ELT Antenna

Hello;

I am in the finishing stages of building a LIVP and realize that I have

forgotten to allow for the elt. What antenna that would be

suitable to place in the tail area (where the top section is

fiberglass). Getting a ground plane will be difficult. Is there an

alternative way to duplicate the effect of a ground plane? Is there any

issue with proximity to com antennas? I have a wire whip 2nd

antenna that is in that section as well.

Thanks,

Gary Rodgers

A lot of the C-150's I've flown just had the ELT with attached whip antenna
mounted to the side of the baggage area. Nothing fancy!
Usually it will be a satelite that will pick up the signal and get earth
bound rescuers to the general area anyway so your not too concerned about
line of sight.
John


Gary wrote:

Hello;

I am in the finishing stages of building a LIVP and realize that I have

forgotten to allow for the elt. What antenna that would be

suitable to place in the tail area (where the top section is

fiberglass). Getting a ground plane will be difficult. Is there an

alternative way to duplicate the effect of a ground plane? Is there any

issue with proximity to com antennas? I have a wire whip 2nd

antenna that is in that section as well.

Thanks,

Gary Rodgers

Thanks for the reply. The issue really is the fact that carbon fiber (lower
part of the tail) is not as good a ground plane as the metal of a Cessna.
Should I be using the 'Z' type com antenna that is for building into the
vertical stabalizer? Seems that type doesn't have the same requirement for
a ground plane.

Gary

"UltraJohn" wrote in message
hlink.net...
A lot of the C-150's I've flown just had the ELT with attached whip
antenna
mounted to the side of the baggage area. Nothing fancy!
Usually it will be a satelite that will pick up the signal and get earth
bound rescuers to the general area anyway so your not too concerned about
line of sight.
John


Gary wrote:

Hello;

I am in the finishing stages of building a LIVP and realize that I have

forgotten to allow for the elt. What antenna that would be

suitable to place in the tail area (where the top section is

fiberglass). Getting a ground plane will be difficult. Is there an

alternative way to duplicate the effect of a ground plane? Is there any

issue with proximity to com antennas? I have a wire whip 2nd

antenna that is in that section as well.

Thanks,

Gary Rodgers

Just a note for all to keep in mind. Beginning in Jan 2009, the
satellites will no longer monitor 121.5/243.0 MHz for distress beacons.
Check out the COSPAS-SARSAT home page
(http://www.cospas-sarsat.org/MainPages/indexEnglish.htm), the FAA Home
Page (http://www.faa.gov/), or NOAA's SARSAT Page
(http://www.sarsat.noaa.gov/)

....zoom


Gary wrote:
Thanks for the reply. The issue really is the fact that carbon fiber
(lower
part of the tail) is not as good a ground plane as the metal of a
Cessna.
Should I be using the 'Z' type com antenna that is for building into
the
vertical stabalizer? Seems that type doesn't have the same
requirement for
a ground plane.

Gary

"UltraJohn" wrote in message
hlink.net...
A lot of the C-150's I've flown just had the ELT with attached whip
antenna
mounted to the side of the baggage area. Nothing fancy!
Usually it will be a satelite that will pick up the signal and get
earth
bound rescuers to the general area anyway so your not too concerned
about
line of sight.
John


Gary wrote:

Hello;

I am in the finishing stages of building a LIVP and realize that
I have

forgotten to allow for the elt. What antenna that would be

suitable to place in the tail area (where the top section is

fiberglass). Getting a ground plane will be difficult. Is there
an

alternative way to duplicate the effect of a ground plane? Is
there any

issue with proximity to com antennas? I have a wire whip 2nd

antenna that is in that section as well.

Thanks,

Gary Rodgers

A j-pole antenna has a built in ground plane and works very well. Just
google on jpole or j-pole antenna. Or maybe a full wave loop if you
have the area for one. A dipole also can be used with up to a 90 degree
angle in the center. Just some ideas. Contact a local ham radio club. A
dipole with a 90 degree angle is fairly omni-directional and easy to
build.

Have the antenna checked BEFORE testing for SWR with an MFJ antenna
analyzer from a local ham club. The ARRL antenna handook for VHF is an
excellent resource.

I would definitely MAKE SURE the antenna cannot come loose. Use 50 ohm
coax to feed if that is what impedence the radio expects.

