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.
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
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.
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 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.
How does this antenna give you an indication of field strength to know if you are compliant?
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
Observed Field Strength + Antenna Correction Factors + Cable Loss = Actual Field Strength
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.
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.
When I said observed, I am refering to the level indicated on the FIM. That "OBSERVED" level is combined with ACF and cable loss.
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.
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
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.
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.