Antenna radiated spurs
 

I'm having a two-way radio. One wide-band antenna was hooked up on a
spectrum analyzer. Video BW and RES BW was adjusted to 3khz and 1kHz
respectively at spectrum analyzer. Span was 300KHz.

When radio was being keyed up, spurs could be seen at around plus
minus 32Khz and 62khz. However, the amplitude were quite low, about
-65 to 70dBc. I was wondering what caused the spurs to appear. If
radio was connected directly to spectrum analyzer, this problem could
not be seen.

When radio was transmitting to different load with different VSWR (up
to 8:1), no spurs were seen. Spurs could only be observed while using
antenna. Could the VSWR of antenna be worse than 8:1? Just could not
figure out the source of the spurs. Please help.

All radios produce these "spurs". The 'source' is in the
design of radio circuits. The service a radio is used in has
requirements as to the level of these 'spurs'. As long as that
requirement is met the 'spurs' are acceptible (not necessarily
desirable, but acceptible).
'Doc

Your spurs sound very strange, too close in 32 kHz, 62KHz, normally they are
further out several MHz if generated by the RF chain. That is like an audio
part/circuit getting into the transmit RF. Try rearranging or twisting up
the power cables to the radio.
If spurs are low, not much power is there, so little damage to radio
rts. -65dBc sounds good for a CB. The farther you look down the more spurs
there. The antenna could be reradiating RF power back onto circuit
components, which could be a cause too.


"Dummy" wrote in message
om...
I'm having a two-way radio. One wide-band antenna was hooked up on a
spectrum analyzer. Video BW and RES BW was adjusted to 3khz and 1kHz
respectively at spectrum analyzer. Span was 300KHz.

When radio was being keyed up, spurs could be seen at around plus
minus 32Khz and 62khz. However, the amplitude were quite low, about
-65 to 70dBc. I was wondering what caused the spurs to appear. If
radio was connected directly to spectrum analyzer, this problem could
not be seen.

When radio was transmitting to different load with different VSWR (up
to 8:1), no spurs were seen. Spurs could only be observed while using
antenna. Could the VSWR of antenna be worse than 8:1? Just could not
figure out the source of the spurs. Please help.

It is probably the way you are coupling the spectrum analyzer to the
radio. How are you coupling it to the radio when the antenna is on the
radio?

If you are just using the antenna on the spectrum analyzer you could
be seeing anything.

You may be overloading the analyzer. It is difficult to see down that
far and not overload the analyzer with the wanted signal. Try
switching in a 10 db attenuater at the front end of the analyzer. Do
the spurs drop 10 db? If they drop more you are overloading the
analyzer.

You mentioned connecting the radio directly to the analyzer. If the
analyzer is an IFR service monitor or another service monitor then all
bets are off as to what you are seeing. They are poor spectrum
analyzers.

73
Gary K4FMX


On 29 Apr 2004 00:34:31 -0700, (Dummy) wrote:

I'm having a two-way radio. One wide-band antenna was hooked up on a
spectrum analyzer. Video BW and RES BW was adjusted to 3khz and 1kHz
respectively at spectrum analyzer. Span was 300KHz.

When radio was being keyed up, spurs could be seen at around plus
minus 32Khz and 62khz. However, the amplitude were quite low, about
-65 to 70dBc. I was wondering what caused the spurs to appear. If
radio was connected directly to spectrum analyzer, this problem could
not be seen.

When radio was transmitting to different load with different VSWR (up
to 8:1), no spurs were seen. Spurs could only be observed while using
antenna. Could the VSWR of antenna be worse than 8:1? Just could not
figure out the source of the spurs. Please help.

More thoughts along the lines of John's comments, 32 kHz is about twice
the frequency of the horizontal line component of TV video. Do you have
any TV transmitters reasonably close by?

