ELT Antenna

I dissagree with your specs. The calculator below can be found at
http://www.hamuniverse.com/jpole.html

I don't have the exact specs of a B-17s rudder, but My C-172 was taller than
5 ft

Enter The Operating Frequency
Freq MHz



A (Long section) dimension is: feet, inches, meters

B (Short section) dimension is: feet, inches, meters

C (Feed point) dimension is: feet, inches, meters

D (Spacing) dimension is: feet, inches, meters

Inside (spacing) dimensions are metal to metal measurements, NOT center to

Begin signatu My Spam filter looks for exclamation points, dollar signs,
and other characters not used in general conversation. To reply, cleanse
your message of all emotion.
"Jim Weir" wrote in message
...
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

My Bad on the html. The long dimension is just over 5 ft.


"AP" wrote in message
...
I dissagree with your specs. The calculator below can be found at
http://www.hamuniverse.com/jpole.html

I don't have the exact specs of a B-17s rudder, but My C-172 was taller
than
5 ft

Enter The Operating Frequency
Freq MHz



A (Long section) dimension is: feet, inches, meters

B (Short section) dimension is: feet, inches, meters

C (Feed point) dimension is: feet, inches, meters

D (Spacing) dimension is: feet, inches,
meters

Inside (spacing) dimensions are metal to metal measurements, NOT center to

Begin signatu My Spam filter looks for exclamation points, dollar
signs,
and other characters not used in general conversation. To reply, cleanse
your message of all emotion.
"Jim Weir" wrote in message
...
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

AP wrote:
I dissagree with your specs. The calculator below can be found at
http://www.hamuniverse.com/jpole.html

I don't have the exact specs of a B-17s rudder, but My C-172 was taller than
5 ft

Just eyeballing it it looks to be about 7' for the B-17 rudder. With the
active part of a J-pole at about 5', just where in your C-172 (assuming it
wasn't made of metal) were you planning to put it? Where do you think the
OP's going to manage to put his? Did you actually think about this problem
or do you just like to argue to hear yourself talk?

The jpole is long for a straight installation on a tail on 121.5 - that
is correct. However, the 1/4 whip without a ground plane IS NOT 50
ohms. Maybe an an 'L' match?

I did not limit the dipole to 90 degrees!. I said it could be installed
with up to 90 degrees offset in the center. Actually more but there is
some signal cancellation. At less than 90 degrees there is no signal
concellation. A dipole that is bent in the middle is called a Vee
Dipole and exhibits a more omni directional pattern.

The J-Pole can be installed with a bend but that would also change the
impedance, but not excessively. I am only putting out some ideas.
EZ-NEC will model the antenna before you build it.

On a metal plane I would opt for the 1/4 wave whip.
On a composite plane the ground plane is gone so basically the feed
coax becomes the ground. This will work but a matching "L' network may
be required to get the antenna to feed at 50 ohms or so. 1.5 to 1 SWR
or less. The radiation pattern will be distorted.

A dipole can be made that is much smaller than the full length by
adding inductance on each side. It will still perform well and be
around 50 ohms. A dipole that is only 30% the normal half wave will
radiate efficiently. So on 121.5 that would be approximately
(3.7 * .30) total length in feet. The inductance may be added at the
feed point. Each side would be (3.7 * .30)/2.

Short dipoles are quite popular in the ham realm especially at lower
frequencies. Several commercial vendors market shortened dipoles.
Rob NV7F

Or you could go to the article I wrote in Kitplanes about making an aircraft
band jpole. July 1988, pp 52-55/



"AP"
shared these priceless pearls of wisdom:

-I dissagree with your specs. The calculator below can be found at
-http://www.hamuniverse.com/jpole.html
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

shared these priceless pearls of wisdom:

-The jpole is long for a straight installation on a tail on 121.5 - that
-is correct. However, the 1/4 whip without a ground plane IS NOT 50
-ohms. Maybe an an 'L' match?

What in the hell are you talking about? A 1/4 wave whip WITH a flat ground
plane isn't 50 ohms, either. More like 32 ohms. As you bend the ground plane
down, it matches 50 ohms at about 30° off horizontal. What do you intend to
L-match to? Certainly not a quarter-wave piece of wire without a counterpoise
of some sort.


