
6 GHz frequency modulated radar - paulgerhardt
http://hforsten.com/6-ghz-frequency-modulated-radar.html
======
jwr
Very impressive. I would have thought that this sort of RF work is outside the
hobbyist realm.

The rest of the blog is equally impressive — 217-ball BGA soldered in a home-
made reflow oven?

~~~
rasz_pl
Soldering BGA is not all that impressive, routing proper pcb for one is.

Contrary to popular belief soldering SMD in general is way easier than dip,
all you need is an oven/hot plate/hotair gun and solder paste/flux. Magic
happens due to molecular surface tension instead of your manual dexterity.

~~~
thrownaway2424
It's a shame the way the impedance-controlled traces on this board turned out
all janky though. I get boards done at Sunstone and I guess the price might be
higher but the results are good.

I think most people under-estimate the degree to which you can click a button
on your computer and get manufactured goods in the mail. Even for small runs
like ten boards I can cost-effectively get the boards fabricated and stuffed
and delivered to my desk in a day or a few days depending on how much I'm
willing to pay.

~~~
conesus
How much are you paying per in^2 to have Sunstone fab a board? Are you paying
for assembly? OSH Park, as used in the article, is only $10/in^2 for 4-layer.

~~~
thrownaway2424
It strongly depends on how many you order, since nearly all the cost is in the
setup. A 10 in^2 2-layer board is ~$300 for ten boards.

------
lifeisstillgood
There is too much interesting cool stuff in the world - I will never get to
try out half of it !

It's like realising that willy wonka's chocolate factory is bigger than you
can walk around in one lifetime.

Darn it.

~~~
JabavuAdams
Step 1: Life-extension

------
ChuckMcM
I really love that you can get a power amplifier for the 5Ghz RF for about $2.

~~~
mng2
What costs money is the spectrum analyzer, network analyzer, and Microwave
Studio license.

~~~
bsder
Exactly.

 _Testing_ RF is a PITA because the test equipment is so stupidly expensive.

~~~
Animats
Yes. Back in the 1990s I built a little LIDAR system using much the same
principle as this system - modulate the output beam with a 20MHz carrier,
down-convert at the receive end using the same local oscillator like an FM
receiver, and measure the phase shift between the down-converted output and
input. That transformation is phase-preserving, and the phase shift at the
lower frequency represents the range.

It wasn't hard to build. But I didn't have the test equipment to adjust it.

------
jkaunisv1
I don't really see how the radar images at the bottom reflect the scene he
showed. Any tips on reading those?

~~~
hwillis
The radio waves are collected at points and don't have any x or y data, so
making an image out of it is pretty challenging and usually requires moving
the radar beam. He just kept the beam still, so the only things you can really
see are things that move.

In the image, the bottom axis is distance(calculated from the time the
reflection took/the frequency output by the mixer), the color is the strength
of the reflection, and the left axis is the time. He walked away from the
setup and then back, visible as the two lines, which fade out almost
completely as he changes direction, because he is far away and therefore looks
smaller to the radar.

[http://hforsten.com/static/img/fmcw/football_field_range.png](http://hforsten.com/static/img/fmcw/football_field_range.png)

From this data you could theoretically build an image of his upper torso and
head, but it would be very blurry. His arms and legs were not static, and the
radar doesn't know how they were moving, so they would just show up as blurs.

~~~
jkaunisv1
Cool, thanks! That helps a lot. Even though I knew what the axes were my
intuition struggled a bit, I think partly because I'm so used to the bottom/X
axis being used for time.

------
lettergram
I'm impressed!

Since, I'm not an EE, is there a better antenna which could reduce the noise
in the signal?

It looks as if the noise comes in a pattern and I was curious if that is
because of the ripples in the antennas (i.e. cans)?

~~~
gcb0
Most highly accomplished EEs can't answer most questions about fields, let
alone antennae design detail.

~~~
hwillis
Good engineers can do dipole approximations in their heads. This would require
a computer- there are just too many internal reflections.

------
lowglow
This really makes me wish I took Antennas as an elective in EE.

