Hello all,
There was a recent thread on this board which began as a discussion of TIS stations and then diverted to the subject of AM part 15.
http://www.radio-info.com/smf/index.php?PHPSESSID=3eb70dcc17ae27bd872eb8fede3e8d11&topic=64249.0
I thought if any are interested in continuing the discussion this would be a better forum.
It seems that we have some who post who predict part 15 AM performance based on physics and modeling and others who believe there is no substitute for experience. I think I am in the middle since I believe both modelling and experiment are valuable.
Here's a bit of my background so you can judge whether to read on or not. My first exposure to AM transmitting was when someone gave me an old phono oscillator which consisted of a 78 RPM phonograph with one tube hidden beneath it in a metal cabinet. I discovered that this thing would broadcast to nearby radios and I was hooked. In 1959, I bought a KnightKit AM transmitter and spent many hours having a great time learning about radio theory and getting my hands dirty and burned in the process, as well as having fun broadcasting around the neighborhood. I even had a listener other than me once.
Ham radio was an interest but I lived in a small rural town and could not get any help with the code. All ended well when in 1977 I learned the code and obtained my extra class license.
During my college years, I still managed to play with AM in a dorm and because of my interests and as an engineering student I was appointed and served as the Chief Engineer of the college carrier current station. This was not an amateurish operaton...it operated 24/7 from studios in an affiliated College Conservatory of Music and had several carrier current transmitters spread around the dorms on campus. I designed, built, tested, adjusted, and operated some of their transmitters.
Now, with the kids gone, I spend some time with part 15 AM as a hobby and I have three AM transmitters which I play with. I have designed and built many transmitters; carrier current, part 15 AM, ham radio so I know the smell of solder flux, and yes burning components. I have also done many a range test on AM and have a sense of what can be done with 3 meter AM antennas.
So, for those who are still with me and not bored to tears, to the point of this post.
The theoretical calculations provided by antenna modelling software such as the NEC program establish predicted performance based on assumptions which may or may not reflect a real world installation. However, as with any engineering model, the choice of assumptions will affect the results of the calculations. If one models a 3 meter base coil loaded antenna assuming 100 mW. into the structure, and one assumes a high ground conductivity then the model will produce a prediction of the performance which exceeds that which is reasonably expected in practice. I view such simulations as a target or standard to achieve rather than a limitation on what can be done.
The models cannot predict all factors which might influence AM range such as power lines, buildings, trees, interference, etc. but they can serve as a useful guide in designing and installing a system. I doubt there is any question that the models accurately predict the advantage of a loading coil on a short antenna. Where it seems to break down is where the interpretation of the model predictions doesn't match experience and if what I have read is an indicaton this is mainly due to violating the assumptions of the model. For example, a subject which comes up frequently in discussions of part 15 AM is the ground lead restriction. If a model predicts a certain result based on a simulation with the antenna at ground level over buried radials why is it realistic to expect that the predictions will hold for a transmitter mounted 20 feet in the air with a 20 foot wire to ground? By the way the models do predict the increase in range which is seen with such installations.
In all my experimenting with part15 AM and ham radio, I have never seen a case where the range I achieved exceeded the theoretical predictions unless there was some confounding variable such as having my receiver near a telephone or power line, so I offer my comments that the models are pretty good.
We, as hobbiests, have two nice tools availabe to us: theoretical models and practical experience. We should see them as complimentary and not competetive.
Thanks for enduring this rather long post and your comments are most welcomed.
Neil
There was a recent thread on this board which began as a discussion of TIS stations and then diverted to the subject of AM part 15.
http://www.radio-info.com/smf/index.php?PHPSESSID=3eb70dcc17ae27bd872eb8fede3e8d11&topic=64249.0
I thought if any are interested in continuing the discussion this would be a better forum.
It seems that we have some who post who predict part 15 AM performance based on physics and modeling and others who believe there is no substitute for experience. I think I am in the middle since I believe both modelling and experiment are valuable.
Here's a bit of my background so you can judge whether to read on or not. My first exposure to AM transmitting was when someone gave me an old phono oscillator which consisted of a 78 RPM phonograph with one tube hidden beneath it in a metal cabinet. I discovered that this thing would broadcast to nearby radios and I was hooked. In 1959, I bought a KnightKit AM transmitter and spent many hours having a great time learning about radio theory and getting my hands dirty and burned in the process, as well as having fun broadcasting around the neighborhood. I even had a listener other than me once.
Ham radio was an interest but I lived in a small rural town and could not get any help with the code. All ended well when in 1977 I learned the code and obtained my extra class license.
During my college years, I still managed to play with AM in a dorm and because of my interests and as an engineering student I was appointed and served as the Chief Engineer of the college carrier current station. This was not an amateurish operaton...it operated 24/7 from studios in an affiliated College Conservatory of Music and had several carrier current transmitters spread around the dorms on campus. I designed, built, tested, adjusted, and operated some of their transmitters.
Now, with the kids gone, I spend some time with part 15 AM as a hobby and I have three AM transmitters which I play with. I have designed and built many transmitters; carrier current, part 15 AM, ham radio so I know the smell of solder flux, and yes burning components. I have also done many a range test on AM and have a sense of what can be done with 3 meter AM antennas.
So, for those who are still with me and not bored to tears, to the point of this post.
The theoretical calculations provided by antenna modelling software such as the NEC program establish predicted performance based on assumptions which may or may not reflect a real world installation. However, as with any engineering model, the choice of assumptions will affect the results of the calculations. If one models a 3 meter base coil loaded antenna assuming 100 mW. into the structure, and one assumes a high ground conductivity then the model will produce a prediction of the performance which exceeds that which is reasonably expected in practice. I view such simulations as a target or standard to achieve rather than a limitation on what can be done.
The models cannot predict all factors which might influence AM range such as power lines, buildings, trees, interference, etc. but they can serve as a useful guide in designing and installing a system. I doubt there is any question that the models accurately predict the advantage of a loading coil on a short antenna. Where it seems to break down is where the interpretation of the model predictions doesn't match experience and if what I have read is an indicaton this is mainly due to violating the assumptions of the model. For example, a subject which comes up frequently in discussions of part 15 AM is the ground lead restriction. If a model predicts a certain result based on a simulation with the antenna at ground level over buried radials why is it realistic to expect that the predictions will hold for a transmitter mounted 20 feet in the air with a 20 foot wire to ground? By the way the models do predict the increase in range which is seen with such installations.
In all my experimenting with part15 AM and ham radio, I have never seen a case where the range I achieved exceeded the theoretical predictions unless there was some confounding variable such as having my receiver near a telephone or power line, so I offer my comments that the models are pretty good.
We, as hobbiests, have two nice tools availabe to us: theoretical models and practical experience. We should see them as complimentary and not competetive.
Thanks for enduring this rather long post and your comments are most welcomed.
Neil