DF'ing in the airport

[Walkman]

Just getting started on my UDF quals. On some training this past weekend, we were in a small airport trying to find the trainer ELT. I was trying to get a reading between the rows of hangars and obviously the signal was bouncing around and giving a false reading. Knowing that many ELT missions will be in a situation like that, how do you determine which hangar the signal is coming from?

Be kind, I'm a newb here...

[JC004]

I've done this many times.  I have a bunch of finds this way.  It's a simple answer - body shield with a scanner.  Turn the scanner to the ELT frequency, hold it close to your body, and turn about in small amounts from different angles.  You're blocking the bounce with your body.  You will need to look where the potential bounces are coming from and get gradually closer to the possible hanger/aircraft. 

A dying ELT with fluctuations in the signal strength with make this harder because you are listening for fluctuations while you pivot your body, following the strongest signal.  In that case, it takes a bit more experience and practice.

Last time I did this, there were two groups - one with an old LPER, one with the new unit and I found it faster with the scanner.

You can also put a little rubber duck antenna on an old LPER unit instead of using the big antennas.  That is why carrying one of these little antennas was part of my standard UDF kit.

[EMT-83]

As the signal gets stronger on the scanner (or VHF portable) you can step down the frequency to reduce receiver sensitivity.

[N Harmon]

1. Set sensitivity such that you are barely picking up the signal.
2. Start moving. If you lose the signal, you're moving in the wrong direction.
3. As you get closer and the signal becomes strong, return to step 1.

[a2capt]

Using a rubber duck antenna on the LPer, with the sensitivity turned way down when you hear it, null/shield it and see what direction it takes to get it to come back on.

Also when you're scanning the walls of hangars - as you get "closer" with the LPer, scanner or aviation band HT off-frequency, (121.6, 121.525, etc), since the transmitter will bleed.

With the unit nulled even with you perpendicular to the building, the signal will "leak" out doors, windows, (windows are easy, but rare).. think of the signal leaking like as if the building were full of water. Where's the water going to seep out?

Door frames, along the leading edge at the bottom of the big door, holes in the walls, etc. Those bi-folding doors tend to leak at the center hinge, and the sides where they're not all the way flush in the middle.  By using the sensitivity and/or going off frequency you get yourself closer to the source.

[Walkman]

Rubber Duck antenna?

[JC004]

Rubber Duck antenna?

A little flexible rubber whip antenna.  You could also go with something like this.  I had one of these on last time.

http://www.radioshack.com/product/index.jsp?productId=2103159

[Thunder]

caution: removing the antenna from a VHF HT will open you to accidentally blowing the the transceiver if you accidentally hit the PTT button. This is why we use a Scanner receiver.

I use a scanner with an adjustable squelch. Increase the Squelch threshold gradually until you can't hear it anymore, and move till it breaks through again. Repeat. When you think you found the hangar, remove the antenna from the scanner and touch the BNC connector to the metal hangar wall/siding. This should tell you if its in there.

The frequency offset method works well too. I typically have only done this in the aircraft to wing-null the location, but the same principle works for body blocking. You can triangulate the location this way. Start on 121.775 (or 121.5 if its real) and rotate as discussed already to block the signal. Do this in three locations not close to each other. Plot the intersection of the three headings. Then change it in 25kHz increments off the center frequency and repeat. You will gradually get closer and closer. To distinguish reflections, remember that constructive interference also comes with destuctive interference, so as you walk between the hangars, the signal will get louder and softer in a regular pattern (periodic) vs linearly

[Star-Maker]

I agree with all the people suggesting body-blocking.  The frequency offset method described by Thunder is also useful.

[RADIOMAN015]

With the new Ltronics Lper, you might be able to just use a BNC connector on the external antenna connector (which turns off those 4 antenna built into the unit)  and this will reduce signficiantly the receive range of the unit.   Also on the old L'Per you just use the unit itself with no antenna connected.

I've found that scanners will work better in some instances in close quarters.   I haven't tried a scanner with "close call capture"/"signal stalker" capability yet to see how close it has to get to the RF emitter before a hit.

RM

[RADIOMAN015]

Sorry I forgot to mention that our Wing ES ground pounder uses an ICOM Receiver model R2 (R5 work well also), since not only do you have a signal strength meter but you can turn the receiver up by 5 khz each time.  He's had much better luck using the R2 at Marinas (with a Marine beacon going off) than using any of the professional DF equipment.

BTW you can also program a regular scanner for 121.5, 121.525, 121.55, to also assist.

RM
   

[EMT-83]

There's some good info on the Ltronics website:

http://www.ltronics.com/publicat1.htm

[coudano]

frequency offset and bodyshield with the jetstream is option #1

but you can do it just as well with the lper with a rubber duck, and the sensitivity dial.

when you get down to the last few feet, you can also do things like ground out the elt antennas on the airplanes

[a2capt]

I also have a Icom A20, and it's antenna gets swapped around with the L-per, which ever is optimal for the scenario.