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Some Field Strength Numbers

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Some Field Strength Numbers

Since installing our EDM FM Transmitter, exactly like the one Tim in Bovey reviewed, set at minimum RF output power with a 1/4-wavelength horizontal Decade GPL32 dipole antenna, we've intended to make field strength measurements around the campus and today was the day.

The time was 11:30 AM, a TECSUN PL-310 with dBu/SN readings was the test instrument, its antenna open to 4-sections for a length of 13" always aligned horizontally on the same plane as the transmission, held at about 5' from the ground.

These numbers form a map of our grounds, all outdoors, X marks the transmitter, the top of the list is NORTH.

00/00   01/02   01/03

--/--   00/02   --/--

--/--   18/19   07/07

36/34   25/28   20/22

27/26   --/--   --/--

35/31   --/--   27/30

40/31   --X--   --/--

52/41   53/49   43/43

--/--    48/41

--/--    42/31

--/--    23/24

--/--    05/06

The clear reception area on this radio is less than 100'.

The same clear reception area is observed on the auto radio in the driveway.

Weak spotty reception is detected on the auto radio in every direction for no more than 600-feet.

Very interesting Carl

I am currently running a C Crane

transmitter that has been modified

to run in mono.

It seems to have about the same

field strength characteristics as

your transmitter.  I have not tried

to take any relative field measurements.

However, I have a Tecsun PL-380 which

is very much like your model 310.  It has

the same numerical strength readout

as your radio.  (Personally, I would

greatly prefer an analog meter any day.)

I am going to try what you are doing and

see what happens.

Very best wishes

Brooce

NOISE AND STATIC RADIO

Speaking on behalf of our

Resident Hobby Agent and the Part 15 Department of the FCC, I have to caution you that the use of non-calibrated Field Intensity Meters (portable radio sets) along with non-calibrated antennas does not provide assurance of compliance with applicable FCC regulations.

I recommend the following for use with your spectum analyzer: http://www.ahsystems.com/catalog/SAS-540.php

You can thank me later.

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Measuring Field Intensity

I recommend the following for use with your spectum analyzer:<snip...>

The PDF describes this antenna as being "omnidirectional" and that it is calibrated for measuring horizontal polarization.

Could anyone please explain how a linear, horizontal dipole antenna can be omnidirectional in the horizontal plane?

Also its SWR gets rather high at the low end of the FM broadcast band (higher than its spec).

Yes that is certainly a typo

Yes that is certainly a typo in the datasheet. 

A biconical antenna is excellent when wide instantaneous bandwidth is needed, but for the most accurate field strength measurements, I would use a tuned dipole. In fact, tuned dipoles are sometimes used in EMC testing when a biconical shows the unit under test to be failing by a couple dB. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

How does this antenna..

How does this antenna give you an indication of field strength to know if you are compliant?

 

To WDCX - I agree

I'm just doing this for the fun.  It's

like a science experiment.

 

My FM transmitter falls within 15.239

as far as I can tell.  This house is on a

very small lot.  Once you get the

car in front of the next house on

the street - the signal (on the car

radio) is about the

same strength as a Providence RI

station - - and I'm in

Hartford CT.

Brooce

NOISE AND STATIC RADIO

The short answer:

Observed Field Strength + Antenna Correction Factors + Cable Loss = Actual Field Strength 

 

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

More

Observed Field Strength + Antenna Correction Factors + Cable Loss = Actual Field Strength

The receive antenna used with an FI meter normally responds either to the electric or the magnetic field of the arriving radio wave (not to both fields simultaneously).

The antenna output does not consist of an "observed field strength," however.  It consists of a finite amount of r-f power in watts, or smaller units thereof such as milliwatts or microwatts.

That power can be related by calculation to the radiated fields that produced it, when the "Antenna Factor," cable loss, and measuring system device impedance/accuracy are known.

The practice and art of producing calibrated test sets for accurately measuring field intensity is not trivial, which is the reason why such commercially-made test sets are so expensive.

Another Transmitter Measured

KDX maintains two transmitters on its main frequency of 89.5 MHz, one is a backup should the other one be unavailable.

The EDM FM Transmitter was the first one tested in an earlier entry, now on to the Ramsey FM30b, also driving a Decade GPL32 Horizontal Dipole from a different location.

The two transmitters are much alike in audio quality and purity of the RF signals, but are different with regard to their RF output power levels: The EDM has two ranges, HI=1-10mW; LO=2-100mW. The FM30b has one range: 5uW-25mW.

