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Helicopter DF to completely wrong location

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#1
Hello,

I was investigating a 121.5 emergency beacon that was going off near the Oakland Airport. We turned on our helicopters direction finder and followed the arrow until passing over a group of antennas on top of a mountain where the arrow would swing from the 12 o'clock to the 6 o'clock position (as it should). After 3 passes from different directions, we were very confident on the location of the source. We later found out that the signal was actually coming from a boat in a marina back near Oakland. I've been reading about radio repeaters and my onboard DF equipment and cannot figure out why this might have happened.


Has anyone experienced this before?
Does anyone have a theory on why this may have happened?
Is there anything I can do in the aircraft to try and counter future false readings?

Thanks
-Sam
 
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#3
If this is a doppler direction finder it is possible the antenna leads are swapped, and/or the calibration phase is off. Best to try DF ing a known transmitter and check that the display is calibrated prooerly. You may need to be airborne to make a proper check as ground reflections will confuse the antennas if belly mounted.
 
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#5
When passing near or over a mountain top repeater site I would expect many radios to become overloaded due the high RF fields and potential Intermod from equipment on the mountain or created in your DF receiver. I think most DF receivers are just glorified police scanners and don't do well in high RF environments.
prcguy

Hello,

I was investigating a 121.5 emergency beacon that was going off near the Oakland Airport. We turned on our helicopters direction finder and followed the arrow until passing over a group of antennas on top of a mountain where the arrow would swing from the 12 o'clock to the 6 o'clock position (as it should). After 3 passes from different directions, we were very confident on the location of the source. We later found out that the signal was actually coming from a boat in a marina back near Oakland. I've been reading about radio repeaters and my onboard DF equipment and cannot figure out why this might have happened.


Has anyone experienced this before?
Does anyone have a theory on why this may have happened?
Is there anything I can do in the aircraft to try and counter future false readings?

Thanks
-Sam
 

902

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#6
I've been able to track an ELT in a 182 with a Becker pretty much dead-on - but I was nowhere near any TV or FM antenna farms at the time.

I'm with prcguy. There may have been an FM station, or a combination of very strong RF at a bunch of frequencies at that mountaintop site that was swamping your DF unit and providing a false reading. Good to know, if that's the case.

You might have tried with your com radio at a lower altitude to cut down on the footprint of the area you're receiving. I would say if you were in a fixed-wing aircraft, you might have tried some banking turns to block the antenna and maybe null out the signal, too.

These days, it probably pays off to cruise the airports and marinas first, although I've heard some wild stories from old CAP guys about chasing garbage trucks after people have thrown their ELTs out.

What kind of equipment do you have, Sam?
 
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#7
My suggestion would be contact a local CAP unit, you may be able to observe one of their practice ELT exercises. If you are affiliated with a Public Safety agency or Emergency Management, you may be able to setup a joint training exercise.
 
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#8
Hello,

I was investigating a 121.5 emergency beacon that was going off near the Oakland Airport. We turned on our helicopters direction finder and followed the arrow until passing over a group of antennas on top of a mountain where the arrow would swing from the 12 o'clock to the 6 o'clock position (as it should). After 3 passes from different directions, we were very confident on the location of the source. We later found out that the signal was actually coming from a boat in a marina back near Oakland. I've been reading about radio repeaters and my onboard DF equipment and cannot figure out why this might have happened.


Has anyone experienced this before?
Does anyone have a theory on why this may have happened?
Is there anything I can do in the aircraft to try and counter future false readings?

Thanks
-Sam
I will try and take a wild a** guess on this and say that there might have been a birdie coming from some transmitter there on the mountain. Doing some simple math, which might or might not come up with the correct answer, I have the following frequencies: 30.375 MHz., 40.5 MHz., 486 MHz. I am sure there are others, but these are the direct hits. Now there could be a transmitter on that hill that is just spilling out trash and generating a signal.

You might try listening to the 121.5 frequency with a receiver and see just what it sounds like. This would provide a hint of where it's originating from. The key here if it's on continuously, then that limits the sources. It could be an intermod hit that might involve a mix of two or more transmitters. If this is the case, good luck on locating what is causing it.
 
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#9
What kind of equipment do you have, Sam?
The DF is a Rockwell DF-301E. 100-400MHz.

Thanks for all the responses. All great ideas. I'll keep playing with it and let you know if I find anything else.

We we're listening to the beacon as we were looking for it. It was intermittent when we were far away (obviously), but as we got close the beeping was relatively consistent and sounded like other emergency beacons that I've searched for.
 

wa1nic

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#10
Assuming you are using a doppler direction finder and can listen to the output of the doppler receiver, listen for a pure tone from the doppler array.

If the tone is distorted you will not get a good direction reading out of it. It could have been hunting for a tone that was actually generated as intermod between the real signal and some other combination of things.
 
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#11
Very well could be a reflection or reflected signal off of the mountain , this can also occur with tall buildings , metal structures , most anything the signal can bounce off of .
 

dlwtrunked

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#12
Very well could be a reflection or reflected signal off of the mountain , this can also occur with tall buildings , metal structures , most anything the signal can bounce off of .
I think the above is likely the correct cause. When approaching the mountain from one direction, the reflected signal may have been stronger due to the path (angle of radiation at the transmitter, etc.) and after going over the mountain, with the reflected signal no longer received, the direct signal was the one being DF'ed/
 
T

Tx4

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#13
Hello,

I was investigating a 121.5 emergency beacon that was going off near the Oakland Airport. We turned on our helicopters direction finder and followed the arrow until passing over a group of antennas on top of a mountain where the arrow would swing from the 12 o'clock to the 6 o'clock position (as it should). After 3 passes from different directions, we were very confident on the location of the source. We later found out that the signal was actually coming from a boat in a marina back near Oakland. I've been reading about radio repeaters and my onboard DF equipment and cannot figure out why this might have happened.


Has anyone experienced this before?
Does anyone have a theory on why this may have happened?
Is there anything I can do in the aircraft to try and counter future false readings?

Thanks
-Sam
use a bandpass filter passing 121.500Mhzn have you tried to find a signals you can hear? local repeaters police of fire practice with your equipment so you trust it. Most are made for antenna array on the rooftop and when its on a helo its on the bottom there is a switch to change modes on ours here. We practice chasing a lot if I can hear the radio I can find it
Don
 
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