Very bizarre antenna function

So with MUCH ado, ./usrp_wfm_rcv.py WORKS!

right now 10dB/div for both settings, and my station is 98.7M.

Now here is the weird part… I get an absolutely crystal clear signal
if I
put my hand near the antenna. Like an inch away!

I can also put the antenna such that it is near my fan or something, and
it
works fine.

Why would this be? I’m sort of flying by the seat of my pants when it
comes
to attaching the antenna (it’s just your basic black hinged one). Are
there
any instructions or diagrams that show how it should be attached and
positioned? And why would placing it near things help some come clearer?
Very confusing! Ideally, I want to be able to pick up any station that a
normal radio could so I can use usrp_spectrum_sense and whatnot.

Your help is appreciated. You have no idea how excited I was when radio
came
over my computer… to paraphrase an article, “congratulations, you just
turned thousands of dollars of equipment into something that you can buy
for
5 dollars.”

Why would this be? I’m sort of flying by the seat of my pants when it comes
to attaching the antenna (it’s just your basic black hinged one). Are there
any instructions or diagrams that show how it should be attached and
positioned? And why would placing it near things help some come clearer?
Very confusing! Ideally, I want to be able to pick up any station that a

What frequency bands does your antenna work in? It sounds like it
only works when it’s being coupled to ground (via your hand, an
appliance, etc) I would try using an appropriately lengthed piece of
wire as an antenna and see if that works.

Jason

On Thu, 2009-07-23 at 15:36 -0400, Jonathan C. wrote:

I’ve attached a picture of my setup w/the antenna. My goal is to be
able to clearly capture as many stations as I can, can anyone
recommend something? How do cars or clock radios do it?

(Your photo is swapped left-to-right–it looked at first like you were
using TX slot B instead of RX slot A until I figured this out.)

Is that an antenna supplied by Ettus R.? It looks like the 900
MHz or 2.4G antenna.

These are worse than inefficient at 100MHz; they might even be close to
the equivalent of a short or open circuit at that frequency.

Try taking a 2.5 foot piece of wire and inserting it into the center pin
of the SMA connector at the front of the USRP.

Johnathan

With some finagling, here are the stations I can get clearly. On a
couple
others have the hint that a station might be there, but in these I can
actually hear things.

106.1
105.3
102.1
98.7
93.3
89.9

I’ve attached a picture of my setup w/the antenna. My goal is to be able
to
clearly capture as many stations as I can, can anyone recommend
something?
How do cars or clock radios do it?

2009/7/22 Jason U. [email protected]

What I really meant was, “What frequency bands are the antenna you’re
using designed to work with”. From the picture it’s just a whip
antenna, meaning for optimum reception it should probably be much
longer.

A quarter wave whip would be 1/4 of the wavelength of the signal you
are trying to receive, 3e8/(100e6) = 3m wavelength, so 3/4 of a meter
or about 2.5 feet (just like the antenna on your car :wink: ). If you
just want to see if it works I might suggest getting a length of wire
and cutting it to size, then strip the end and push it into the
connector slot. You could also get one of the collapsy antennas you
find on boomboxes and the like and hook it up. The problem you will
run into is that the SMA connectors are usually used in HF/RF
applications, so you will need some sort of adapter.

If you want to build a fancier antenna (still easy to do), just google
‘FM antenna’ and you will get lots of sites explaining both the theory
and construction of FM antennas

Using an appropriately sized antenna will greatly increase your signal
strength.

Jason

Thank you very much. As per Johnathan’s response also, I think I grabbed
the
wrong antenna when I was picking up all the equipment (they also do work
in
higher frequency stuff, so I think I grabbed an antenna they use for
that.
Antennas are the most mysterious part of this whole thing to me so I
didn’t
know better).

I’ll do exactly what you suggested, thanks!

2009/7/23 Jason U. [email protected]

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