Part 15 AM System Calculator

[rfry]

Lately I've been working on an Excel spreadsheet to calculate Part 15 AM system parameters (see http://i62.photobucket.com/albums/h85/rfry-100/Part15AMSystemCalculator.gif.

If anyone has any suggestions/comments I'd be glad to get them. My email address is shown in upper right corner of the linked image.

When I'm finished with the spreadsheet I may post it for download.

Thanks.

 

[Black_Shire]

Thank you for taking the time to put together the spreadsheet. It is a simple and "intuitive-to-use" analysis tool that I think will be very useful to Part 15 AM experimenters & broadcasters as well as Part 15 AM MedFER beacon operators.

For determining the loading coil resistance, what coil size did you use? It varies pretty widely among available Part 15 AM transmitters, from axial RF chokes (in kit transmitters such as the Vectronics VEC-1290K) to toroids (the Rangemaster and the old Panaxis design use/used them) to air-core solenoidal coils (the TalkingSign TS100) to slug-tuned ferrite loopsticks (the Talking House TH II, which has two of them [just one is used for frequencies above 1000 kHz]).

When you selected 25 ohms for the typical RF ground resistance, what kind of ground system were you assuming (one 8' ground rod, twelve 3 meter buried copper radial wires, etc.)? Metal rooftop, vehicle roof, and boat (especially metal boat) installations should be able to reduce the ground resistance figure significantly. Two or three "crossed rolls" of chicken wire laid out on the ground (with a radius about equal to the antenna height) is also an easy and effective way to cut the RF ground resistance.

The only parameter I would disagree with is your chosen transmitter output power. The Part 15 AM kit transmitters (and toy AM transmitters such as the "Wild Planet Radio DJ") do tend toward the low end of the power output range as you indicated, but the commercial-grade transmitters (Talking House, TalkingSign, LPB AM2000, Rangemaster, etc.) usually operate at 80 mW - 100 mW output as adjusted at the factory. The Rangemaster can actually be adjusted (without "souping it up" in any way) to put out slightly more than 100 mW, and it has a power output meter so that the user can make sure it is radiating within the legal limit.


-- Black Shire

 

[rfry]

For determining the loading coil resistance, what coil size did you use? (and) When you selected 25 ohms for the typical RF ground resistance, what kind of ground system were you assuming.

I had to choose some specific parameters to load the spreadsheet example I posted -- not to imply that those values are sacred, or even (necessarily) typical.

The spreadsheet permits entering whatever values apply to a given installation. Knowing what those values truly are takes a fair amount of education, experience, skill and instrumentation to determine.

But even if those values are unknown for a particular installation, the spreadsheet will give some direction toward the understanding and planning of Part 15 AM system performance, including its inverse distance, groundwave field strength at 1 km, and another (user-defined) range.

As for the allegation that the Rangemaster is capable of 100+ mW of legal r-f output power -- are you including the AM sideband power? I'm sure you know that the DC input power producing the unmodulated carrier is limited to 100 mW, per FCC Part 15.219. And no circuit or other device is more than 100% efficient.

Amplitude modulation of a legal Part 15 AM tx can produce output powers that exceed 100 mW, but that is not exclusive to the Rangemaster. That can be the result of the amplitude modulation of a carrier produced by 100 mW of DC input power to the r-f output stage of any legal Part 15 AM transmitter.

For example, if any Part15 AM tx was set for 100 mW input power for its unmodulated carrier, and its r-f output amplifier was 80% efficient (probably on the high side), then its total r-f output power when modulated +/- 100% by a sine wave would be 1.5 x 80 mW = 120 mW.

//

 

[kc8gpd]

For example, if any Part15 AM tx was set for 100 mW input power for its unmodulated carrier, and its r-f output amplifier was 80% efficient (probably on the high side), then its total r-f output power when modulated +/- 100% by a sine wave would be 1.5 x 80 mW = 120 mW.

I have 2 prototypes here made by Jack Gittings that are 95% Effecient.

 

[Black_Shire]

R. Fry wrote:

<As for the allegation that the Rangemaster is capable of 100+ mW of legal r-f output power -- are you including the AM sideband power? I'm sure you know that the DC input power producing the unmodulated carrier is limited to 100 mW, per FCC Part 15.219. And no circuit or other device is more than 100% efficient.>

The Rangemaster can put over 100 mW (but not by much, perhaps 1 mW - 3 mW at most) into the final RF stage (for unmodulated carrier).

That feature surprised me as well, but it does allow the transmitter to be adjusted for the maximum legal field strength under the Part 15.209 rules.


-- Black Shire

 

[rfry]

The Rangemaster can put over 100 mW (but not by much, perhaps 1 mW - 3 mW at most) into the final RF stage (for unmodulated carrier). That feature surprised me as well, but it does allow the transmitter to be adjusted for the maximum legal field strength under the Part 15.209 rules.

As I recall, the tx output power needed with antennas even shorter than 3 meters is much less under 15.209 than under 15.219, isn't it?
//

 

[Black_Shire]

The Rangemaster can put over 100 mW (but not by much, perhaps 1 mW - 3 mW at most) into the final RF stage (for unmodulated carrier). That feature surprised me as well, but it does allow the transmitter to be adjusted for the maximum legal field strength under the Part 15.209 rules.

As I recall, the tx output power needed with antennas even shorter than 3 meters is much less under 15.209 than under 15.219, isn't it?
//

Could be.