Part 15 FM Coverage

[rfry]

Part 15 for FM limits field strength to 250 uV/m when measured 3 meters in any direction from the transmit antenna. But there is no Part 15 limit on the height above ground for that transmit antenna.

Antenna height is important at FM frequencies, because VHF radio waves travel on a line-of-sight path between the transmit antenna and the receive antenna. Unlike AM broadcast frequencies, groundwave propagation is not used (or usable) on FM broadcast frequencies.

The following table shows the effect on field strength at about 9 meters above ground level, 2 miles from the transmit site, for three elevations of the transmit antenna above average terrain, using legal Part 15 FM radiation limits in each case. These field strengths were calculated using the FCC's algorithm for FM coverage, which is based on measured data.

Transmit Antenna Height > Field Strength at 2 miles
30 meters > 0.0301 uV/m
40 meters > 0.0402 uV/m
50 meters > 0.0514 uV/m

Two conclusions are possible from the above data.

1. The higher the FM transmit antenna, the greater the distant field strength it will produce.
2. Claims of "2 mile" coverage by Part 15 FM operators cannot be justified by Part 15 FM radiation limits and the laws of physics. FM receivers need field strengths at least 10 times greater than the values above. That would require increasing the Part 15 FM transmitter radiated power by a factor of at least 100.

Again, I have no axes to grind... I am only reporting on the engineering realities involved in Part 15 FM applications.

 

[kc8gpd]

and here's the loophole.....

obviously for safety and insurance reasons the agent is not going to climb a 50 ft tower or rooftop, etc. to get F/S readings, don't picture them renting a bucket truck either.

Well their only choice is to stand 3 meters from the base of the pole or tower
to take readings.

Well when you use a gain antenna such as a comet 5/8 wave mounted high in the air you can push signifiganty more power than the P15.239 allows before it produces legal limit f/s on the ground.

I remember one forum member on here who actually hired a guy to come out and measure his feild strength using a calibrated FIM and he was pushing 25mw into a comet 5/8 wave and still not producing 250uV/m @ 3m from base of the tower.

this is why canada law probably states a measurement be made at 30m instead of 3m. i forget the actual f/s in CRTC law but it is the equivalent of USA.

one other thing we have to praise the FCC for is canadian only allows you to use one Transmitter. forget about syncing multiple sites. not allowed in canada.


> Part 15 for FM limits field strength to 250 uV/m when
> measured 3 meters in any direction from the transmit
> antenna. But there is no Part 15 limit on the height above
> ground for that transmit antenna.
>
> Antenna height is important at FM frequencies, because VHF
> radio waves travel on a line-of-sight path between the
> transmit antenna and the receive antenna. Unlike AM
> broadcast frequencies, groundwave propagation is not used
> (or usable) on FM broadcast frequencies.
>
> The following table shows the effect on field strength at
> about 9 meters above ground level, 2 miles from the transmit
> site, for three elevations of the transmit antenna above
> average terrain, using legal Part 15 FM radiation limits in
> each case. These field strengths were calculated using the
> FCC's algorithm for FM coverage, which is based on measured
> data.
>
> Transmit Antenna Height > Field Strength at 2 miles
> 30 meters > 0.0301 uV/m
> 40 meters > 0.0402 uV/m
> 50 meters > 0.0514 uV/m
>
> Two conclusions are possible from the above data.
>
> 1. The higher the FM transmit antenna, the greater the
> distant field strength it will produce.
> 2. Claims of "2 mile" coverage by Part 15 FM operators
> cannot be justified by Part 15 FM radiation limits and the
> laws of physics. FM receivers need field strengths at least
> 10 times greater than the values above. That would require
> increasing the Part 15 FM transmitter radiated power by a
> factor of at least 100.
>
> Again, I have no axes to grind... I am only reporting on the
> engineering realities involved in Part 15 FM applications.
>
<P ID="signature">______________
Rev. Robert P. Chrysafis
Universal Life Ministries (ULC)

http://www.freecycle.org
Join the FreeCycle Revolution</P>

 

[radiopilot]

FCC's only compliance for antennas...