I've had problems with detected video on a couple of occasions, getting
into audio circuits. After I redesigned the audio circuits to look more
like VHF/UHF circuits -- laid out, bypassed, and filtered for VHF/UHF --
the problems disappeared. A working hypothesis is that there's a strong
TV transmitter getting into your transmitter via the antenna and/or its
transmission line, then getting detected and modulating your
transmitter. In the case of my audio problems, the vertical scan
component was worse, resulting in a "hum" that varied with the picture
of the offending TV station. You might take a close look at the spurs
and see if they vary with a local station's picture.

Roy Lewallen, W7EL

John Smith wrote:
Your spurs sound very strange, too close in 32 kHz, 62KHz, normally they are
further out several MHz if generated by the RF chain. That is like an audio
part/circuit getting into the transmit RF. Try rearranging or twisting up
the power cables to the radio.
If spurs are low, not much power is there, so little damage to radio
rts. -65dBc sounds good for a CB. The farther you look down the more spurs
there. The antenna could be reradiating RF power back onto circuit
components, which could be a cause too.

"Dummy" wrote in message
om...
I'm having a two-way radio. One wide-band antenna was hooked up on a
spectrum analyzer. Video BW and RES BW was adjusted to 3khz and 1kHz
respectively at spectrum analyzer. Span was 300KHz.

When radio was being keyed up, spurs could be seen at around plus
minus 32Khz and 62khz. However, the amplitude were quite low, about
-65 to 70dBc. I was wondering what caused the spurs to appear. If
radio was connected directly to spectrum analyzer, this problem could
not be seen.

When radio was transmitting to different load with different VSWR (up
to 8:1), no spurs were seen. Spurs could only be observed while using
antenna. Could the VSWR of antenna be worse than 8:1? Just could not
figure out the source of the spurs. Please help.

I'm having a fine day.
There are too many possibilities to decide with this medium. If you are
making these kinds of measurements without an experienced RF Tech or
Engineer you will see many many strange things which defy explanation in
this forum where many assumptions must be made - and we all will make
assumptions based on our experience.


Things to consider:
Location of the antenna - RF could be getting into the radio
- RF could be getting into the Spec Analyzer
Do some radio or antenna moving or even some hand waving to see if a RF
field is a contributor.
Get the analyzer as FAR away as possible and if the spurs are gone, forget
it and delete the rest of this.

The radio synthesizer SHOULD produce such signals. There are reference
frequency spurs. It is rare that you won't see this - however, I realize
that your antenna vs. no antenna result is odd and I have no idea what the
reference frequency is or frequencies are.
Signal level into the analyzer.
Is the analyzer working properly.
Can you duplicate it?
Can you make any other changes which produces an expected result and get
what is expected or does something strange happen. Such as signal level
into the analyzer or power out of the radio.
SWR could be an issue as you speculate.
Without being there and trying other "crazy" things I can't speculate
further...
--
Steve N, K,9;d, c. i My email has no u's.

Roy Lewallen wrote in message ...
More thoughts along the lines of John's comments, 32 kHz is about twice
the frequency of the horizontal line component of TV video. Do you have
any TV transmitters reasonably close by?

I've had problems with detected video on a couple of occasions, getting
into audio circuits. After I redesigned the audio circuits to look more
like VHF/UHF circuits -- laid out, bypassed, and filtered for VHF/UHF --
the problems disappeared. A working hypothesis is that there's a strong
TV transmitter getting into your transmitter via the antenna and/or its
transmission line, then getting detected and modulating your
transmitter. In the case of my audio problems, the vertical scan
component was worse, resulting in a "hum" that varied with the picture
of the offending TV station. You might take a close look at the spurs
and see if they vary with a local station's picture.

Roy Lewallen, W7EL

John Smith wrote:
Your spurs sound very strange, too close in 32 kHz, 62KHz, normally they are
further out several MHz if generated by the RF chain. That is like an audio
part/circuit getting into the transmit RF. Try rearranging or twisting up
the power cables to the radio.
If spurs are low, not much power is there, so little damage to radio
rts. -65dBc sounds good for a CB. The farther you look down the more spurs
there. The antenna could be reradiating RF power back onto circuit
components, which could be a cause too.

