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AM Tower question

Ed Nielson

Ed Nielson

OK, this is one of those things I've been curious about for a while. Inquiring minds want to know. :)

We know that touching a live AM tower is bad for you. But if the transmitter is off, how much danger is there from surrounding RF energy? Is seems to me that the tower would essentially be acting as a giant antenna.
 
Ed Nielson said:
We know that touching a live AM tower is bad for you. But if the transmitter is off, how much danger is there from surrounding RF energy?
Click to expand...
If it's near other AM towers, it will pick up energy from their signals and will need to be grounded before it can be safely worked on.
Ed Nielson said:
Is seems to me that the tower would essentially be acting as a giant antenna.
Click to expand...
Indeed, and it can pick up quite a bit of static charge just from the atmosphere.
 
None, especially if nothing else is nearby

This is me leaning against the WKBI-AM 1400 Saint Marys, PA tower while electricity was turned off for transmitter replacement in 2012

516645293_10233000625255295_3729633687855523211_n.jpg
 
I built a modified tuned radio circuit one transistor AM preamplifier from Tom Kneitel's 103 Simple Transistor Projects. I built it on a wooden board and connected it with Fahnestock clips for experimentation and modification. I lived in an area of West Central Michigan for a few years where there was a single AM radio station with 1000 watts about one mile away. The area had very poor ground conductivity. All the rest of the Day signals were well below 0.5 mV/m, and couldn't be heard well on any typical AM radio. The second strongest station was a low band 50000 watt station close to 200 miles away in Chicago, and I estimate about 0.2 mV/m Daytime. I used the twin lead from a 20 foot outside TV antenna and twisted the conductors together. That was the vertical antenna. It was grounded to the center screw of a metal outlet box. It worked great on those weak signals. I coupled the ferrite core output coil inductively to the internal ferrite rod of a Sony portable radio. It worked great. I got about 60 AM stations well in the Daytime. One time it started to arc across the 365 pf air dielectric variable capacitor, and I heard it clicking. There was no storm yet, but I figured that the electric field across the air was exceeding the dielectric strength of the air gap in the capacitors. I quickly removed the antenna all the time when not in use. I think I burned out the Semitronics AT20H transistor once, and I quickly learned to almost always remove the twin lead vertical antenna. I listened to WLS a lot in the Daytime. The operators of the carrier current station nearby were amazed. I picked up the second harmonic of the carrier current station. WIND blocked me from hearing the carrier current station on 560.
 
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Ed Nielson said:
But if the transmitter is off, how much danger is there from surrounding RF energy? Is seems to me that the tower would essentially be acting as a giant antenna.
Click to expand...
As they say, it's not how big your stick is ...

AM broadcast towers are designed to be very efficient at their intended frequency. That usually means the antenna is tuned for its usual frequency. If I got to shut down WWL 870, and use its towers as a receive antenna, it would be good on 870, but there is no one else nearby to receive on 870. The nearest 870 with night authorization is in far northern Colorado, some 1100 miles away. But WWL's antenna probably would not be good to DX 670 WSCR/Chicago.

How tightly tuned a tower is to its frequency will vary a lot from station to station, so I can't put good numbers to it.
 
PTBoardOp93 said:
As they say, it's not how big your stick is ...

AM broadcast towers are designed to be very efficient at their intended frequency. That usually means the antenna is tuned for its usual frequency. If I got to shut down WWL 870, and use its towers as a receive antenna, it would be good on 870, but there is no one else nearby to receive on 870. The nearest 870 with night authorization is in far northern Colorado, some 1100 miles away. But WWL's antenna probably would not be good to DX 670 WSCR/Chicago.

How tightly tuned a tower is to its frequency will vary a lot from station to station, so I can't put good numbers to it.
Click to expand...

I once hooked up a transciever to an AM tower in SE TX... it didnt work very well at all
 
PTBoardOp93 said:
As they say, it's not how big your stick is ...

AM broadcast towers are designed to be very efficient at their intended frequency. That usually means the antenna is tuned for its usual frequency. If I got to shut down WWL 870, and use its towers as a receive antenna, it would be good on 870, but there is no one else nearby to receive on 870. The nearest 870 with night authorization is in far northern Colorado, some 1100 miles away. But WWL's antenna probably would not be good to DX 670 WSCR/Chicago.

How tightly tuned a tower is to its frequency will vary a lot from station to station, so I can't put good numbers to it.
Click to expand...
An AM tower is tuned before the associated station is fed into the tower. If you connect a receiver directly to the tower it will certainly be most efficient where it is close to a quarter wave or even an eighth wave. But it will be a good non-directional antenna up and down the AM band.

I occasionally DXed from the tower of HCRM 570 AM in Quito, Ecuador, after I had finished Sunday morning, preventative maintenance on the station. The real problem was that it took too much work to set up and tear down for only a brief period of DXing. I had to run a coax from the tower into the transmitter building at a length of about 100 m and that took time to place and remove. I finally found that using a box roof antenna from home gave me more options and target DX locations than a non-directional tower.
 
kevtronics said:
If it's near other AM towers, it will pick up energy from their signals and will need to be grounded before it can be safely worked on.

Indeed, and it can pick up quite a bit of static charge just from the atmosphere.
Click to expand...
I have heard the spark gap flashing over on an AM mast where it was disconnected from the transmitter, just from the static charge it obtained from the atmosphere. That is going to hurt if you were to touch it.
 
Ed Nielson said:
OK, this is one of those things I've been curious about for a while. Inquiring minds want to know. :)

We know that touching a live AM tower is bad for you. But if the transmitter is off, how much danger is there from surrounding RF energy? Is seems to me that the tower would essentially be acting as a giant antenna.
Click to expand...
A station I did some work for in the Florida Keys was on an island adjacent to Radio Marti's 100 kW. If I worked inside the transmitter or phasor I would have to put a ground jumper on the antenna circuit. Even with the transmitter off, Radio Marti put enough energy into the system to burn me.
 


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