3rd harmonics of FM stations in the 327
the 104.1,102.9 fundamental with a dipole
30may13:measuring 104.1, 102.9 harmonics,
intermod products from route 10
30apr12: measuring 104.1,102.9 harmonics from
2005-2006:measurements of FM station
harmonics through the telescope, az dependence,.
04jun13: measuring fm stations
fundamental with a dipole
The log periodic that we had used to
measure the fundamental and 3rd harmonic of the fm stations
only went down to 190 MHz. Since the fm stations are at
102.9 and 104.1, we did not get an accurate measurement of
the fundamentals. We made two dipole antennas covering
310.5 and 103.5 to try and improve the measurement.
The dipoles were made by:
- We strung wires on a fiberglass post . we cut the lengths
to the approximate lambda/4.
- The network analyzer was used to tune the lengths to get a
maximum null in S11. This was down outside.
- A ferrite clamp was placed around the coax that fed the 2
halves of the dipole.
On 4jun13 we drove out to the same site
as 30may13 and tried to measure the signal strengths using the
dipoles and the log periodic antenna.
The first plot shows the 3rd harmonic measured
with the dipole and the anritsu spectrum analyzer (.ps)
- analyzer setup: preamp on, rbw 100 Khz, vbw 30 Khz,
avg 20 traces.
- Black, red trace: the are avg 10, peak hold when the
antenna was held up on the 5 foot dipole pole.
- blue,brown: avg10 and peak hold when the dipole was
extended about 12 feet on the van's mast.
- all measurements had a stronger signal at lower
frequencies (unlike what we later measured with the log
periodic at 310 MHz).
- the 12 foot mast measurement is also about 35 db stronger
than the 5 foot hand held measurement.
- the 12 foot measurement had about 15 feet of cable while
5 foot extension had a much shorter cable.
- The ferrite clamp we used was probably not doing a very
good job of isolating any currents on the feed coax
shield. We probably need to use a balun to have a
correctly connected the dipoles to the coax line.
The second set of plots
shows the fundamental measured with the dipole while the 3rd
harmonic was measured by the log periodic (.ps)
- black is an average of 10 traces while red is a peak hold.
- Top: The fundamental measured with the 103 MHz dipole
- These values are about 30 db higher than those measured
with the log periodic back on 30may13.
- The noise floor is also about 30db higher than the
- We did have a 20db external amp, but i'm pretty sure it
wasn't connected for this measurement (but maybe i'm
- You can also see some other fm stations below
102.9. These were barely visible using the log periodic.
- 2nd frame: The 3rd harmonics measured using the log
- These values are close to those measured on 30may13 with
the log periodic.
- 3rd frame: I removed 2db from the fundamental measurement
to remove the dipole gain
- bottom frame: i removed 6.5 db from the 3rd harmonic
measurement. This is the gain of the log periodic antenna
- The table in frame 3 shows that
- 102.9 3rd harmonic is 96 db down from the fundamental
- 104.1 3rd harmonic is 80db down from the
- the dipole for the 3rd harmonic had large changes in value
when we put it 12 -15 feet in the air on a mast.
- The dipole was probably not fed correctly.
- Using the 103 MHz dipole for the fundamental and the
log periodic for the 3rd harmonic we measured:
- 102.9 3rd harmonic down 96 db
- 104.1 3rd harmonic down 80 db
- Since the 103 dipole was fed the same was as the 312 MHz
dipole, the dipole fundamental measurement was also probably
- We need to put a correct a balun on each dipole, or just
buy the calibrated dipole set from AHsystem.
harmonics,intermods from route 10.
We (dana and phil) measured the fundamental and 3rd
harmonics of the fm stations 104.1 and 102.9 from route 10. The
- The towers are located at 18.2887, -66.6606
- The van was located at 18.3157, -66.6830 (lat,long)
- This is at the turnoff to 6612 from route 10.
- See Google maps
image showing van and tower (.pdf)
- The van is at marker A, the towers are at marker B.
- this was a few hundred meters closer to utuado
(along route 10) from the 30apr12 measurement.
- The distance from van to tower was about 3.9 km.
- We used the log periodic antenna for the measurements
(same as 30apr12).
- anritsu spectrum analyzer was used. Its setup was:
- preamp on, rbw=30 Khz for fundamental, 100 Khz for 3rd
harmonics, average 10 and peak hold.
