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Old 14-11-2014, 10:50   #1
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How do you search for a 121.5 signal?

How do you set up a search pattern for someone in the water with an epirb or plb?
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Old 14-11-2014, 13:02   #2
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Re: How do you search for a 121.5 signal?

I dont think you can. Its a special receiver, I think. But someone ekse may know better.
Thats why AIS can be better. Some AIS transmitter attach to your life vest ir shove in your pocket for activation when you hit the water. Then everyone can see you
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Old 14-11-2014, 14:12   #3
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Re: How do you search for a 121.5 signal?

Normally a directional antenna (yagi) is used to locate the signal. A method can be used where you keep a course until the signal is getting weaker then turn 180 degs until the signal is strongest then turn 90 degs and keep that course & the signal will either get stronger ( the correct dirrection ) or weaker. If weaker turn 180 degs to locate the signal. The use of the squelsh control can be used to find the strength of the signal. (Use a handheld or antenna on the back rail if close would reduce the search area.) 121.5 is AM.
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Old 14-11-2014, 14:37   #4
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Re: How do you search for a 121.5 signal?

I don't think 121.5 is an regular AM radio signal. I know it is quite clear on FM about 100 because I've tested a whole bunch of ELT's on 121.5. on my boom box in the shop.
I am wondering about how you would do a logical search at night. Using your chart plotter course backwards. When you pick up the signal would you steer right or left for stronger signal or do a 360? I would be in a hurry. The 360 thing is for a more complicated search with mountains
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Old 14-11-2014, 14:44   #5
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Re: How do you search for a 121.5 signal?

The OH-58 helicopter had a receiver just for this, it had two antennas, one on each side of the aircraft, and I believe it worked based off of relative signal strength of the two antennas, but you could home in on the military distress freq., which I think was 40.50 FM?
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Old 14-11-2014, 15:09   #6
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Re: How do you search for a 121.5 signal?

The CAP has some good instructions but they lose you when they get into reflection and DF steers. I sounds like a small cheap portable fm radio would almost be the best thing.
Boy I need to say, having safety gear and not knowing how to use it is a big waste of money. I think this lack of knowledge causes people to buy even more stuff sometimes because they don't understand the stuff they all ready have.
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Old 14-11-2014, 16:08   #7
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Re: How do you search for a 121.5 signal?

It is AM & the destress freq for military is 243.0 (2nd harmonic) & the 121.5 is made to transmit the 2nd harmonic as well. It is likely the real search aircraft setup is for null & hence the dual antennas. But the question was how could Guy use something he had without going to a lot of trouble. (I thought) Square search pattern would get to visual range at least.
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Old 14-11-2014, 16:25   #8
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Re: How do you search for a 121.5 signal?

Maybe this will be helpful in understanding what happens when an EPIRB is set off, The Trawler Beach House: You activated your EPIRB... now what? . Chuck
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Old 14-11-2014, 16:48   #9
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Re: How do you search for a 121.5 signal?

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It is AM & the destress freq for military is 243.0 (2nd harmonic) & the 121.5 is made to transmit the 2nd harmonic as well. It is likely the real search aircraft setup is for null & hence the dual antennas. But the question was how could Guy use something he had without going to a lot of trouble. (I thought) Square search pattern would get to visual range at least.
I'm puzzled by your saying it's an am frequency? It is 121.5 on a vhf aircraft radio.
So square pattern turning towards what? You would turn 90 deg, but if the signal became weaker immediately turn 90 back to the direction you were going This would set up your square pattern?
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Old 14-11-2014, 17:19   #10
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Re: How do you search for a 121.5 signal?

Aircraft use AM. The turn at the strongest point 90 degs puts you in line but you don't know which side of the line until you travel on that line for long enough to work out if the signal is increasing i.e. you are going the right way or if the signal is decreasing i.e. the wrong way & thus 180 deg turn.
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Old 14-11-2014, 17:28   #11
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Re: How do you search for a 121.5 signal?

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How do you set up a search pattern for someone in the water with an epirb or plb?
If you want to do it from only one vehicle the you better have some radio direction finding kit that will make a noise (or stop making it) when you point in the right direction. It works by exploiting difference in distance between PLB and each end of a quasi-folded dipole antenna. Google "ham foxhunt equipment" for details. Those ham nerds have worked it all out!

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Old 14-11-2014, 17:28   #12
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Re: How do you search for a 121.5 signal?

121.5 VHF civilian "guard" freq 243.0 military "guard" freq. 282.8? if I remember right was voice freq on our PRC-90 radios, but there was a military FM distress freq,. all ground units of the day has FM only radios, usually PRC-77, if I remember right?

Gotta be Vietnam Vets that remember, I was actually a very young, PVT or so at the time, and it has been a long time ago.

Point being to "home" in on a signal, your going to need a directional type of antenna, or antenna array, really old days has an antenna, usually a circular one that took the pilot to manually rotate to determine the "null", again by memory.
Next level was dual antennas that could electronically determine a bearing to, or from, again old memory, but I think both were the same and you could only tell you were going away from the signal by gradual weakening? Anyway you couldn't tell distance either, only new when you passed as the needle would quickly point the other way.
Been a long time since Aero Scout training
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Old 14-11-2014, 17:29   #13
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Re: How do you search for a 121.5 signal?

Terra Nova,
Does this sound right?
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Old 14-11-2014, 18:00   #14
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Re: How do you search for a 121.5 signal?

Quote:
Originally Posted by a64pilot View Post
121.5 VHF civilian "guard" freq 243.0 military "guard" freq. 282.8? if I remember right was voice freq on our PRC-90 radios, but there was a military FM distress freq,. all ground units of the day has FM only radios, usually PRC-77, if I remember right?

Gotta be Vietnam Vets that remember, I was actually a very young, PVT or so at the time, and it has been a long time ago.

Point being to "home" in on a signal, your going to need a directional type of antenna, or antenna array, really old days has an antenna, usually a circular one that took the pilot to manually rotate to determine the "null", again by memory.
Next level was dual antennas that could electronically determine a bearing to, or from, again old memory, but I think both were the same and you could only tell you were going away from the signal by gradual weakening? Anyway you couldn't tell distance either, only new when you passed as the needle would quickly point the other way.
Been a long time since Aero Scout training
Not according to the civil air patrol and they do a pretty good job. A simple fm radio works great . The ships radio is to loud so you need to move the freq off a bit or squelch it.
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Old 14-11-2014, 18:20   #15
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Re: How do you search for a 121.5 signal?

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Not according to the civil air patrol and they do a pretty good job. A simple fm radio works great . The ships radio is to loud so you need to move the freq off a bit or squelch it.

What does this mean, CAP DF's an ELT
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