For purposes of these measurements at 1:30 PM the FM 30b is set at its lowest 5uW output with the dipole=1/2Wavelength. Top of the list is North. X marks the transmitter. Numbers reflect dBu/SN from TECSUN PL-310 Radio, taken on the Internet Building Campus, home of KDX.

06/08   00/00   00/00

--/--   07/10   00/01

--/--   24/25   05/07

--/--   29/33

34/36  36/37   32/34

44/44  --X--   27/28

41/40

36/32   --/--   27/28

21/23

24/25

--/--   11/12   08/08

--/--   10/09

--/--   12/13

--/--   06/07

Both of these transmitters were measured out to a distance of 100' and are easily well within the 200' allowed by FCC bulletin.

Carl Blare

When I said observed, I am

When I said observed, I am refering to the level indicated on the FIM. That "OBSERVED" level is combined with ACF and cable loss.

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Why I'm Planning to Raise Power on FM

It is obvious from the numbers shown on my charts that I am well under the 200' typical range stated by FCC document for Part 15 FM.

Distance from these transmitters to the far boundry is right at 100' and we see that power is down in the zeros.

We made an interesting observation about this TECSUN PL-310 FM Radio.

It picks up a clear, listenable signal all the way down to a reading of 07 07, after which signal level drops off abruptly as the numbers reach 00 00, but even at that point the station can still be heard in the fizzle.

Power increase plans are being mapped and will be announced.

Carl Blare

Ya know Carl

I think this is very

interesting.

Science, electronic theory,

and mathematics would all

fit in here.  We would have to

know about all of the reflections,

refractions, and attenuations that 

would occur in the path, 

I suppose.  That is WAY

beyond me.

Brooce

NOISE AND STATIC RADIO

Interesting Explorations

You are very right Brooce, the reflections happening within the signal field are there, but not possible to take into account with the limited kind of measurements we are taking.

Since our transmission antenna is a horizontal dipole we expect that the lobes are extending at right angles from the antenna leaving nulls at the tips of the dipole, but readings in the immediate field don't show much of a null on either side of the building. Maybe the null increases in size with distance, except we have no measurable signal on any distant path. And we can imagine that walls are bouncing the signal at angles which is probably very different from what would be experienced if the antenna were up high out in open space.

Today I happened to drive around and noticed that our FM signal becomes discernable right at the foot of the driveway, suggesting that our useful signal is within the confines of the property and virtually non-existant at any other property.

Carl Blare

What a Jump

Field measurement of the EDM FM Transmitter at the northern boundry between private and public land using the dBu / SN read-out from a TECSUN PL-310 radio, it's antenna at 13" horizontal:

EDM RF out into DecadePL32 Horizontal Dipole power level set minimum:

With transmission dipole set at 1/4 wave:

00 00   01 02   01 03

With transmission dipole set at 1/2 wave:

15 17   22 25   23 25.

Large difference!

We are aiming for a border power of 08 08, so we need to re-figure things.

It's gotten cold around here, 30-degrees after temps in the 50s, 60s and 70s.

The best part of working outdoors is coming indoors.

 

Carl Blare

This Might Be the Right Power

Following up on yesterday's power reading... 22 25 at the campus boundary, which is a public sidewalk... we wondered how much farther out we'd find the target minimum field reading of about 08 08, and we walked out 30-feet to the streetsign island that divides two confluencing streets, where the reading was 06 09, which equates with a reducewd signal submerged in FM fizz-noise.

Therefore 08 08 would be found in the main lane of the street and there's no audience there so the signal loses its meaning by being there.

With a "useful" signal range of 125-feet we are safely under the FCC's published allowance of 200-feet, so we'll stop here and make no more adjustments.

Besides using the TECSUN radio for our own compliance with FCC part 15, we have shown that these dBu/SN type radios CAN be used to setup for compliance.

We therefore now REQUIRE that all part 15 stations obtain and keep a radio with dBu and SM readouts. Deadline for compliance is 12 noon Decewmber 25th.

Carl Blare

FM NOUOs

It may appear to some that many/most of the FCC §15.239 NOUOs are issued to operators of unlicensed systems using "high power" transmitters running  tens or hundreds of watts, and that otherwise, there isn't much need be concerned. 

But there are cases where NOUOs have been issued for systems radiating far less power (see graphic below).

Note that the radiated powers in the table apply only to free space fields.  Signal reflections from the earth and other objects can either reinforce, or reduce them at some locations in the propagation environment.

Unclear Blizzard of Numbers

Hobby Agent says:  "There are cases where NOUOs have been issued for systems radiating far less power (see graphic). Signal reflections from the earth and other objects can either reinforce, or reduce them at some locations in the propagation environment."