-
-I did not limit the dipole to 90 degrees!. I said it could be installed
-with up to 90 degrees offset in the center. Actually more but there is
-some signal cancellation. At less than 90 degrees there is no signal
-concellation. A dipole that is bent in the middle is called a Vee
-Dipole and exhibits a more omni directional pattern.

You really need to take a few classes on this stuff. Signal cancellation is not
the correct term. Pattern modification perhaps.


-
-The J-Pole can be installed with a bend but that would also change the
-impedance, but not excessively. I am only putting out some ideas.
-EZ-NEC will model the antenna before you build it.

You can put out all the ideas you wish. However, it would be best if you had
some actual experience with aircraft antennas before coming in here and spouting
stuff of which you have no clue. Eznec is a fairly good antenna modeler; why
don't you foreshorten and/or bend that jpole to something that will fit inside
of a 3' high structure and then tell us how to match it over a 17% bandwidth.
You **do** realize that the jpole won't match decently over a 5% bandwidth,
don't you? Then the match reduces the bandwidth still further.



-
-On a metal plane I would opt for the 1/4 wave whip.
-On a composite plane the ground plane is gone so basically the feed
-coax becomes the ground. This will work but a matching "L' network may
-be required to get the antenna to feed at 50 ohms or so. 1.5 to 1 SWR
-or less. The radiation pattern will be distorted.

Oh, Lord. The old sleeve dipole trick. Please, sir, take your L-match over to
rec.radio.amateur and rachetjaw with the fuddies over there about HF antennas.
I have no idea what you mean about the radiation pattern being distorted.
Distorted relative to WHAT?



-
-A dipole can be made that is much smaller than the full length by
-adding inductance on each side. It will still perform well and be
-around 50 ohms. A dipole that is only 30% the normal half wave will
-radiate efficiently.

Efficiently relative to a strand of wet spaghetti in a copper septic tank,
perhaps. You ever mess around with Reg Edwards calculations over in the UK?



So on 121.5 that would be approximately
-(3.7 * .30) total length in feet. The inductance may be added at the
-feed point. Each side would be (3.7 * .30)/2.
-
-Short dipoles are quite popular in the ham realm especially at lower
-frequencies. Several commercial vendors market shortened dipoles.

Don'cha just love ham radio "experts" in HF coming in here telling us how to do
stuff at VHF in aircraft?

Jim


-Rob NV7F




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

wrote:

The jpole is long for a straight installation on a tail on 121.5 - that
is correct. However, the 1/4 whip without a ground plane IS NOT 50
ohms. Maybe an an 'L' match?



Like one of my comments. !!! (that's to catch the guys spam "emotion
filter")
KISS
If you want it simple go get a rubber ducky antenna such as for a Icom A21
or so and stick it on it. When your half buried it the ground you really
don't care if you have a 3 to 1 VSWR or not only that the satalite will
pick it up and the CAP can DF you down! Down get too carried away with
"High Tech" just KISS.
John

I wasn't going to put it anywhere. I was just supplying actual dimensions
of the proposed antenna.


Begin signatu My Spam filter looks for exclamation points, dollar signs,
and other characters not used in general conversation. To reply, cleanse
your message of all emotion.
"Ron Natalie" wrote in message
m...
AP wrote:
I dissagree with your specs. The calculator below can be found at
http://www.hamuniverse.com/jpole.html

I don't have the exact specs of a B-17s rudder, but My C-172 was taller
than
5 ft

Just eyeballing it it looks to be about 7' for the B-17 rudder. With the
active part of a J-pole at about 5', just where in your C-172 (assuming it
wasn't made of metal) were you planning to put it? Where do you think the
OP's going to manage to put his? Did you actually think about this
problem
or do you just like to argue to hear yourself talk?

Not sure I have been too enlightened; will try to sort it out.

Gary
"UltraJohn" wrote in message
news
wrote:

The jpole is long for a straight installation on a tail on 121.5 - that
is correct. However, the 1/4 whip without a ground plane IS NOT 50
ohms. Maybe an an 'L' match?



Like one of my comments. !!! (that's to catch the guys spam "emotion
filter")
KISS
If you want it simple go get a rubber ducky antenna such as for a Icom A21
or so and stick it on it. When your half buried it the ground you really
don't care if you have a 3 to 1 VSWR or not only that the satalite will
pick it up and the CAP can DF you down! Down get too carried away with
"High Tech" just KISS.
John