The antenna was pointing to any angle and direction randomly while
transmitting. One strange behaviour observed. The spurs could only be
seen at certain angles of transmission only.

Spurs at 32khz and 62khz away from carrier would cause an interference
at adjacent frequencies. A simple test was being carried out. Two
radios. First radio had 502.025MHz as Tx and Rx frequency. While
second radio had 502.057MHz as Tx and Rx frequency. Transmitting at
502.025MHz would cause a interference on second radio, in which we
could hypothesize that spurs at 32kHz away was too high in amplitude
and thus causing interference to other frequency. So, the spurs were
real! It wasn't some kind of 'illusions' produced by spectrum
analyzer. I guess this is highly undesirable.

Besides, I'm sure those spurs weren't produced by radio internal
circuit as I couldn't see any spurs if radios were being transmitted
into spectrum analyzer directly using 50 Ohm coaxial RF cable. Even
with 4:1 and 8:1 VSWR load, there's no existence of spurs. Could the
antenna's match worse than VSWR 8:1? Perhaps the spurs were part of
the intrinsic characteristic of any antennas?

Maybe the culprit was TV transmitter nearby. I have yet to look into
that. Another test that can be done is to do transmission in a sealed,
interference-free room. If spurs still could be seen, I would say it's
the antenna's imperfection that produced the spurs. If spurs gone,
it's a good news that the spurs were coming from outer space, Mars
maybe.

I would be grateful if somebody could check this out on few radios to
see if this problem exists.

Dummy wrote:
The antenna was pointing to any angle and direction randomly while
transmitting. One strange behaviour observed. The spurs could only be
seen at certain angles of transmission only.

Spurs at 32khz and 62khz away from carrier would cause an interference
at adjacent frequencies. A simple test was being carried out. Two
radios. First radio had 502.025MHz as Tx and Rx frequency. While
second radio had 502.057MHz as Tx and Rx frequency. Transmitting at
502.025MHz would cause a interference on second radio, in which we
could hypothesize that spurs at 32kHz away was too high in amplitude
and thus causing interference to other frequency. So, the spurs were
real! It wasn't some kind of 'illusions' produced by spectrum
analyzer. I guess this is highly undesirable.

Besides, I'm sure those spurs weren't produced by radio internal
circuit as I couldn't see any spurs if radios were being transmitted
into spectrum analyzer directly using 50 Ohm coaxial RF cable. Even
with 4:1 and 8:1 VSWR load, there's no existence of spurs. Could the
antenna's match worse than VSWR 8:1? Perhaps the spurs were part of
the intrinsic characteristic of any antennas?

Maybe the culprit was TV transmitter nearby. I have yet to look into
that. Another test that can be done is to do transmission in a sealed,
interference-free room. If spurs still could be seen, I would say it's
the antenna's imperfection that produced the spurs. If spurs gone,
it's a good news that the spurs were coming from outer space, Mars
maybe.

I would be grateful if somebody could check this out on few radios to
see if this problem exists.

Could it be you're just expecting a little more from your IF filters
than they can realistically eliminate?

73, AC6XG

Dummy wrote:
"One wide-band antenna was hooked up on a spectrum analyzer."

Years ago, from time to time, I`d hear the sheriff`s dispatcher in the
background, and sometimes TV Channel 8 audio, when monitoring a certain
2-meter amateur repeater. It stopped when the Sheriff changed
frequencies.

Turns out the difference between the sheriff and Channel 8 frequencies
matched the IF of my Icom radio. When the repeater opened my squelch,
there the other signals were.

In 1949 I used to turn on the Mighty 790 here before 5:00 a.m. when we
signed on back then.

