Re: NEC-LIST: DF Antenna for 121.5 MHz EPIRB

Hi Dan,

    I used to do volunteer SAR in the Cascade Mountains in the Seattle
area for about 6 years when I lived there. I don't know what type of
a system you are using, but, will tell you about the system that King
County Search and Rescue Association (KCSARA) used in the State of
Washington. I will avoid technical details at this point, since they
would complicate my description.

    We used an antenna with a sharp null in it to find the supposed
direction of the sigal. We used a basic two antenna interferometer to
varify that the wave front was indeed perpendicular to the direction
of the signal. We found that using this system that we could have up
to a 95 percent confidence level in our bearing if the wavefront was
perpendicular to the direction.

    If an aircraft were to crash into a rocky mountainous area, we
found that the DF system we used would not be effective due to a very
high level of multipath when in close to the transmitter. Once we
felt we had to be pretty close, we would use a combination of people
searching on foot with radios and a helicopter searching visually
above, weather allowing. The people on foot used a device callled an
ELPer which had an antenna with a null and a rough order of magnitude
signal strength meter.

    The most important aspect of using the DF devices was that the
searchers practice regularly, at least once a month, in mock searches.
The high level of multipath is indeed a big problem which I think
would be easy to solve if ELTs transmitted GPS coordinate information.

    By the way, the frequency of 406 MHz is new to me. Civilian
aircraft use 121.5 MHz. Military aircraft use 243 MHz. What type of
vehicle uses 406 MHz?

Sincerely,
Roland Mueller
KG7XX

Dan Bathker wrote:

> EPIRB -- emergency position indicating radio beacons -- 121.5, 243.0
> and 406 MHz --
>
> Looking for advice on or design of a simple, proven DF antenna for
> volunteer Search and Rescue Team use at 121.5 MHz.
>
> It is understood multipath is a major problem in the mountainous
> forest region to be served yet portability is paramount (these guys
> break their bottoms saving lives and I feel we can help them).
>
> All comments appreciated.
>
> Thanks
> Dan Bathker
Received on Mon May 10 1999 - 14:46:44 EDT