> and here's the loophole.....
>
> obviously for safety and insurance reasons the agent is not
> going to climb a 50 ft tower or rooftop, etc. to get F/S
> readings, don't picture them renting a bucket truck either.
>
> Well their only choice is to stand 3 meters from the base of
> the pole or tower
> to take readings.
>
> Well when you use a gain antenna such as a comet 5/8 wave
> mounted high in the air you can push signifiganty more power
> than the P15.239 allows before it produces legal limit f/s
> on the ground.
>
> I remember one forum member on here who actually hired a guy
> to come out and measure his feild strength using a
> calibrated FIM and he was pushing 25mw into a comet 5/8 wave
> and still not producing 250uV/m @ 3m from base of the tower.
>
>
> this is why canada law probably states a measurement be made
> at 30m instead of 3m. i forget the actual f/s in CRTC law
> but it is the equivalent of USA.
>
> one other thing we have to praise the FCC for is canadian
> only allows you to use one Transmitter. forget about syncing
> multiple sites. not allowed in canada.
>
>
> > Part 15 for FM limits field strength to 250 uV/m when
> > measured 3 meters in any direction from the transmit
> > antenna. But there is no Part 15 limit on the height
> above
> > ground for that transmit antenna.
> >
> > Antenna height is important at FM frequencies, because VHF
>
> > radio waves travel on a line-of-sight path between the
> > transmit antenna and the receive antenna. Unlike AM
> > broadcast frequencies, groundwave propagation is not used
> > (or usable) on FM broadcast frequencies.
> >
> > The following table shows the effect on field strength at
> > about 9 meters above ground level, 2 miles from the
> transmit
> > site, for three elevations of the transmit antenna above
> > average terrain, using legal Part 15 FM radiation limits
> in
> > each case. These field strengths were calculated using
> the
> > FCC's algorithm for FM coverage, which is based on
> measured
> > data.
> >
> > Transmit Antenna Height > Field Strength at 2 miles
> > 30 meters > 0.0301 uV/m
> > 40 meters > 0.0402 uV/m
> > 50 meters > 0.0514 uV/m
> >
> > Two conclusions are possible from the above data.
> >
> > 1. The higher the FM transmit antenna, the greater the
> > distant field strength it will produce.
> > 2. Claims of "2 mile" coverage by Part 15 FM operators
> > cannot be justified by Part 15 FM radiation limits and the
>
> > laws of physics. FM receivers need field strengths at
> least
> > 10 times greater than the values above. That would
> require
> > increasing the Part 15 FM transmitter radiated power by a
> > factor of at least 100.
> >
> > Again, I have no axes to grind... I am only reporting on
> the
> > engineering realities involved in Part 15 FM applications.
>
> >
>

According to the rule below, other than what is stated for requirements of Part 15.239, there is nothing to suggest high gain antennas cannot be used, in fact if the manufactures of these FM/AM transmitters omit the antenna all together, the the user may be able to add whatever antenna he may have at his disposal since the unit was not certified with the antenna or the kit not supplied with the antenna... The Rangemaster... I believe was certified with a 3 meter CB whip antenna, therefore the rule below applies.

My FCC certified tranmitter does not have an antenna, but has a unique coupler for an antenna, therefore I must add one, so it will be whatever design I can use to effect the 250uv/3m limit, whatever it may be and use the 'unique' coupler to the unit. Simple as that, should the FCC inquire about the use of the antenna/coupler he will see that the unit is legal.

Radiopilot


[Code of Federal Regulations]
[Title 47, Volume 1]
[Revised as of October 1, 2004]
From the U.S. Government Printing Office via GPO Access
[CITE: 47CFR15.203]

[Page 805]

TITLE 47--TELECOMMUNICATION

CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION

PART 15_RADIO FREQUENCY DEVICES--Table of Contents

Subpart C_Intentional Radiators

Sec. 15.203 Antenna requirement.

An intentional radiator shall be designed to ensure that no antenna
other than that furnished by the responsible party shall be used with
the device. The use of a permanently attached antenna or of an antenna
that uses a unique coupling to the intentional radiator shall be
considered sufficient to comply with the provisions of this section. The
manufacturer may design the unit so that a broken antenna can be
replaced by the user, but the use of a standard antenna jack or
electrical connector is prohibited. This requirement does not apply to
carrier current devices or to devices operated under the provisions of
Sec. 15.211, Sec. 15.213, Sec. 15.217, Sec. 15.219, or Sec. 15.221.
Further, this requirement does not apply to intentional radiators that
must be professionally installed, such as perimeter protection systems
and some field disturbance sensors, or to other intentional radiators
which, in accordance with Sec. 15.31(d), must be measured at the
installation site. However, the installer shall be responsible for
ensuring that the proper antenna is employed so that the limits in this
part are not exceeded.

