TV Antennas

You mean transmitting towers? They're usually in groups because all the channels want to be on the highest hill. The tower is supportive - not energized. Often the amplifier is in a small house at the base of the antenna, and receives the signal via microwave from the TV studio, although there are certainly other schemes as well.

Perhaps you can clarify your question since I am a little confused. You said you saw the familiar TV antenna arrays. Then you went on to say you saw six or eight, and then you asked if the entire tower is energized or an element you couldn't see.

What did you see, towers with nothing on them? or an antenna at the top?

The typical TV antenna is called a Log Periodic and is a wide bandwidth directional antenna. This antenna looks like a bunch of small horizontal rods of varying length on a single boom spaced apart by about a foot.

Other alternatives could of been that the towers had a single vertical antenna at the top which would not of likely been for TV. Another alternative as you point out is that the tower its self was the antenna which would of been for a much lower frequency. You also mention a bunch of towers, and the word array. One scheme is what is called a phased array, where you use a bunch of antennas either horizontal or vertical set up in a pattern. You can control the direction of your signal with a phasing box so you have a directional antenna that doesn't move. It is unlikely that any of these alternatives would have been for TV reception.

Your last question about gain loss, do you mean loss from the transmitting station? or loss of signal down the tower? Regardless the signal does degrade over distance quite a bit which is why you need good antennas to pick it up. By linear amplifiers along the way are you referring to what would be called a repeater that receives and re transmits the signal? If you are referring to TV signals the answer is probably not. If this is some other sort of radio then possibly, but it depends on what it is and what frequency it is.

I hope that answers your questions, with out more specifics its the best I can do. The other alternative of course is that these towers at one time had TV antennas on them, and now were not in use. Since cable and small satellite dishes have come into the market many people in rural areas have switched and just left the tower there.

frankd20 - Thanks for your response. The clarity of my question is directly related to my knowledge of the subject and my confusion.

I was referring to the long skinny towers which I asumed were TV towers. These are much taller than the micowave or cell phone towers and appear very skinny. I don't remember seeing any elements on them (does not mean they were not there) and I have never seen just one (always in groups). So I assumed they were an array of some sort. I see little buildings around the group but not necessarly at each tower. I again assumed that these were connected to some transmitter far away and was wondering about the losses from the transmitter to the antenna and the power that it would take to energize the entire tower.

These sound like they are transmitting towers for an AM radio station. The little buildings do likely contain the transmitter and amplifier. The content is usually sent to the transmitting equipment via a digital signal on a cable or a high frequency link. They are as you point out an array used to direct the signal in what ever way they have been licensed.

I am not sure the range now, however, they used to have a range about 50 miles. Every 50 miles or so was required a retransmition tower. Today the towers are much higher so I am sure they get longer ranges.

As Blankii said, the numerous towers are typically owned by different stations.

Some are, back-up towers. Typically the shorter (older) towers are kept as a backup.

One tower can hold many transmitters and receivers

Here is a great site showing alot of different towers and their transmitters

http://www.well.com/user/dmsml/look.html

Ok, now here is a stupid question! How do you get a directional array from what looks to be just vertical towers? Are they using some towers as passive reflectors (like the ground planes from my anger9 days)?

This is done with the magic of phasing, and can be rather complicated. If you want to know more search google for "antenna phasing". It can be like using one as a reflector but can also be set up in other ways to achieve other directions.

The simplest radiator is a 1/4 wave vertical. It radiates a field that is circular and the equipotential field looks like a donut. That is fine, and it works, but if you are sending out 500,000 watts and all your clients are non the ground a large % of the power goes into the air where there are no viewers. How to solve this? make a 3/8 or 5/8 wave vertical array. This squashes a lid on the donut so more goes close to the ground and less in the air. You can save 50% of your energy this way, and 500,000 watts transmitted at 50% efficiency is 1 million watts power in. This is not cheap. You save 500 Kilowatt hours of electricity every hour and at 7 cents/KWH = $35/hour 24/7. In addition you save tube life etc.

Now if you want just a single beam, if your antenna is on the coast and you want the beah going inland, you make a phased vertical array. Little goes out to see and most goes inland.

Now phased array patterning is a complex craft and stations often make strange arrays to fit their demographics and tune them to fit with guys whi drive aroubg with field strength meters to plot the field.

As always, search is your friend

http://www.google.com/search?hl=en&q=%22phased+vertical%22+%2Bantennas&btnG=Search

http://www.google.com/search?hl=en&q=%22phased+vertical%22+%2Btelevision+%2Bantennas

In the San Francisco Bay Area a lot of our broadcast television comes from Sutro Tower, which has become somewhat of a local landmark. Almost all VHF channels are transmitted from this tower. The control station is located some distance below the active array. It's said that when a technician goes up to the array for maintenance, he first puts on a stainless steel suit of armor. Supposedly to protect him from the strength of the radio wave at the arrays.

How does he see? Or do they just let his eyeballs boil?

military radar techs have these mesh suits as well.

No. We are smart enough to shut the power off first.

The visor has a lot of little holes drilled in it. Each is far lees than a 1/4 wave wide, so no problem.

WOW Guys

Thanks for the good info. This is a lot more complicated than just aluminum foil on my rabbit ears.