For Part 15 transmitting in the AM band, does the diameter of the antenna matter?
I am interested in the electronic side of things. Would a larger diameter antenna be more broadband? Is there any transmission efficiency gain using a large diameter pipe over a piece of wire?
If I had a 9’ tall 12’ diameter metal tube on end, insulated from earth, I could drive it from the bottom as an antenna, but if they showed up, how would the FCC measure that, would they measure the 9’ from the ground to the top of the tube, or would they measure the diameter of the tube?
The diameter of the antenna will affect bandwidth if the radiating element is in a resonant state but in practice the bandwidth of the antenna on audio response isn't noticeable.
Ground loss generally dominates in the transmission efficiency so any effects from the radiator diameter will not matter.
Regarding antenna dimensions as you cite it would be up to the inspector.
Neil
Is there any transmission efficiency gain using a large diameter pipe over a piece of wire? etc.
The radiation resistance of an electrically short monopole antenna relative to the other antenna system resistances such as the ESR of the loading coil, and the resistance of the r-f ground path will determine the radiation efficiency of that antenna system.
The radiation resistance of an electically short vertical monopole depends on its height, the operating frequency, and the location of its feedpoint and loading coil above the ground plane.
The diameter of that monopole, itself, has relatively little affect on antenna system radiation efficiency for typical Part 15 AM transmit installations at the carrier frequency and its useful upper/lower sidebands.
But how an FCC field inspector would judge the §15.219(b) compliance of a base-fed and base-loaded vertical monopole with a height of 9' and an OD of 12' would be difficult to predict.
Is it true to say...
.... that measuring the audio response received on a radio using different widths of transmission antenna would give a picture for the bandwidth differences as physical transmission widths increase (?)
I will probably experiment with this.
FURTHER, from previous readings I seem to recall that narrower width can translate to better distance coverage, whereas wider-width antennas, although better for audio response, are compromised as to distance.
... I seem to recall that narrower width can translate to better distance coverage, whereas wider-width antennas, although better for audio response, are comprimised [sic] as to distance.
With respect to r-f bandwidth and for a given radiated power -- and even if the transmitted bandwidth is infinite, it is the bandwidth of the receive system that determines the intelligibility of a given modulated waveform, along with received field intensity, local r-f noise, and interfering signals from other transmit systems that are present in that r-f spectrum.
There is little point in AM transmissions having more r-f bandwidth than that permitted by the receiver, and/or receiving conditions.
For most consumer-level AM receivers, that r-f bandwidth is less than 10 kHz, which limits their audio output reponse to less than 5 kHz.
What you say (Rich(F)) about the receiver's bandwidth being the determinant with regard to radio reception, and not the bandwidth of the transmission antenna, I comprehend and agree.
But, going further, and referring back to another detail of my post, would it be true that a narrow bandwidth, slim-width, transmitting antenna would project farther all things not withstanding (?)
... would it be true that a narrow bandwidth, slim-width, transmitting antenna would project farther all things not withstanding (?)
If by "project" you mean that a "slim-width' Part 15 AM antenna system would radiate usefully greater fields than a fat one at/near its resonant frequency for a given applied power and other things equal -- that belief is not supported by science and experience (sorry).
O.K. then, I believe you, Rich(F), but I still have a problem to carry in life...
Something gave me the notion of the opinion I had, and now I need to reverse my knowledge to dismiss those incorrect thoughts and replace them with the new revelation from today.
My "True" and "Not True" columns must be updated.
For say, a copper pole, vs. a wire?
Probably negligable here???
Best wishes to all
Bruce
IMHO the only things one would gain ( no pun here) with a fat antenna for amplitude modulation is bandwidth both AF and RF.
Skin effect is determined by the wire or pole diameter, or, more correctly, the surface area of the radiator. This is included as part of the formulas I've seen for determining antenna efficiency at a given frequency. Some think it won't make much difference, but at such incredibly low power as we use, I think every tiny bit more we can achieve in anything will make some difference. This is why I'm building a cage monopole, along with some other reasons.
Having said that, yes, the most important feature for, in particular, AM radio, because it is daytime groundwave-based, is ground conductivity. With these micro-watt transmitters, good ground system for them to work against, along with ERP transmitter efficiency, antenna peak resonance, and location, are the most critical things.