Jamming Enforcement Doppler radar
This is something I've always been curious about , I guess more
for the technical challenge than the practical use of such a device ( I
drive a diesel ok !) . Now I make this plain as day ,to use one of these devices is totally
illegal and would land you in a heap of crap when you get caught. It would be
a double whammy cos two government departments would come down on you like a (
metric) tonne of bricks , the law enforcement division and the radio
regulatory people as well . So don't say you have not been warned!
KNOW THINE ENEMY!
Just about all enforcement speed radars work on the Doppler principle when
a eg radio wave ( or sound ) at a frequency of "n" Hertz strikes
a reflecting moving surface some of its signal ( or all what ever doesn't get
absorbed ) changes frequency, the amount of frequency change is directly
proportional to the relative speed of the reflecting surface.\ and the incident
wavefront . This is described by the formula below
F doppler = 2R.Fo
R is speed in meters/sec. Fo frequency Hz c is
speed of light m/s-1
c
Law enforcement Doppler radars work at three common frequency bands,
known as X band typically 10.525 Ghz, K band 24.150 GHz
and Ka band 33.5 Ghz and a few frequency variants
up to 36 GHz ( 3 GHz wide so there's a lot of room to hide in up here!). They
are all typically Gunn oscillator based devices and operate as direct
conversion receivers with a "Zero" Intermediate frequency .The
fact that they are Gunn oscillators is the first problem as they are not a
particularly stable RF source (but that is of no consequence for their successful
operation ) The IF is in fact in the audio range from 50 Hz up to maybe 10 Khz
depending on the frequency of the Gunn oscillator and the maximum speed the
device is designed to display . So there in lies another hurdle to
overcome, the sensitive D.C receiver is essentially narrow band ( there is
a constraint to this statement and I will explain it later as you can also
argue that it is in fact a wideband receiver as well !) the detection range of all
radars is described with the familiar Radar Range Equation. I guess we are
all familiar with the inverse square law for radio signals ie, where
to double the transmission distance you have to increase the radiated RF
power by a factor of four ?, well in the radar equation, the detection
range of a system is related to the fourth power ! that's why commercial radars
eg ships, aviation , land based , have such powerful transmitters
(Kilowatts) to achieve good range!
2 2
Pr
=
Pt
. G . l
. s Radar
Range Eqn
2
4
(4p)
. R
Think of it this way, there is the inverse square law operating when the
radar transmitter sends a signal out to a remote reflecting object OK ? Now the
signals bounce off that distant object . Think of that reflecting surface
again as a second transmitter( it is sending out a signal is it
not?? the fact that it receives its transmitting power from
the initial illuminating source is only of interest) so its sends
out its signal ,( even if it is only a reflection) It still obeys the
inverse square law but it is radiating its signal back to the initial signal
source ( which at this stage is receiving signals ) so there is two
inverse square laws involved here add them together and you get the Fourth
power in the equation. That's heaps of potential signal LOSS, The take
home message is: to get good range, a radar transmitter needs lots
of transmitting grunt! and because of the fourth power constraint
in the radar range equation , also means you can
pick them up way before they can pick you up !
Enforcement Doppler radars don't have a lot of grunt . The Gunn oscillator
output is probably at maximum of about 100 mW . However at these frequencies you
can have a lot of antenna gain in a small volume. ( typically 15-25
dB) of directional antenna gain associated with that 100 mW and end up with an
EIRP of up to 10Watts down the boresight of the antenna .
(+20 dBm and 20dB gain
gives erp of 40dBm (10 W) !
The antennas typically have a 3dB beamwidth of
about ~ 20 degrees . These radars are used in a directional manner ,so the
high gain antennas are a plus ! you can pick off your target , the
detection beam is
highly directional ! (that is a "reciprocal" statement for receiving
the signal too)
The outer reaches of range of the typical stationary Doppler radar is probably
approaching a mile ( 2 Kms) on a good sized large flat nosed vehicle
i.e large truck/tractor unit. ( lots of plane reflecting surface,) . Given all the constant variables ie one type
of Doppler radar, the range is entirely dependant on the size and shape of the
reflecting vehicle. The range against a small motorcycle ( small frontal reflecting
area) is poor, maybe 300 hundred meters, a low sporty "aerodynamically shaped "sharp" cars give less Doppler reflection
that large "squarer" cars . ( go to google.com ,search and read all about how
stealth aircraft achieve
low radar signatures) The detection range is somewhat less in a
moving situation due to the large amount of signal clutter reflected off
the road directly ahead of the speed detection vehicle. this backscatter, swamping weak
target high
frequency Doppler shift. However the advent of Fast Fourier Transformations
(FFT) algorithms in DSP processing chips processing the incoming audio, do
go a long way to minimise this . There in lies another problem . The newer
Doppler radars are getting smart, they can do spectral analysis on their
targets and pick out the fast ( higher frequency Doppler shifts) from the slower
speeds and also with the microprocessors do mathematical manipulation so a moving
Doppler detection unit can show the actual "ground speed" of an
approaching or receding vehicle, another potential problem!
