On Sat, 7 Jan 2012 08:50:51 -0000, "N_Cook" wrote:
I've no idea if it would work but can you make a portable FM receiver
directional by placing in a metal tube?
Not really. The tube would need to be about 1/2 wavelength long
(about 1.5 meters) and would totally block the signal. If you run a
1/4 wave lengthwise slit down the length of the tube, you would be
able to hear something (at reduced sensitivity). However, it wouldn't
be very directional. I've built rotating tube type direction finders
using this principle, but at 2.4Ghz, where the size of the antenna is
more practical.
Perhaps with a metal plate under a
shoe with a grounding wire. Would have to be speaker output , so an earpiece
did not fed RF down
You can stop the earpiece from becoming an antenna with a simple
ferrite bead at the EP jack.
Much of what you suggest has already been done by hams for transmitter
hunts.
http://www.homingin.com
There's quite a bit of existing equipment available that will work.
Something cheap and crude are the various "homer" type direction
finders. Also known as TDOA (time difference of arrival).
http://www.handi-finder.com
http://www.arrl.org/direction-finding
http://www.dxzone.com/catalog/Operating_Modes/Radio_Direction_Finding/
etc..
AN/SRD-21 homer type direction finder manual (4.5MB)
http://802.11junk.com/jeffl/AN-SRD-21/ANSRD-21%20manual.pdf
Much of the technology use for the 2meter (145Mhz) ham band is
directly applicable to the 88-108 band if scaled. Unfortunately, all
the problems and headaches are also applicable (interference from
other stations, reflections, insufficient directionality, antenna side
lobes, etc).
Direction finding on the FM broadcast band is problematic because of
the potential for interference from adjacent channel stations. The FM
receiver IF bandwidth usually slops into the adjacent channels. In
the US, we have 200KHz channels, with allocations every alternate
channel. This reduces the problem, but doesn't eliminate it. In EU,
it's on 100KHz channels, possibly with alternate channel allocations.
However, methinks all this is overkill for finding a noise source. The
first thing I would do is turn off the power to the house, except for
the FM receiver. Many noise sources are conducted (through the power
lines) rather than radiated (over the air). If the noise goes away,
it's coming from something in the house. From there, it's just a
matter of turning off breakers until the location is isolated. Then,
turn off individual pieces of equipment. I've found an amazing
assortment of noise sources this way. If killing the power doesn't
find it, ask the neighbors to do the same (while carrying a portable
FM receiver). You'll be amazed at how many consumer devices generate
disgusting levels of RFI/EMI. From the description, the 10 sec to 1
hr duration is an important clue. That points to a thermostat, air
flow, or water demand operated device. It might also be an electric
heater. Killing the house power should be able to isolate it.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558