Directional antenna to locate RFI

Sorta. Transmitter hunting is an art. Under ideal conditions, it's almost trivial with a decent antenna and receiver. However, under typical conditions, it's a nightmare. Reflections, multipath, multiple sources of RF, limited rx dynamic range, antenna side lobes, and such cause nothing but trouble.

The "snoop" program scans at 3 speeds. None are ideal. If you have a CW or FM source, that's always on, then it might be possible. However, if the transmitter is on only erratically, or moves around, or drifts (as in a microwave oven), the time that it will take the snoop program to move across the band is far too long to be useful.

Quite reasonable, under ideal conditions. It really depends on what you're chasing, what else is around, and how much time do you have to do this. The method I use is to direction find on the move. I pick a location, and draw a line of position on a map to where the antenna points. I then move several hundred feet in hopefully a perpendicular path to the source, and take another line of position. Repeat a few dozen times and you will find the map has one location where most of the lines converge. There will be some lines off into some strange directions. These are reflections and can be discarded. Where the lines converge is the transmitter.

However, you're not there yet. I've done this and found myself facing the outside wall of an apartment building with no clue which apartment is the source of the interference. The signal is strong enough that the directional high gain antenna overloads the receiver. So, I have to use an attenuator in the receive path. 2.4GHz step attenuators are not cheap or easy to build. For paying work, I use an MMDS downconverter and a borrowed portable spectrum analyzer.

For close in work, a simple 2.4GHz field strength meter will be sufficient if used with a high gain antenna. However, walking down the hallway with a 3ft diameter dish antenna is not a good idea. I use a butchered microwave oven leakage detector, RF amplifier, and

19dBI dish antenna.

Parabolic dish. The bigger the better. 24dBi gain and 5-7 degree beamwidth.

Look for the horizontal (azimuth) -3dB beamwidth specification. The narrower the better for direction finding. The beamwidth is roughly proportional to the gain. The more gain, the narrower the beamwidth.

Yeah, something like that. 5-7 degrees is a very narrow beamwidth. You need the gain at long ranges, where you start hunting. However, the narrow beamwidth will make it difficult to point the antenna. However, as you get closer, the gain is less and less important, while the beamwidth becomes more critical (as in the apartment building example).

Been there, done that, etc. One aspect of ham radio are transmitter hunts. Although not directly applicable to 2.4Ghz hunting, read:

book is generally worth a read although microwave xmitter hunting isn't mentioned.

Not to hurt Marlon's influx of cold cash, but I bought a TT PCMCIA card just for the SA functions, never could get a damn peep out of it. That was early on in getting into wireless for me and I ended up throwing it in the junk drawer with a bunch of other worthless crap.

The sentiment is appreciated. But it is/was one of the 1-2 meg units, pre 802.11b. And I didn't mean to imply I bought one of Marlon's kits, I seem to remember it was from Electromm. It was a "kit" but they didn't have all the parts. What was missing was a little patch antenna, I already had one so that didn't matter.

If I remember correctly, I thought maybe the antenna connector wasn't soldered on correctly and I did some surgery. I don't think it'd be worth the effort to try to bring it back to life, but I'll take another look at it when I'm at the office.

Without getting into expensive equipment, is there a better option out there for a poor man's spectrum analyzer?

Thanks, I'll check it out.

And thanks for all the other info as well (I wasn't going to quote it just to say Thanks... no sense wasting the bandwidth)

-chris

No. What you have (Proxim 7200 or 7400 card) is about the cheapest and most functional spectrum analyzer around. However, it's not the only card that will play spectrum analyzer. See:

uses a Prism 1 chipset card. I've never gotten it to work, but others have reported success. The big advantage is that it scans only

11 channels instead of 80, and is therefore faster.

There's also the Teletronics WL2000 PCMCIA card. It only does 802.11 (1 and 2 mbits/sec) but comes with a nifty spectrum analyzer program.

Y'er welcome. Undying gratitude, endless praise, and cold cash, are always welcome.

Sweet, at least I now know other brands should I decide to get more of them (I looked on ebay for the Proxim RangeLan2 series... YIKES, I guess now that people can use them for cheap analyzers, they aren't cheap any more! I'm glad the box I found contained a few so I'm at least set for the time being)

Alas, you will have to just have my gratitude and praise... if I had cash, I wouldn't need the poor man's analyzer ;-)

-chris

I bought 6 of the 7400 cards on eBay for $25 + $10 shipping recently. The catch was that there were no antennas which was exactly what I wanted. Unfortunately, 3 of the cards failed to survive someones attempt at reflashing the cards with receive only firmware. I fixed that but it was pure hell and a big waste of time. Incidentally, the "snoop" program is part of the Windoze driver, not a stand alone program.

