Unlicensed Transmissions on 1710 kHz

[R. Fry]

The field intensity on 1710 kHz that legally may be radiated by an unlicensed transmitter system at a distance of 30 meters from its antenna is 15 µV/m (see FCC § 15.223 pasted below).

This means that a transmitter system FCC-certified for use in any part of the AM broadcast band 530-1700 kHz while using 100 mW DC input power to the final r-f amplifier, and including the antenna used in the certification submitted to the FCC, would need to greatly reduce its radiation in order to meet the legal limit given in § 15.223.

For example, a legally operating, FCC-certified, unlicensed transmit system on 1700 kHz with the base of its antenna at earth level, a total of 35 ohms loss in the loading coil and r-f ground connection, and 35 mW output power Z-matched to the antenna system can produce a groundwave field intensity of about 3,462 µV/m at a distance of 30 meters.

Legal use of that same setup on 1710 kHz would require reducing the output power of the transmitter to about 0.00065 mW (a reduction of about 47.3 decibels).

If the means of accurately making this power adjustment are not provided for in the transmitter design and documentation (FCC certified or not), then any operator wanting to use 1710 kHz in compliance with § 15.223 will be on his/her own.

The field intensity limit under § 15.223 for a 1710 kHz AM broadcast signal at 30 meters is well below the ambient r-f noise level in all environments outside a screen room -- so even if an operator had an accurately calibrated field intensity meter, legal setup would be "difficult."

It should also be noted that such a poor signal-to-noise ratio would not be very useful in serving the general public.

This may not be good news, but it is another reality given to us by Part 15.
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e-CFR Data is current as of January 9, 2012

Title 47: Telecommunication
PART 15—RADIO FREQUENCY DEVICES
Subpart C—Intentional Radiators
Radiated Emission Limits, Additional Provisions

§ 15.223   Operation in the band 1.705–10 MHz.

(a) The field strength of any emission within the band 1.705–10.0 MHz shall not exceed 100 microvolts/meter at a distance of 30 meters. However, if the bandwidth of the emission is less than 10% of the center frequency, the field strength shall not exceed 15 microvolts/meter or (the bandwidth of the device in kHz) divided by (the center frequency of the device in MHz) microvolts/meter at a distance of 30 meters, whichever is the higher level. For the purposes of this section, bandwidth is determined at the points 6 dB down from the modulated carrier. The emission limits in this paragraph are based on measurement instrumentation employing an average detector. The provisions in §15.35(b) for limiting peak emissions apply.

(b) The field strength of emissions outside of the band 1.705–10.0 MHz shall not exceed the general radiated emission limits in §15.209.

[WRPO]

And the beat goes on..........

[pianoplayer88key]

So what happens if you set your RF bandwidth to 171 kHz (10% of 1710 kHz), then brick-wall lowpass your audio chain above, for example, 8 kHz, like some IBOC stations do (but without running the IBOC)?  (Also maybe you run IBOC-like sidebands at -4.5 dB modulation at 150 kHz off center frequency.)  Would you be able to do 100 µV/m @ 30 meters then?
Also you mention ambient noise level ... Any idea what the ambient noise level is in a few situations, like low vs high end of the band in a rural very quiet location with no nearby thunderstorms (1000s of kms from storms & electrical power grids), vs. an urban noisy area during a thunderstorm close enough for the lightning and thunder to be perceived simultaneously, as well as maybe a couple steps in between?
As for a station whose field strength is right at that noise level, what would be the expected reception quality?  Would most of the words still be understandable (assuming you speak the language) just difficult to hear, using standard NRSC AM, or would you be required to use QRSS CW and a BW to detect any trace of a carrier (and still not be able to identify the transmission)?
Also, speaking of a higher frequency (mentioned in 15.225 (a)) like 13.56 MHz, is it possible to use an electrically short antenna (1° or shorter) in a portable device resting on a table/podium/etc, and have radiation that primarily uses the skywave (like shortwave) or direct wave (like FM)?  (For FM I can understand if it'd help to mount the antenna higher if possible, but for me much of the time that would be impractical as my feet would still be in firm contact with the ground when I set it up.)
Also how would you calculate the power required with such a short un-grounded (unless the chassis of the device counts) antenna to meet the field strength requirement?  (There are other situations I would like to figure out as well, so rather than telling me "you have to run 500 milliwatts with a 0.01° antenna at 1710 kHz" or "if you want to run 50 watts you must use an antenna shorter than 0.0001°" or whatever it is, I'd like to learn how to calculate it myself if possible.)

[R. Fry]

So what happens if you set your RF bandwidth to 171 kHz (10% of 1710 kHz), then brick-wall lowpass your audio chain above, for example, 8 kHz, like some IBOC stations do (but without running the IBOC)?  (Also maybe you run IBOC-like sidebands at -4.5 dB modulation at 150 kHz off center frequency.)  Would you be able to do 100 µV/m @ 30 meters then?

Some short answers:

Even a field of 100 µV/m would not provide an acceptable S/N for a typical AM receive setup over a free-space path of 30 meters.

Also, speaking of a higher frequency (mentioned in 15.225 (a)) like 13.56 MHz, is it possible to use an electrically short antenna (1° or shorter) in a portable device resting on a table/podium/etc, and have radiation that primarily uses the skywave (like shortwave) or direct wave (like FM)?  (For FM I can understand if it'd help to mount the antenna higher if possible, but for me much of the time that would be impractical as my feet would still be in firm contact with the ground when I set it up.) Also how would you calculate the power required with such a short un-grounded (unless the chassis of the device counts) antenna to meet the field strength requirement?  (There are other situations I would like to figure out as well, so rather than telling me "you have to run 500 milliwatts with a 0.01° antenna at 1710 kHz" or "if you want to run 50 watts you must use an antenna shorter than 0.0001°" or whatever it is, I'd like to learn how to calculate it myself if possible.)

I admire the interest you show in learning about these subjects, but this isn't the best venue to do that.

Suggest you get a copy of Radio Engineers' Handbook by F.E Terman, which has comprehensive chapters on antennas and the propagation of radio waves.  Reference Data For Reference Engineers is another good resource.  I think both can be ordered on line from various sources.

The Terman work is readable (and downloadable) on line at http://www.scribd.com/doc/23847007/Radio-Engineers-Handbook-1943-Terman .