From my monitor loudspeaker I could hear KTRH, the 50 KW station about
15 miles away which was already broadcasting. The tiny 740 KHz signal
was mixing with the carrier in my final and being rebroadcast. Now both
stations are owned by Clear Channel so it would make no difference. It
made no difference then either because the 740 KHz signal on the 790 KHz
signal was more than 50 dB down. That`s under the FCC requirement at the
time.

I think the spectrum analyzer antenna is clean and the "spur" is a
simple mixing product.

Best regards, Richard Harrison, KB5WZi

"Dummy" wrote in message
om...
Roy Lewallen wrote in message
...
More thoughts along the lines of John's comments, 32 kHz is about twice
the frequency of the horizontal line component of TV video. Do you have
any TV transmitters reasonably close by?

I've had problems with detected video on a couple of occasions, getting
into audio circuits. After I redesigned the audio circuits to look more
like VHF/UHF circuits -- laid out, bypassed, and filtered for VHF/UHF --
the problems disappeared. A working hypothesis is that there's a strong
TV transmitter getting into your transmitter via the antenna and/or its
transmission line, then getting detected and modulating your
transmitter. In the case of my audio problems, the vertical scan
component was worse, resulting in a "hum" that varied with the picture
of the offending TV station. You might take a close look at the spurs
and see if they vary with a local station's picture.

Roy Lewallen, W7EL

John Smith wrote:
Your spurs sound very strange, too close in 32 kHz, 62KHz, normally
they are
further out several MHz if generated by the RF chain. That is like an
audio
part/circuit getting into the transmit RF. Try rearranging or
twisting up
the power cables to the radio.
If spurs are low, not much power is there, so little damage to radio
rts. -65dBc sounds good for a CB. The farther you look down the more
spurs
there. The antenna could be reradiating RF power back onto circuit
components, which could be a cause too.

The antenna was pointing to any angle and direction randomly while
transmitting. One strange behaviour observed. The spurs could only be
seen at certain angles of transmission only.

Spurs at 32khz and 62khz away from carrier would cause an interference
at adjacent frequencies. A simple test was being carried out. Two
radios. First radio had 502.025MHz as Tx and Rx frequency. While
second radio had 502.057MHz as Tx and Rx frequency. Transmitting at
502.025MHz would cause a interference on second radio, in which we
could hypothesize that spurs at 32kHz away was too high in amplitude
and thus causing interference to other frequency. So, the spurs were
real! It wasn't some kind of 'illusions' produced by spectrum
analyzer. I guess this is highly undesirable.

Besides, I'm sure those spurs weren't produced by radio internal
circuit as I couldn't see any spurs if radios were being transmitted
into spectrum analyzer directly using 50 Ohm coaxial RF cable. Even
with 4:1 and 8:1 VSWR load, there's no existence of spurs. Could the
antenna's match worse than VSWR 8:1? Perhaps the spurs were part of
the intrinsic characteristic of any antennas?

Maybe the culprit was TV transmitter nearby. I have yet to look into
that. Another test that can be done is to do transmission in a sealed,
interference-free room. If spurs still could be seen, I would say it's
the antenna's imperfection that produced the spurs. If spurs gone,
it's a good news that the spurs were coming from outer space, Mars
maybe.

I would be grateful if somebody could check this out on few radios to
see if this problem exists.

I think somebody else mentioned this in passing, but it is worth considering
whether RF is getting into the radio via the microphone cable, or whatever
the driving source is. You say the spurs change when you rotate the antenna.
Do they go away when the radio is at an antenna nul? Can you key the radio
with the microphone unplugged? If not, have somebody look at the spectrum
analyzer while somebody else is moving the mic cable around.

This might be grasping at straws, but worth copnsidering.

Tam/WB2TT

"Tam/WB2TT" wrote in message ...
"Dummy" wrote in message
om...
Roy Lewallen wrote in message
...
More thoughts along the lines of John's comments, 32 kHz is about twice
the frequency of the horizontal line component of TV video. Do you have
any TV transmitters reasonably close by?