Are there any good books/resources to get up to speed on things like this?

~~~
Jach
The lecture notes on the MIT site these projects are inspired from has some
useful PDFs: [http://ocw.mit.edu/resources/res-ll-003-build-a-small-
radar-...](http://ocw.mit.edu/resources/res-ll-003-build-a-small-radar-system-
capable-of-sensing-range-doppler-and-synthetic-aperture-radar-imaging-january-
iap-2011/lecture-notes/)

When I was building my own version, Kai Chang's _RF and Microwave Wireless
Systems_ was a really great resource for the basics. Beyond that, you'll want
to have a good foundation in signal processing to make use of the data from
the device. The author doesn't mention it, but the radar he has is capable of
creating SAR images. I'd love to see the results of a 6 GHz radar -- Dr. Greg
Charvat (one of the MIT guys) has a demo on his website of a 10 GHz version
that's pretty impressive. His thesis can help with some signal processing
details, but the book _Spotlight Synthetic Aperture Radar: Signal Processing
Algorithms_ by Carrara, Goodman, and Majewski is even better.

------
fegu
Very impressed. Solid knowledge demonstrated. The blog makes it seem so easy,
but imagine the years of trial and error needed to produce this knowledge
(which of course the author gets "for free")

------
madengr
Nice work. Could ditch the Wilkinson though since your coupling to the mixer
is -10 dB, so just use a resistive tap instead of 3 dB splitter and 7 dB
attenuator.

------
darkarmani
I'm not sure this is legal in the US w/r/t FCC. I know the author isn't in the
US, but pointing this out for US hackers.

~~~
hwillis
It would be kind of grey. He was aiming for 5.8 which is an open band in the
US, although his bandwidth was way above the width of that channel. Generally
though unless you are actively impeding someone else or, god forbid, you're on
government or medical channels, you're pretty okay and you are unlikely to get
hunted down. People get nabbed for jamming cell phones and such but if you are
using it rarely and not using a massive amount of power, nobody will notice.

Good practice is to stick to the open bands:
[http://en.wikipedia.org/wiki/Amateur_radio_frequency_allocat...](http://en.wikipedia.org/wiki/Amateur_radio_frequency_allocations#ITU_Region_2)

Also, he says he's aiming for a radar band: "The reason I chose to use 5 – 6
GHz band was because there is a band reserved for radars around 5.8 GHz and
this range overlaps with 5 GHz WLAN."

~~~
Johnythree
The issue is not Licensing, but Type Approval.

Even on the ISM bands
([http://en.wikipedia.org/wiki/ISM_band](http://en.wikipedia.org/wiki/ISM_band))
any transmitter must still be submitted for Type Approval Testing.

The only exception to Type Approval is on the Amateur bands where "homebrew"
is permitted (and encouraged). However the Amateur Regs specifically rule out
any commercial use.

The only way this could legally be operated is in a professional quality
screened room, or if the University has an "Experimental" class radio license.

~~~
hwillis
It's in a legal band for any use, but you are correct, the output power is
300-600 times too high to be unlicensed.

------
ck2
This makes me wonder if you could map a building using existing 5ghz wifi n/ac
signals.

------
aidenn0
The best antenna upgrade for least work would probably be to build a Yagi

~~~
hwillis
Actually no, a yagi would be very bad. The Radar has a 700MHz bandwidth,
almost 12% of the center frequency, and yagis work for a fraction of that. A
horn is pretty simple to make.

~~~
aidenn0
Oh! I didn't see the bandwidth was so wide. You are correct. Wouldn't the
resolution still be decent with a narrower bandwidth (i.e. the 10s of MHz that
a yagi could manage)?

Also, as an alternative to a horn antenna, a parabolic reflector should work
well in this band without being too large, right?

~~~
hwillis
The range resolution would go from 8.5" to five yards if you changed the
bandwidth to 30 MHz. Currently the radar could figure out the direction a
person is facing to about 45 degrees. 5 yards would be good to find the
orientation of a house.

Parabolic antennas are just horns with lenses. The bigger the parabola, the
better the focus and the wider the bandwidth. A horn would be a few inches
across, but a 700 MHz reflector would be a couple feet.