[54 FR 17714, Apr. 25, 1989, as amended at 55 FR 28762, July 13, 1990]

 

[LeeSmith]

Just to clarify the Canadian requirements of Industry Canada (equivalent to FCC) here is a copy and paste from the current version of RSS-210 which details licence exempt operations similar to the FCC Part 15.

Which states in part.....The field strength shall comply with the following:
(1) not exceeding 250 microvolts/m measured at 3 metres with an averaging meter (equivalent to 19 nW e.i.r.p.). Any type of modulation (and carrier frequencies within the 88-108 MHz band) may be used for this category; or
(2) not exceeding 100 microvolts/m measured at 30 metres (equivalent to 1000 ìV/m measured at 3 metres, equivalent to 300 nW e.i.r.p. ) only if the modulation is FM and the carrier frequencies are chosen from the following set: 88.1; 88.3; 88.5; 107.7; 107.9 MHz (i.e. spaced every 200 kHz).
The occupied bandwidth shall not exceed 200 kHz.
Outside this 200 kHz band (as well as outside the band 88-108 MHz), Table 2 limits apply.

Hope this may be helpful to someone.

Regards,
Lee Smith

 

[rfry]

> and here's the loophole..
>
> Well when you use a gain antenna such as a comet 5/8 wave
> mounted high in the air you can push signifiganty more power
> than the P15.239 allows before it produces legal limit f/s
> on the ground.

This is no loophole. Part 15 FM does not limit field strength "on the ground," only at a 3-meter radius from the transmit antenna, itself.

Obviously, if a Part 15 FM transmit antenna can produce the 250 uV/m limit in some direction at points further away from it than 3 meters, then the field strength in that direction AT a 3-meter distance from the transmit antenna must be above that value -- which is absolute proof of illegal operation.

> I remember one forum member on here who actually hired a guy
> to come out and measure his feild strength using a
> calibrated FIM and he was pushing 25mw into a comet 5/8 wave
> and still not producing 250uV/m @ 3m from base of the tower.

Even if the FIM measured 249.999... uV/m at the tower base, that would still be absolute proof that the operation was illegal, unless the transmit antenna was 3 meters away from the antenna of the FIM. Probably not the case if the transmit antenna was on top of a tower. The distance reference here is from the transmit antenna itself, not from the base of the tower.

Radiating 25 mW from a Comet 5/8 wave antenna will produce a peak field strength far, far above the Part 15 FM limit. That limit can be achieved by applying around 12 nanowatts to a simple 1/2-wave dipole. Using 25 mW with a 1/2-wave dipole would be over 2 million times more power than it takes to produce a legal Part 15 field. The ratio would be even higher for a Comet 5/8 wave.

 

[rfry]

Re: FCC's only compliance for antennas...

> According to the rule, other than what is stated for
> requirements of Part 15.239, there is nothing to suggest
> high gain antennas cannot be used, ...
> My FCC certified tranmitter does not have an antenna, but
> has a unique coupler for an antenna, therefore I must add
> one, so it will be whatever design I can use to effect the
> 250uv/3m limit, whatever it may be and use the 'unique'
> coupler to the unit. Simple as that, should the FCC inquire
> about the use of the antenna/coupler he will see that the
> unit is legal.
___________

For Part 15 FM (Rangemaster makes only AM, I think), the Rules allow any combination of antenna input power and peak antenna gain that will not produce a field in excess of 250 uV/m at a distance of 3 meters in any direction from the transmit antenna. The further requirement for commercially manufactured, complete transmitters is that it, and its supplied or specified antenna must be FCC/IC certified as a system.

A user-supplied Part 15 FM antenna that differs from the one used in the tx certification means the user becomes responsible for meeting Part 15 radiation limits for that system -- which is a technically difficult process. Replicating the kind of "coupler" (RF connector type) on the input of some other antenna doesn't mean that such an antenna will produce legal fields when connected to that transmitter. That will depend on the electrical performance of the replacement antenna, and the power that the tx can deliver to it. The legal performance of such a system will need to be proven by field measurements (properly taken, and properly interpreted).