There's one more I forgot about. I bought a Symbol medical computah with a Spectrum24 card installed for $150. |

got my attention was that it included a stand alone spectrum analyzer program. Well, it turned out to be the same as the snoop program in the Spectrum 24 7400 card driver. The computah is a 486/25 with 4Mbytes of RAM with a 640x480 VGA color display. It accepts a PCMCIA memory card (or a CF card with an adapter). Unfortunately,

16MBytes maximum for the card. Meanwhile, I managed to kill the battery and the stupid pen doesn't turn off eventually killing the AAAA battery. (yes, it's the overpriced $3/ea AAAA battery). I'll fix it eventually, but it's apparently no better than using the 7400 card and a laptop. Bummer.

If you're serious about playing direction finder and wanna go the spectrum analyzer router, there's no need to buy a 2.4GHz analyzer. A much cheaper model that only goes to perhaps 500Mhz will work just fine with a downconverter. I've butchered a Pacific Wireless MMDS downconverter and my really ancient HP141T spectrum analyzer into a useable system. It's certainly not portable, but it does work around my neighborhood. The MMDS converter downconverts the 2.4Ghz to an IF frequency of about 140Mhz. The converters normally receive 2.6-2.7Ghz and might contain a 2.4Ghz notch filter, which is easily removed with an xacto knife. I'll post photos when I do my next "site survey" or antenna test. |

| | more MMDS converter modifications on the web.

I never bought any of Marlon's kits. I'm too cheap. I bought just the cards from Marlon and made my own kits. The nifty thing about the cards was the built in and quite rugged R-SMA connector. There are 3 WL1000 cards sitting in a box at a former client's waiting for me to pick them up and do something useful with them. They all work.

The problem you may have had with them was that Teletronics was screwing around with the firmware when they came out with the 802.11b version. The 802.11 (1-2mbits/sec) cards worked just fine, but I could never get the spectrum analyzer to work with the 802.11b cards. As soon as the driver would load, the whole laptop (or desktop with PCMCIA adapter) would lock up and hang. Lonnie was doing battle with Teletronics at the time and of course, nobody at Teletronics would talk to me. I sold the card to someone cheap and never bothered dealing with it again.

Normally, I would volunteer to re-flash it for you, but I'm a bit buried in paying work and don't wanna deal with it. Don't toss it as I think it can be resurrected.

Yeah, I looked at those and realized it was nothing more then a tablet computer. I actually thought of buying a hardened Walkabout for $80 and using it as an ebook reader as well (and hardened is good, I break things easily... I'm kind of like Lenny, always in search of a rabbit farm). But the dealer on ebay isn't selling them with power supplies and wants another $60 for it. I couldn't tell from the specs and pics if it was standard enough to be something I could replace with any of the dozens in my box of orphaned wall warts.

So I just passed on the whole thing (besides, a Clio would be a nicer looking ebook reader, and has a keyboard to boot... but then those are WinCE, and the RangeLan2 7200 doesn't have WinCE drivers... dang nabbit!)

Well, when I started this quest, I had a particular site in mind to use it. But the more I learn about it, the more I realize it just isn't going to be practical to attempt where I wanted to attempt it (indoors in a Drs office, where I'm surrounded by potential sources of RFI trying to find the one that *might* be the source of the wireless problems). And of course, I'm crossing my fingers their problem turns out to be the power saving settings on their Intel 2200BG cards (they

*tell* me they are already set to CAM, but I just found out, they may be lying... the hazards of debugging over the phone for a site 3 hours away)

So portable was important, but now much less so as I'd likely use it for kicks to go wifi hunting... but that implies I have time for fun... every time I think I do, my wife corrects me on the issue :-)

-chris

I bought quite a bit from Electrocomm at the time, and may have gotten it from them instead of Marlon. I don't recall for sure. If it's the

1-2Mbits 802.11 version, it *SHOULD* work (famous last words). Don't toss it.

I think it was a Comtelco patch antenna, worth about $35 at the time, plus a pigtail of sorts.

I think it might be useful as a spectrum analyzer. However, as I recall, the driver was awful. I think it uses a Prism 2 chipset, and should therefore work with WLANExpert. I'm not so much interested in the spectrum analyzer display as the VSWR display. If that works, I can use it to test antennas and cables. My bench direction coupler setup works, but it's not portable.

really enjoy the part where it suggests the card warm up 15-20 minutes before measuring VSWR. Shades of tubes and filaments?).