While it is true that small numbers can be broken down into many many many smaller numbers into infinity, it is not clear whether this is a warning that an NOUO will be sent to KDX for being recievable to a drop-off distance of 125'.

It is not obvious whether the graphic table relates to the TECSUN line of radios with their dBu/SN indicators.

Possibly the posting does not address this thread in any particular way and should be given its own thread.

Carl Blare

My question is...

How did the FCC make measurements at a distance of 3 meters from the source? Were they trespassing? What if the antenna is on a 20 foot pole? Did they use a high reach? Something looks suspect here.

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Explaining (again)

From Reply 18 above:  ... It is not obvious whether the graphic table relates to the TECSUN line of radios with their dBu/SN indicators. ...

No, it does not.

The Tecsun PL-310 and similar receivers do not measure/display the net field intensity of the e-m wave(s) arriving at the antennas of those receivers.

S Type meter accuracy

"Most S meters on traditional analog receivers are not calibrated and in practice can only provide a relative measure of signal strength based on the receiver's AGC voltage. Some S meters on traditional analog receivers are calibrated to read S9 for an input of -73 dBm but do not provide the correct 6 dB per S unit correspondence.Often the correlation between a radio listener's qualitative impression of signal strength and the actual strength of the received signal on an analog receiver is poor, because the receiver's AGC holds the audio output fairly constant despite changes in input signal strength."

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

The Conversation Takes a Branch

DHR branches onto a path not previously taken in this thread about the field number readings on TECSUN-type radios.

Heretofore there has been no mention of "S" meters, but it's not unreasonable to take them under consideration as being less accurate signal-level indicators, in that they produce no numbers, other than the inaccurate number associated with "S9", as indicated.

However, to make use of both signal reading methods, that is by combining dBu indicators with "S" meters, a low power radio station expands its view of the RF field within the few feet being observed.

Carl Blare

Followup

RE:  ... this thread about the field number readings on TECSUN-type radios. ...

Once again, Tecsun PL-310 and similar radios do NOT display "field number readings."

Rich is correct:

The numbers Carl and others see is a function of voltage across the antenna terminals. In most cases as with a portable radio the impedance may be unknown.

Also to further rebuff Carl's comment: S9 is generally accepted to 50 uV across 50 ohms.

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Hobby Talk

DHR used the expression: "To further rebuff Carl's comment..."

The practice of rebuffing can really bring out a shine in marble floor tiles.

But I have not been rebuked!

And no matter what gets said, the readings I took WERE taken outdoors IN A FIELD!

Carl Blare

Carl is a Lucky Man!

Carl's home is large enough to have a field inside. I am guessing that the grass grows slowly.

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Got Grass?

Grass roots radio.

Carl Blare

About Reply 24 of This Thread

RE:  ... S9 is generally accepted to (be) 50 uV across 50 ohms.

However that does not mean that the net field intensity arriving at the receive antenna equals 50 µV/m.

The reason for that (Carl), mostly is due to the unknown/uncalibrated characteristics of the receive antenna system at the received frequency.

A web-sourced definition of field intensity is "... electric fields created by electric charges, and by time-varying magnetic fields. The units of the electric field in the SI system are newtons per coulomb (N/C), or volts per meter (V/m)."

The electric fields of interest here exist between two different physical  points in space -- not a conducted voltage existing between two terminals inside a radio receiver.

The FCC uses units of field intensity in several of its Part 15 rules applying to unlicensed use of the AM & FM broadcast bands.  Examples are §15.209 for the AM band and §15.239 for the FM band.

Measuring such electric fields _accurately_ requires calibrated (expensive) test equipment used correctly by a skilled and experienced operator.

Beyond Reach

As has been said:  "The FCC uses units of field intensity in several of its Part 15 rules applying to unlicensed use of the AM & FM broadcast bands.  Examples are §15.209 for the AM band and §15.239 for the FM band.  Measuring such electric fields _accurately_ requires calibrated (expensive) test equipment used by a skilled and experienced operator."

And again, while it is good for the hobbyist to realize this in principle, the "calibrated expensive test equipment" that he will never own must be done without, leaving miniature radio stations with whatever make do solutions they can concoct.

Carl Blare

Concoctions

RE:  ...leaving miniature radio stations with whatever make do solutions they can concoct.

Preferably also with an understanding that such an approach is very unikely to provide the hoped-for "proof" or assurance desired for it.

Dangerous Hobby

Said:  "Preferably also with an understanding that such an approach is very unikely to provide the hoped-for "proof" or assurance desired for it."

That kind of remark could discourage anyone from pursuing the radio hobby.

Carl Blare

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