Rob

Did you really read what the guy had to say? Do you know that a j-pole at 121.5
is longer than a B-17's rudder assembly? Do you understand that you have to
radiate equally efficiently at both the fundamental and second (NOT third)
harmonic?

Did you read that he has only a small amount of fiberglass in the tail, and that
most of it is carbon fiber? Did you read that there is another com antenna in
that same location?

I read it and concluded that they guy didn't have a chance in hell of putting
his ELT antenna in the tailfeathers, but waited around to see if somebody that
knew what they were talking about came up with a clever idea...I don't claim to
have cornered the market on clever.

Do you understand that a j-pole does NOT have a built-in ground plane, but is a
matched half wave radiator? Why do you limit the dipole to a 90 degree angle?
If SWR is all you are concerned with, I can give you an antenna that is good
from DC to 20 GHz.. It is called a 51 ohm resistor.

Jim (still waiting for clever) Weir



shared these priceless pearls of wisdom:

-A j-pole antenna has a built in ground plane and works very well. Just
-google on jpole or j-pole antenna. Or maybe a full wave loop if you
-have the area for one. A dipole also can be used with up to a 90 degree
-angle in the center. Just some ideas. Contact a local ham radio club. A
-dipole with a 90 degree angle is fairly omni-directional and easy to
-build.
-
-Have the antenna checked BEFORE testing for SWR with an MFJ antenna
-analyzer from a local ham club. The ARRL antenna handook for VHF is an
-excellent resource.
-
-I would definitely MAKE SURE the antenna cannot come loose. Use 50 ohm
-coax to feed if that is what impedence the radio expects.
-
-Rob


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

Jim Weir wrote:

Did you really read what the guy had to say? Do you know that a j-pole at
121.5
is longer than a B-17's rudder assembly? Do you understand that you have
to radiate equally efficiently at both the fundamental and second (NOT
third) harmonic?
I still contend what I said earlier.
Most older C-150's that were retrofited with an ELT use an ELT with a whip
(1/4 wave) fixed to the ELT with no groundplane. Would I recommend this for
a com radio? no! but it does work in thousands of C-150's (and probably
others I'm just familiar with 150's). anyway KISS is the best.
John

And I don't disagree with you. For this guy, in this situation, an ELT with the
whip mounted on the ELT is probably the optimum solution if his airworthiness
inspector will let him get away with it.

I'm constantly amazed at the number of calls I get asking whether an ELT antenna
should be mounted horizontal or vertical polarization. The stock answer is that
if you can guarantee me the orientation of the airframe if you really need the
services of the ELT, I can give you that advice.

Jim



UltraJohn
shared these priceless pearls of wisdom:


-I still contend what I said earlier.
-Most older C-150's that were retrofited with an ELT use an ELT with a whip
-(1/4 wave) fixed to the ELT with no groundplane.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

I believe that the C-150 has a metal skin that acts like a ground plane
automatically.

"UltraJohn" wrote in message
link.net...
Jim Weir wrote:

Did you really read what the guy had to say? Do you know that a j-pole
at
121.5
is longer than a B-17's rudder assembly? Do you understand that you
have
to radiate equally efficiently at both the fundamental and second (NOT
third) harmonic?
I still contend what I said earlier.
Most older C-150's that were retrofited with an ELT use an ELT with a whip
(1/4 wave) fixed to the ELT with no groundplane. Would I recommend this
for
a com radio? no! but it does work in thousands of C-150's (and probably
others I'm just familiar with 150's). anyway KISS is the best.
John

You weren't reading what he was saying, Cy. In the very early days of ELTs,
then didn't have to have external antennas. The contemplation is that whatever
tore the airplane up would have torn it up enough that the ELT would have been
exposed to the point where an antenna connected directly to the ELT body itself
sufficed for the installation. No external antenna.

I haven't seen an approved design like this for thirty years or so, but at one
time, it was all the rage. One manufacturer actually had a steel tape measure
segment spring loaded to uncoil at some g load and act as the antenna.

Jim


"Cy Galley"
shared these priceless pearls of wisdom:

-I believe that the C-150 has a metal skin that acts like a ground plane
-automatically.


-
-"UltraJohn" wrote in message
hlink.net...
\

- Most older C-150's that were retrofited with an ELT use an ELT with a whip
- (1/4 wave) fixed to the ELT with no groundplane. Would I recommend this
-for
- a com radio? no! but it does work in thousands of C-150's (and probably
- others I'm just familiar with 150's). anyway KISS is the best.
- John
-

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com