I've had problems with detected video on a couple of occasions, getting
into audio circuits. After I redesigned the audio circuits to look more
like VHF/UHF circuits -- laid out, bypassed, and filtered for VHF/UHF --
the problems disappeared. A working hypothesis is that there's a strong
TV transmitter getting into your transmitter via the antenna and/or its
transmission line, then getting detected and modulating your
transmitter. In the case of my audio problems, the vertical scan
component was worse, resulting in a "hum" that varied with the picture
of the offending TV station. You might take a close look at the spurs
and see if they vary with a local station's picture.

Roy Lewallen, W7EL

John Smith wrote:
Your spurs sound very strange, too close in 32 kHz, 62KHz, normally
they are
further out several MHz if generated by the RF chain. That is like an
audio
part/circuit getting into the transmit RF. Try rearranging or
twisting up
the power cables to the radio.
If spurs are low, not much power is there, so little damage to radio
rts. -65dBc sounds good for a CB. The farther you look down the more
spurs
there. The antenna could be reradiating RF power back onto circuit
components, which could be a cause too.

The antenna was pointing to any angle and direction randomly while
transmitting. One strange behaviour observed. The spurs could only be
seen at certain angles of transmission only.

Spurs at 32khz and 62khz away from carrier would cause an interference
at adjacent frequencies. A simple test was being carried out. Two
radios. First radio had 502.025MHz as Tx and Rx frequency. While
second radio had 502.057MHz as Tx and Rx frequency. Transmitting at
502.025MHz would cause a interference on second radio, in which we
could hypothesize that spurs at 32kHz away was too high in amplitude
and thus causing interference to other frequency. So, the spurs were
real! It wasn't some kind of 'illusions' produced by spectrum
analyzer. I guess this is highly undesirable.

Besides, I'm sure those spurs weren't produced by radio internal
circuit as I couldn't see any spurs if radios were being transmitted
into spectrum analyzer directly using 50 Ohm coaxial RF cable. Even
with 4:1 and 8:1 VSWR load, there's no existence of spurs. Could the
antenna's match worse than VSWR 8:1? Perhaps the spurs were part of
the intrinsic characteristic of any antennas?

Maybe the culprit was TV transmitter nearby. I have yet to look into
that. Another test that can be done is to do transmission in a sealed,
interference-free room. If spurs still could be seen, I would say it's
the antenna's imperfection that produced the spurs. If spurs gone,
it's a good news that the spurs were coming from outer space, Mars
maybe.

I would be grateful if somebody could check this out on few radios to
see if this problem exists.

I think somebody else mentioned this in passing, but it is worth considering
whether RF is getting into the radio via the microphone cable, or whatever
the driving source is. You say the spurs change when you rotate the antenna.
Do they go away when the radio is at an antenna nul? Can you key the radio
with the microphone unplugged? If not, have somebody look at the spectrum
analyzer while somebody else is moving the mic cable around.

This might be grasping at straws, but worth copnsidering.

Tam/WB2TT

Even with microphone unplugged, the spurs still could be seen. At
certain angle of transmission, the spurs would seem to be disappeared.
I have no idea where the spurs came from.

Are you using a switch mode power supply to run your radio's?

You can get modulation products produced by switching noise on the supply rail.

Regards

Robin Cassidy

"Robin Cassidy VK3AYZ" wrote in
message om...
Are you using a switch mode power supply to run your radio's?

You can get modulation products produced by switching noise on the supply
rail.

Why do switchers have such a reputation in ham radio?

I realize it's possible to hose up a switcher design pretty thoroughly, (you
can also make a 7805 into about a 2W transmitter on 160M if you're not
careful), but I've never seen the sorts of problems that I keep hearing
associated with them..

BTW, I design switchers, and use them for power supplies in low noise
systems, without shielding or other expensive "voodoo" of any kind.

When designed properly, they are very efficient, and very nearly "silent".

Dave VanHorn wrote:

"Robin Cassidy VK3AYZ" wrote in
message om...

Are you using a switch mode power supply to run your radio's?