See what I mean by "Know thine enemy" if you are going
to make a Doppler jammer then you must take into account every
conceivable variable you may come across. You must account for them
or your final result my not work ,or end up with a number of limiting
constraints or you may end up doing exactly the opposite to what you wish to achieve!
( end up with an enforcement notice for twice the speed you "were
doing" ). it is not an easy project (it was when enforcement radars
were simple devices) but not now !
CAN WE FIX IT ?
There are several ways, some are ideas I have not tried and others have suggested.
There is the Brute force method .This I have tried. You synthesise a Doppler
shift and transmit it
to the enforcement radar on the same or close frequency . This is easy to
do, simple electronically and can be done using a ne555 timer
running as a variable square wave audio oscillator generating the required
"Doppler" shift frequency. You run the 555 off a 6 volt
regulated supply and supply the gunn oscillator from pin 3 of the
555 , (it will source up to 200 mA max), this is enough current for the average 10mW
gunn diode. Simply point the gunn osc with its 20 dB horn antenna, at the
enforcement radar and bingo! you can make it read any speed you want it to read,
by just turning the variable resistor that controls the square wave frequency
Hawk being "suckered"
Stalker being "Suckered"
on
the 555 oscillator .
However, you must use the correct frequency verses Doppler shift
i.e. at 10.525 Ghz 33Hz of doppler shift gives 1 mph eg 3300 Hz
square wave into the gunn osc will show 100 mph on the enforcement radar.
There are other scaled "Doppler constants" for 24 GHz and 33 Ghz
. easy huh ! and yes it works, Ive tried it (against a unit operating
at 24 GHz) when you point it at the radar! BUT in practice its about as
useless as "titts on a bull" ( sorry to pop your bubble) I have seen
variations of these sort of circuits for sale on the 'net. Some are quite
sophisticated with digital readouts for setting the "jamming speed"
and are triggered into action by the receipt of a signal by a conventional radar
detector.
I give them full marks for the Concept of "electronic
countermeasures" but I wouldn't trust one of those devices to protect me
against enforcement radar here! All the jamming circuits / units I've
seen are low power devices, maybe 50mW output ? into a 20 dB horn . I made up a simple device as described
above with the NE555 and a 5mW 24 Ghz gunn osc and a 25db horn .
Output spectrum of the 555 modulated gunn osc
24 Ghz gunn osc and horn antenna
The maximum
"jamming" range I could get was about 250m (you're excited about
this aren't you ! I can tell ) that was the maximum range at which I could make
the 24 GHz radar read the speed I wanted . HOWEVER when we tried it in a moving
car coming towards the Doppler radar it was a dismal failure, the received
"true Doppler"
signal swamped the effect of the synthesised Doppler jamming signal
completely ,not enough grunt!. In order for it to work, we would have to increase
the power of my
jamming oscillator to a level such that the Doppler radar could pick up my
jamming signal before it picked up its true reflected signal.
This
brings the variables into the equation like frontal area of the jamming vehicle verses
detection range etc. Note that the above simple experiment shows that the closer
you are to the Doppler radar the harder it is to jam! Lets do some
simple maths and try to work out how much jamming power I need to hit the radar
before it gets me.
Assume my 250m jammer puts out eirp of +7 dBm and a 25
dB horn gives eirp of +32 dBm ( just over 1 Watt ) To double the
jamming range, I increase my gunn osc by 6dB ( 4X ) to eirp of +38 dBm, we now jam at 500m
. Increase another 6 dB , this takes it out to 1000m ( eirp of 44dBm ) and to be safe we
increase another 6 dB and take it up to an eirp of +50 dBm ! .( almost 100 Watts
EIRP.) That
will enable us to make the Doppler radar read at 2 Kms away line of sight ( if we
point straight at it AND it is looking straight down the antenna boresight
at us) ( this is in an optimum configuration hardly what you might find in real
life or in a worst case scenario) But what about the antenna gain.?
Well we cant really increase it much and there is a limit to the amount of gain we want to go to, as the higher
the gain, the larger ( physically) and more directional it is , the less is the
"spread" or coverage of the "protective" signal )
We
have had to increase the output of our signal source ( gunn osc) by +18 dB!
we would have to use a +25 dBm output oscillator , we are now into the relm of high power
impatt diode oscillators and this translates to mucho $$$$$$ and high current high
voltage sources 30-70 volts (depending on the manufacturer and the device etc.)
its looking like its much cheaper to drive at a legal speed isnt it !.
