenomenon along the enclosed avenues of the City, mankind follows the mute partitionings of the hour hand.
The expressways empty, the noise of life sinks, the moving mob along the colossal alleys melts away; New York City lies in Earth's unnoticed shadow, and its population sleeps.
The Caves of Steel, Isaac Asimov, 1953

OK, here we are living in the future. Others have written at length about the absence of flying cars but what about wildly artificial environments? Space habitats? Underground cities? Huge isolated skyscrapers? Floating cities? Cities at the bottom of the ocean? Any sort of archology at all? The sort of places that the geeky set can have adventures in (or at least be respected for their ability to function in high tech environments). A science fiction cliche. The only reason Asimov bothered to describe his Cities in any detail was because he wanted to do a detective mystery there. We all knew what he meant.

Classic science fiction about such places can now be considered to be incomplete in light of our recent knowledge about circadian light. The required means of regulating circadian light and the associated rules would change a lot of narrative.

The interesting real life questions are:

Does the circadian light idea allow us to create entirely artificial environments?

and (less important)

Does the circadian light idea allow us to improve the sort of indoor environments we face today?

Unintended Consequences

2011-09-06T17:58:36Z

I sold my car the other day. I wasn't really using it. This has forced me to deal with some day to day issues in a different way. Every month or so I would take the car to a large supermarket and load it full of all the heavy stuff I did not feel like carrying home from a more local store. This is no longer possible.

They make something for this application. A classified ad web site and $5 got me this:

It worked. It lacked both the capacity and the load carrying capacity of my car trunk. The solid wheels meant that I had to pay attention to avoid having the cart hit discontinuities in the pavement. Having to pay attention to the surroundings seriously detracts from a walk. It also had an associated old lady vibe. I am an old man. That is entirely different.

Here is my current grocery hauler after a triumphant trip out and back:

It is rated at 360 KG (800 lb). The pneumatic tires simply glide over discontinuities in the pavement. Once you get it up to speed you can hardly tell it is there. This is how a guy gets the groceries home from the store.

The wagon shown was originally sold as something to help with the incomprehensible things people do to interfere with the plant life in their yards. As a result it would be quite reasonable for people to assume that my super hot and high maintenance trophy wife had simply taken the BMW and left me with the shopping.

On the way home from my first shopping trip with the "garden cart" I had a realization that made me feel a little less superior to conventional shopping cart users. A light, two wheeled cart was simply essential to grandma's mission. The environment had changed in a significant way and I had not noticed. Grandma had to deal with curbs.

The local disabled community fought long and hard in the battle against curbs. Today there is not a single significant discontinuity from my stairs to the aisles of a supermarket 1.4 km away. That is kind of remarkable and represents a triumph. I now feel a bit bad about sitting out the conflict.

The morals to this story:

Sometimes different communities have more in common than they realize.

Some unintended consequences are good.

The pneumatic tire is a really clever and significant invention.

The 12+12 DTV Solution: Gathering Information

2011-08-19T03:48:00Z

OK, what do we need to know to design our over the air (OTA) TV antenna system?

What channels do we wish to receive?

There will likely be compromises. It is a good idea to spend some time thinking about what sort of compromises you would be willing to make. If you are doing this entirely for the technical challenge, then the answer to the question might be "All the channels that are humanly possible to receive from my location." If you are all or partially doing this for the TV then you will likely end up with three lists; Mission Critical (I need these to consider doing the project), Optional, and Don't care.

Where are the transmitters for these channels?

This is normally a matter of public record. The appropriate government agency web site will likely have the coordinates of the local TV transmitters that you can plot on a map. You want to have some idea of where you have to aim the antenna.

How strong are the signals from these transmitters?

The signal gets stronger when; the transmitter is closer, the transmitting antenna is higher, the transmitted power (measured as ERP in Watts) is greater. Signal strength is hard to predict so this is going to be a guess even with the best available information. If you have any TV antenna using neighbours you can ask them what channels they see. Online OTA enthusiast forums can be sources of reception reports for your area. In general you need larger antennas placed higher above the ground to receive weaker signals.

In places with enlightened government agencies that provide TV transmitter data in a usable form people will create web sites that display this data in a way most useful to those messing around with TV antennas. As an example, my favourite site of this kind is TV Fool. After giving the site my location it gave me this:

This site does some sort of modelling and comes up with a signal strength as well as a direction and distance. This signal strength information can be quite handy even if it is not entirely accurate but for the purposes of this example I will ignore it.

What channels can we expect to receive? The people who make TV antennas often claim that their very best efforts can receive signals from as far away as 100km. This is under conditions few will ever see but it makes a nice extreme limit. The distances given on the site are in the delightfully quaint units of "miles" so I will translate into something a little more mainstream as I go along. There are 6 transmitters within 100 km on channels 7, 13, 27, 35, 40, 51. Annoyingly, KNRR on channel 12 is more or less exactly at the 100 km distance (63.2 mi). I happen to know from the reports of others and personal experience that this is a very difficult signal to receive at my location ... so we will drop it. Receiving such a marginal signal is well beyond the level of this discussion anyway. We will place all 6 channels on the Mission Critical list to keep things challenging.

There is a tendency to locate TV transmitters that serve a particular area in more or less the same place. That makes it possible for everyone in that area to point a single antenna to receive everything. Let's see how they did in my area...

So ... not so great. There are transmitters directly to the east, transmitters directly to the south, and channel 13 by itself directly to the west. This is starting to look bad. But let us now group the transmitters in terms of band to get an idea of what sort of antennas we might need. We only have two bands in use. VHF-hi (channels 7 and 13) and UHF (channels 27, 35, 40, 51). That's good. Let us now consider each band separately. On VHF-hi we have channel 13 directly west and channel 7 directly south. In other words, at right angles. That's bad. On UHF we have channel 35 pretty much at right angles to all the other channels. This is also bad but I note that the UHF transmitters other than channel 35 are only about 3km (1.7 mi) away. At that distance it is unlikely that we could do anything to prevent the reception of those channels.

We now understand the problem. We must have enough information...

Edit: channel 13 -> channel 35
Edit: removed extra period