You can get modulation products produced by switching noise on the supply

rail.

Why do switchers have such a reputation in ham radio?

I realize it's possible to hose up a switcher design pretty thoroughly, (you
can also make a 7805 into about a 2W transmitter on 160M if you're not
careful), but I've never seen the sorts of problems that I keep hearing
associated with them..

BTW, I design switchers, and use them for power supplies in low noise
systems, without shielding or other expensive "voodoo" of any kind.

When designed properly, they are very efficient, and very nearly "silent".

1) Leakage Inductance from the core allows a small magnetic field to be
radiated.

2) By their nature they require a 'small' ripple voltage to exist on the
output as a consequence of the switching principle.

3) The input power, the raw power, is being modulated by the switching
cycle. This generates a transient pulse power on the input lines that
easily couples noise via various susceptibilities into the receivers.

4) The variable switching cycle, or the varying switching duty cycle,
creates broadband noise.

5) All of which are potential sources of trouble in a communications
receiver that wants to find a 0.16 uV/meter signal in the presence of noise.

As a designer of switchers, have you ever had to Qualify a switcher to
MIL-STD-461 and 462?. There is a conducted interference test on the
input power lines to the switcher that is brutal. The ripple on the
output fails the requirement of MIL-STD-462. The radiated emissions have
to be controlled, etc. The switching transients from ALL sources cause
EM Susceptibility in related equipment.

It's not impossible to design switchers for a quiet environment, but
I've spent $millions on getting them quiet enough to meet
MIL-STD-461/462 requirements.

And can get EVEN WORSE! Tho realize that these type supplies seldom
used,nowdays, back in dark ages, had SQUARE WAVES-- generate infinate order
of (if memory serves me) odd harmonics!! when filters went bad, baseband
looked like a picket fence on spectrum anaylizer! ant, at 600 channels,
that loaded to about 2.5 MHz!! But you could hear these heck of a lot
higher with low band rcvr!! Jim NN7K


"Dummy" wrote in message
om...
(Robin Cassidy VK3AYZ) wrote in
message . com...
Are you using a switch mode power supply to run your radio's?

You can get modulation products produced by switching noise on the
supply rail.

Regards

Robin Cassidy

I think there are no tests that can be conducted to determine the
source of the spurs. RF radiated from antenna could cause interference
and non-linearity at spectrum itself. Unless I could 100% shield the
spectrum from RF, all tests would be invalid. Even if I stood faraway
from spectrum, the spurs could also be observed. However, by
transmitting into spectrum using 50 Ohm coaxial cable, no spurs were
seen. Possibly because RF was transmitted into spectrum without any
leakage which could have large effect in causing interference.

3) The input power, the raw power, is being modulated by the switching
cycle. This generates a transient pulse power on the input lines that
easily couples noise via various susceptibilities into the receivers.

Only if you fail to decouple the input properly.

4) The variable switching cycle, or the varying switching duty cycle,
creates broadband noise.

5) All of which are potential sources of trouble in a communications
receiver that wants to find a 0.16 uV/meter signal in the presence of
noise.

Potential, yes I suppose.
As I said, I know you can make them perform badly, but it's just not that
hard to make them perform well enough that they won't be noticed on your
receiver, assuming you don't make it a practice to connect the power supply
directly to the antenna inputs.

The receiver itself may create more noise than a properly designed switcher.

As a designer of switchers, have you ever had to Qualify a switcher to
MIL-STD-461 and 462?.

That one I haven't. My hamshack dosen't require MIL-STD-461 either. That's
a large complicated battery of tests, that by it's very nature is expensive
to test to, even if you were testing a D-Cell battery.

I have several switchers in use, all commercial designs, and they are barely
detectable on my R-8500 or FT-847. I haven't had to go to any extremes (or
even any measures at all) to quiet them.

My PCs are another matter, they have needed ferrites on the cables, and EMI
absorption material inside the case, but that has proved to be managable.