 

[radiopilot]

Re: FCC's only compliance for antennas...

> > According to the rule, other than what is stated for
> > requirements of Part 15.239, there is nothing to suggest
> > high gain antennas cannot be used, ...
> > My FCC certified tranmitter does not have an antenna, but
> > has a unique coupler for an antenna, therefore I must add
> > one, so it will be whatever design I can use to effect the
>
> > 250uv/3m limit, whatever it may be and use the 'unique'
> > coupler to the unit. Simple as that, should the FCC
> inquire
> > about the use of the antenna/coupler he will see that the
> > unit is legal.
> ___________
>
> For Part 15 FM (Rangemaster makes only AM, I think), the
> Rules allow any combination of antenna input power and peak
> antenna gain that will not produce a field in excess of 250
> uV/m at a distance of 3 meters in any direction from the
> transmit antenna. The further requirement for commercially
> manufactured, complete transmitters is that it, and its
> supplied or specified antenna must be FCC/IC certified as a
> system.
>
> A user-supplied Part 15 FM antenna that differs from the one
> used in the tx certification means the user becomes
> responsible for meeting Part 15 radiation limits for that
> system -- which is a technically difficult process.
> Replicating the kind of "coupler" (RF connector type) on the
> input of some other antenna doesn't mean that such an
> antenna will produce legal fields when connected to that
> transmitter. That will depend on the electrical performance
> of the replacement antenna, and the power that the tx can
> deliver to it. The legal performance of such a system will
> need to be proven by field measurements (properly taken, and
> properly interpreted).
>

Rich,

As I read your statements, it's clear such measurements will need to be made, under reasonable engineering practices.. the below states what needs to be furnished to the FCC upon request or notice, subpart (b) is all that is required I believe reagarding and such information.... as far as extensive field tests? I feel that is beyond the operator of such devices, therefore simple field strengths are all that may be required.

Radiopilot


[Code of Federal Regulations]
[Title 47, Volume 1]
[Revised as of October 1, 2004]
From the U.S. Government Printing Office via GPO Access
[CITE: 47CFR15.29]

[Page 760]

TITLE 47--TELECOMMUNICATION

CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION

PART 15_RADIO FREQUENCY DEVICES--Table of Contents

Subpart A_General

Sec. 15.29 Inspection by the Commission.

(a) Any equipment or device subject to the provisions of this part,
together with any certificate, notice of registration or any technical
data required to be kept on file by the operator, supplier or party
responsible for compliance of the device shall be made available for
inspection by a Commission representative upon reasonable request.
(b) The owner or operator of a radio frequency device subject to
this part shall promptly furnish to the Commission or its representative
such information as may be requested concerning the operation of the
radio frequency device.
(c) The party responsible for the compliance of any device subject
to this part shall promptly furnish to the Commission or its
representatives such information as may be requested concerning the
operation of the device, including a copy of any measurements made for
obtaining an equipment authorization or demonstrating compliance with
the regulations.
(d) The Commission, from time to time, may request the party
responsible for compliance, including an importer, to submit to the FCC
Laboratory in Columbia, Maryland, various equipment to determine that
the equipment continues to comply with the applicable standards.
Shipping costs to the Commission's Laboratory and return shall be borne
by the responsible party. Testing by the Commission will be performed
using the measurement procedure(s) that was in effect at the time the
equipment was authorized or verified.

 

[rfry]

Re: FCC's only compliance for antennas...

> As I read your statements, it's clear such measurements will
> need to be made, under reasonable engineering practices..
> the below (omitted here) states what needs to be furnished to
> the FCC uponrequest or notice, subpart (b) is all that is required
> I believe reagarding and such information.... as far as
> extensive field tests? I feel that is beyond the operator of
> such devices, therefore simple field strengths are all that
> may be required.
_________

The field tests that are required by the Rules are those that can demonstrate with scientific and defensible accuracy that the system actually MEETS the Rules. Those tests are not simple. I agree that they probably are beyond the skill level and test equipment inventory of almost every Part 15 user.