There will be those out there who will be saying "bollocks" I
can talk on my 5 mW gunn oscillator transceiver to someone else over a
line of sight distance of greater than 10 miles ! and that's true, yes you can .
I mentioned before that the Doppler radar receiver can be treated as a narrow
band or a wide band receiver and I will clarify this, When we send our synthesised pulsed Doppler signal
to the radar, we are "punching" into the receiver in the wideband mode . I'll
explain .When we flood the enforcement radar with our signal, the diode in the
receiver is operating in "video detector" mode ie converting pulsing
rf into pulsing dc at our generated audio rate , It is NOT MIXING signals
to produce an audio IF, so our radar receiver is just a crystal set. Much like
those early passive radar detectors we used to make years ago that had a chopper
diode ahead of the video detector diode in the tuned cavity . This video
detected chopped DC
signal was fed into a high gain amplifier and then a tone at the chopping rate
was heard on a loudspeaker . In essence you "heard" the enforcing
radar. They were not very sensitive compared to the superheterodyne
receivers we have now (which incidentally still don't have an RF amp before the diode
mixer, so they can be multi frequency)
A straight video detector diode operated
in an optimal mode with correct bias matching etc has a TSS (Tangential
sensitivity) minimum detectable signal of about -50 to -60 dBm ! that's still a
lot of signal . Remember that is a diode with good matching and optimum bias current ,
NOT a heavily biased mixer diode sitting in a cavity swamped with a couple of
milliwatts of RF from the gunn oscillator cavity next door! In video
detector mode and the gunn osc going , the diode in "TSS mode" as deaf as a post!
(There are some enforcement radars that use this principle to see if they are
being jammed , if there is still audio signal coming out of the detector/mixer
diode when the gunn oscillator is turned off then this must be an interfering
signal and not a true Doppler generated one !) and the read JAM on the digital display
!
Clever Hawk detecting Jamming !!!!!!!!
This hopeless lack of sensitivity in broadband mode,
is why we have to hit the Doppler receiver with so much RF to
make it read our synthesised speed .
This is why I wouldn't have much
faith in those Internet passive radar jammer plans..... just not enough
returned signal !
The other problem is that the Doppler receiver diode is hugely more
sensitive in MIXING MODE and we are talking microvolts of signal (-100 dBm
) so its at least ~50 dB more sensitive in "narrowband" mode ( crystal
set verses zero IF "direct conversion" ) it is much more sensitive to its own
synchronised reflected Doppler signal , that's why I couldn't jam or over
ride the radar with my gunn
oscillator.
YES WE CAN!
The only way we can have any real cost effective effect on the Doppler radar
is to get into its receiver in "Narrowband mode" . This is very tricky and if we could
do it in "narrowband " mode we could knock it off with the
described 5mW gunn osc and antenna from a couple of miles line of
sight. ! it is not easy and I haven't tried this idea yet. You need to phase lock
your jamming oscillator in CW mode to within a couple of Kilohertz, of the
doppler radar signal , this will
enable a mixing process to occur within the radar receiver and produce a very strong
interfering signal , There in lies the difficulty, Varactor controlled Gun
oscillators with wide tuning range, particularly at 33 GHz would present a
problem . also the doppler radar gunn oscillator is not particularly
stable either. Now we have two independent variables to account for ! to
lock the jamming oscillator, one could produce a simple Direct conversion receiver at the frequency
of interest with a very wide bandwidth IF , this IF could be fed into a
frequency/ phase detector PLL ? operating at ~2 Khz ,the amplified
voltage out put of the PLL phase detector drives the frequency controlling
varactor of the jamming oscillator? . Thus one attempts to lock the jamming gunn oscillator to the Doppler
radar . This system would have to be modulated by a ramp voltage on the tuning,
to sweep the jamming receiver across a range of frequencies of interest. The ramp voltage
would be inhibited when the wanted signal appeared within the bandwidth of the
receiver allowing "lock" to be achieved . In the interest of
rapid acquisition of lock, one may have to start with a very wide band IF of many MHz
then switch to narrower bands to "zoom in" on the wanted signal .This
would have to be done incrementally as his is similar to having the sweep speed on the spectrum analyser too fast when
in too narrow a bandwidth IF filter, at too great a dispersion , The display
response is so distorted and at a much reduced level ,this is what would happen
with a narrow audio IF and a fast wide sweep by the search ramp
voltage. starting to get complicated isn't it ? ( its easier
just to drive slowly !!) but not nearly as much an electronic challenge!
