Re: [Discuss-gnuradio] Antenna Questions

[Eric Blossom]

On Sat, Aug 26, 2006 at 10:29:24AM -0600, Jim Borynec wrote:
> Hi All,
> 
> I've installed/compiled gnuradio on an Ubunto box.  Together with a USRP 
> and a 3 foot length of wire,
> I've managed to turn my computer into an FM radio.
> I really appreciated all of the tutorials and how-to's that people had 
> prepared for me.
> It made things a *lot* easier.

Great, glad to hear that it's working.

> Next, I'd like to play with two antennas and compare the time latencies 
> between them.
> (this would give me some primitive direction finding abilities.)
> I can put the antenna's out in the back yard, but I don't want to move 
> my computer,

OK.

> Are there any faq's on "roll your own" FM antennas?
> Is it simply a matter of taking a coax cable,  cutting bare the center 
> wire for a metre and
> plugging it into my basic RX daughter board?

I'd start with a couple of 1/4 wave monopoles.

1 wavelength at 100 MHz is about 3m.
1/4 of that is 75cm.  

You'll want to space the antennas 1/2 wavelength apart.

Mount your monopoles vertically over some kind of ground plane.
I used "hardware cloth" from Home Depot.
http://comsec.com/2x2-antenna.jpg

> The daughterboard has a jack for the antenna, but it looks too small for 
> a normal cable TV sized coax cable. what type of connector do I use?

SMA.  Jameco carries a variety of SMA-to-foo connectors.
http://www.jameco.com/webapp/wcs/stores/servlet/CategoryDisplay?langId=-1&storeId=10001&catalogId=10001&categoryId=304550

> If I keep the wires the same length then any time differencial in 
> signals must be due to the different speed of light latencies in 
> reception, correct?

Yes, assuming the group delays through all the analog h/w are the
same (reasonable assumption for Basic RX).

> How far from the computer can I move the USRP and still get decent
> throughput?  can I put the antennas in the back yard and run them to
> a USRP by the side of the house (where there is a plugin), then run
> a USB wire to my computer inside the house?

I think USB 2.0 is spec'd to 2 meters.

You can run the coax from the antennas a pretty good way depending on
the type you use.  There are tables that will give you the
loss/distance as f(freq).  http://www.ocarc.ca/coax.htm

> Is there an easier way to do this stuff?  I saw a picture of 4
> antenna on the gnuradio passive radar page. That is sort of the
> configuration I'll eventually want to work towards.

Two people built it in an afternoon out of pvc pipe, zip ties,
hardware cloth and a few odds and ends from Radio Shack.

> Thanks in advance for any help.

You're welcome!  Have fun and let us know how it goes!

FYI, there's code to gather data from 4 antennas
in gnuradio-examples/python/multi-antenna/*.py

If you want to try to DF in the 2-meter band, take a look at using a
Ramsey PR100 on each input.  They should work fine in front of the
Basic Rx and will provide gain and selectivity that you'll want.
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=PR100

You'll need to calibrate the system to handle the differences between
the two inputs.

Eric