The plots show the
fundamental and 3rd harmonics of the fm stations (.ps)
- Top: fm station fundamentals:
- Black is an average of 10 traces, red is a peak hold
- The RBW is 30 Khz.
- HQ = 102.9 MHz
- Radio Redentor= 104.1 MHz
- The difference between the two frequencies is 1.2 MHz
- Dana pointed out that the log periodic did not have an
element long enough for 104 MHz.
- The peak amplitude occurred when we pointed a little
off of the tower direction.
- Because of this, the amplitudes measured are probably
less than the actual values.
- Bottom: 3rd harmonics of the fm stations.
- The rbw is 100 Khz.
- The log periodic had an element that covered this
frequency. The maximum amplitude occurred when we
pointed at the towers.
- green trace:308.7: 3rd harmonic of 102.9. Peak value:
- blue trace: 312.3: 3rd harmonic of 104.1. Peak
value -96 dbm.
- purple traces: Let Frq1= 102.9, Frq2=104.1
- These are intermod products between the two
- 309.9 MHz = 2*Frq1 + frq2
- 311.1 MHz =2*Frq2 + frq1
- these intermod products are also seen in the
dynamic spectra recorded at AO (see
2005-2006 dynamic spectra)
- The 3rd harmonics are clearly seen in the spectrum
analyzer 4 km from the towers.
- So the harmonics we see in the 327 receiver are not
being created inside our receiver.
- The measured ratios of measured 3rd Harmonic/fundamental
- I used the average of 10 traces.
- 102.9: -50dbm - (-109dbm) = 59 db down
- 104.1: -79dbm - (-107dbm) = 28 db down
- The log periodic did not cover the fm band
adequately so the absolute difference is probably off.
- The relative difference is probably meaningful:
- 102.9's 3rd harmonic is 60 db down
- 104.1's 3rd harmonic is only 28 db down.
- If 102.9 is ok, then 104.1 probably has problems with
it's 3rd harmonic filters.
- We should go back out to the site and measure the
fundamental and 3rd harmonic with an antenna that covers
both frequencies. We could then have a correct ratio of
fundamental to 3rd harmonic.
- Looking at the azimuth swings done back in 2005-2006
- see: (strength
azimuth dependence of the harmonics (.ps) (.pdf)
- the 104.1 3rd harmonic has a definite azimuth dependence
that repeats for the 5 azimuth swings.
- The102.9 3rd harmonic does not have a fixed azimuth
- This also points to 104.1 as the greater problem of the
two stations (and maybe the source of the
30apr12: 104.1,102.9 harmonics from route
On 30apr12 we (angel and i) took the rfi van
up route 10 to look at the 3rd harmonics of the FM stations:
The 327 MHz receiver has seen:
- 104.1 (radio redentor). 3rd harmonic: 312.3
- 102.9 (HQ) 3rd harmonic: 308.7
- the 3rd harmonic of these stations.
- A comb about these 3rd harmonics spaced by 3*(the difference
of the fundamentals). This comb varies with the strength of the
The setup for the measurements:
The plots show the
spectrum analyzer measurements (.ps) (.pdf):
- antennas are located within a few hundred meters of each other
at: 18:2895,-66.6604 (lat,long)
- the van was parked on route10 at:18.3164,-66.6804
- We were just before the turnoff to 6612 (that heads west)
- I then used Google maps to get this longitude on route 10.
- The distance to the towers was about 4km.
- the log periodic antenna was placed on a mast and the mast was
then placed in the holder at the back of the van to give a
stable pointing direction.
- The anritsu model:Ms2713E spectrum analyzer was used to record
- rbw: .03 MHz, toavg:100
- a peak hold was taken at the same time
- Top: the fundamentals of the radio stations:
- black: average 100 sweeps, red: peak hold
- Bottom: the 3rd harmonics of the stations:
- black: average 100 sweeps
- red peak hold:
- The vertical dashed line is the computed location of the 3rd
- When monitoring the 3rd harmonic, the strength of the 100
sweep average was changing with time. It would go from "buried
in the noise" to sticking way up
- The strength of the 100 sweep average of the 3rd harmonics was
changing in time. It's probably better to compare the peak
- The 3rd harmonic of 104.1 is the worse. Only 40 db below the
fundamental. It's peak hold fundamental also looks a little
- The azimuth dependence for the 104.1*3 MHz data (see 2005/2006
az dependence plots bekiw) has the peak at az=110. This has the
azimuth pointing pretty close to the towers.