The Third idea suggested by a friend Peter Williams is in the same vein
as passive radar jammers ( that work in theory but DON'T work in practice
for the ratio of reflected to modulated signal strength reasons
I've mentioned above) . This
technique gets around the problem of synchronising the return signal to the
transmit signal so we can get into the radar receiver in
"narrowband" mode . We use the received signal as our source of our
transmit signal . like a transponder principle? Pick it up on a horn antenna
amplify it by 50-70 dB (by what ever maximum level cheque book and rf isolation
allows,) then retransmit it back to the Doppler radar BUT chop the final amplifier
off and on at an appropriate audio rate to OOK (off on keying) modulate the returned
signal , This will put sidebands on it! I'm not sure how this will go as the
return signal already has a Doppler shift on it and we are adding another double
sideband signal on top of that as well ! god only knows how the Doppler radar
will interpret it (we could make matters worse for ourselves )
The Fourth idea I had is a variation on the one above. Use a grunty
super regenerative detector quenching at around 3 Khz or so.(I dont know if you
have seen on a spectrum analyser how wide the noise is that
comes out of one of those things!) to lock to the
received signal and re radiate a dirty strong signal back to the
doppler radar . Super regens at this frequency may not be all that sensitive
and from the research I have done on them, maybe only in the
order of -60 to --70 dBm for lock to occur . However they are a very simple receiver
. I have had a 10 Ghz gunn oscillator self super regenerating but the
sensitivity was very poor! though it did quiet when it received a strong
signal !
Seen on the dashboard of the average patrol car
!!
This is the 33 Ghz
casing
close up shot of PC board with low noise op amp
(LT1115) and AGC
chip LP2951
Business end Tx cavity and Rx cavity face each other note
small phase shift probe to convert to circular
polarisation 33.5 Ghz gunn assly 5.5 v
perhaps 100 mw ??
The unit operates off 12 volt , so i wired up 12v gell cell
to the doppler module, also I connected an Audio amp with speaker to
the audio output . The unit is very sensitive , you can
"hear " people walking down the road 100m away
and it
reminded me of a heart beat through a doppler heart beat monitor , the
swish swish of the legs moving .I guess the shorter wavelength gives better
resolution ? Anyway I drove to a good long clear
length of road to assess the maximum range of the doppler unit
.
The first thing I notice is the frequency of the doppler shift is very
high, about 100 Hz per mile an hour at 33 Ghz , so vehicles at 50-60 Kph
have a very high doppler pitch ,,too fast and they are hard to hear
,( but no problem to the DSP electronics ) the slow 30 Kph sounds much
more audible . The max range for average size motor vehicles , I can
hear with my own ear, is around 1000 m and you must have the unit aimed
down the boresight at this range. Going by the "directivity " of
the unit Id say the horn has quite a high gain , so to get good detection
range, the officer using the device has to aim it well ! I
may try and work out what the beamwidth is ?? pity it did not
have a varactor diode in the gunn cavity as it would be cool to hake a
CWFM radar out of it !
So there are a few ideas on the electronic countermeasures , as you can see
to actually make any impact takes a huge amount of clever circuitry and $$$$
. it is much easier and cheaper to drive responsibly !!
however one could take another approach as used by stealth aircraft and
absorb the reflected microwave signal !! , There are available carbon
impregnated foam polyethylene and conductive rubber that can be layered over the
vehicle to reduce its radar signature............... its not cheap
$$$$.....
Well the next test is to take a large piece of conductive
foam 1000 mm by 1200 mm as used to insert
IC into for static protection .and wrap it around the front of my
motorcycle ! we will do a simple range test deriving reflected
signal strength using the AGC voltage derived from the agc
chip in the direction conversion Rx board with the
motorcycle naked ( no foam ) and then do another run with the foam
wrapped around the front area .
Unfortunately the foam I have is of relatively high resistance ,
several K ohms / cm so may not work as well as foam with an
impedance/resistance of 377 ohms per centimeter . The problem is
although the carbon impregnated conductive foam will absorb microvaves
well , it is a rapid impedance transformation between free space and
the material . When you have mismatches , you get reflections
! these are what we are trying to minimize so i am not sure
how well this experimet will work . Real microwave absorbant foam is of
the correct resistance and usually has pyrimids shapes cut into it so
reflected signals bounce around in the vallys of the pyramids and eventually get
absorbed with very little signal being reflected . This could
be impractical on a vehicle however would work but would be a bastard in
the rain , it would act like a sponge your motor bike would end up
very front heavy. to be continued ...................
....... but that is another story !! drive
safely !!