- Mike nolan the following link for the allowable emitted power
- for power > 600 khz from carrier:
- attenuation must be 43 + log10(kwPwr) or 80 db down from
unmodulated carrier (whichever is less).
- The 104.1 is a 50 Kw station so the first section gives
about 89 db down.
- So they should be 80 db down from the unmodulated carrier.
- Unfortunately i don't have a measurement of the
2005-2006: measurements made on the
Links to plots/images:
azimuth dependence of the harmonics (.ps) (.pdf)
of first azimuth spin(.gif)
2005/2006 Summary (top)
Third harmonics of FM radio stations are showing up in the
327 MHz receiver.
They are not being created in the receiver since they remain
when rf filters are placed in front of the first amp.
Some show a constant azimuth dependence, some are more
variable in time.
Some of the harmonics are weaker because they fell at the
edge of the 25 MHz filter that we were using.
there is a comb 1.2 MHz apart around the 308.7 and 312.3 3rd
harmonics of 102.9 and 104.1.
The FM stations:
The whip antenna and the spectrum analyzer was
used to measure the strength of the FM stations at the observatory
(the whip is not on the platform so there may be a difference). The
spectrum analyzer was set to a span of 10 MHz (about 300Khz rbw).
* these frequencies are outside the 327 band we measured.
The azimuth swing data from 28oct05 was
used to check out the strength of the harmonics (see 327
swings). There were 5 complete azimuth spins between -90 and
+270 deg azimuth. The dome was at 19 deg. A 25 MHz band was centered
at 320 MHz with 2048 channels. The azimuth moved at .3 deg/sec.
The plots show the strength
azimuth dependence of the harmonics (.ps) (.pdf):
The strongest harmonic is 312.3 from FM 104.1
followed by 321.9 (107.3), 308.7 (102.9), and then 319.5 (106.5).
spectra of the first az spin also shows that 312.3 is the
strongest/most spread out signal
- Fig 1 has the average spectra for each az spin.
Each azimuth spin is plotted in a different color. The top plot
is the mean, the center plot is the median, and the bottom plot
is a peak hold for each spin. The dashed lines are the 3rd
harmonics of the FM stations. The vertical units are Tsys. The
308.7 birdie is small because it is on the edge of the 25 MHz
- Fig 2 plots harmonic strength versus azimuth for all 4
signals. 20 channels about each FM harmonic were averaged
and then plotted versus azimuth angle. Each color is a
different azimuth spin. Each plot is a different FM station
harmonic. The azimuth repeatability is not perfect (probably
because of the time variation of the signal).
- Fig 3: 308.7 (FM 102.9) strength vs az in 90 degree
sections. This blows up the azimuth scale a bit.
- Fig 4: 312.3 (FM 104.1) strength vs az in 90 degree
- Fig 5: 319.5 (FM 106.5) strength vs az in 90 degree
- Fig 6: 321.9 (FM 107.3) strength vs az in 90 degree
2005/2006: Some observations:
- The 321.9 and 319.5 harmonics were still present when
filter were placed in front of the first amp
(the other two were outside the filter band). So the birdies are
coming from outside the receiver (most likely from the FM
- We listened to the 321.9 MHz and 312.3 MHz birdie in the 2nd
IF with a radio receiver. We could hear the FM station on the
radio at the 3rd harmonic.
- The dynamic
spectra showed that occasionally a large number of
birdies occurred together
- frequencies:308.7 309.9 311.07 313.1 314.3
- These are spaced by 1.2 MHz. This is the same
difference as 104.1-102.9 (the 2nd and 3rd strongest FM
- 308.7 is one of the FM harmonics.
- At another time we heard the 107.3 FM station at 317.5 (it
should be at 321.9). The difference (321.9-317.5) is 4.4 MHz:
the same difference between the FM stations fundamentals.
- 308.7, 312.3, 319.5, and 321.9 are 3rd harmonics of FM
stations that are not created in the receiver.
- We should do field measurements to see if the transmitters
are within specs. We might want to think about buying them some
filters. We should start with 312.3 (104.1 radio redentor whose
transmitter is in utuado). (see above measurements).
- What's going on with the bursts of 7 frequencies at once. Why
is one of them the 3rd harmonic of the FM station shifted by 4
MHz. Did the filters make this go away are were we just unlucky
to have them there on the 28oct05?