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Pitot tube for the antennae



 
 
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  #1  
Old May 23rd 06, 04:43 AM posted to rec.aviation.homebuilt
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Default Pitot tube for the antennae

I've been going over my Delta, trying to find a way to hide the
antennaes in what is basically a rag and tube aircraft. (some of the
rags are soaked in epoxy, and some of the tubes ain't round...but,
anyway) I was beginning to think it wasn't possible when I happened
across this guys website:

http://contrails.free.fr/instruments_ant_sonex.php

Looks like it has possibilities. Has anyone ever tried this.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."
  #2  
Old May 23rd 06, 05:40 AM posted to rec.aviation.homebuilt
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Default Pitot tube for the antennae

Is it available in English? Technical articles lose a little, when you can
only look at the pictures! g
--
Jim in NC


  #3  
Old May 23rd 06, 06:06 AM posted to rec.aviation.homebuilt
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Default Pitot tube for the antennae

If it is the pitot tube shown on the top of the vertical stabilizer, I
would be reluctant to use that location. Pitot tubes are normally
placed in a location that affords exposure to an undisturbed airstream,
and atop the vert. stab. just isn't that.

However, inside the plastic tip of the vert.stab is a likely location
for an antenna. The Lockheed T-33 used that location for the UHF . Of
course, replacing the coax was a bear -G.

Flash

  #4  
Old May 24th 06, 12:12 AM posted to rec.aviation.homebuilt
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Default Pitot tube for the antennae

Morgans wrote:
Is it available in English? Technical articles lose a little, when you can
only look at the pictures! g


You can use google to get a rough english translation:

http://translate.google.com/

http://translate.google.com/translat...language_tools

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."
  #5  
Old May 25th 06, 04:39 AM posted to rec.aviation.homebuilt
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Posts: n/a
Default Pitot tube for the antennae

Regarding translation - - - scroll down to the bottom, in the
kefthand tool-pile, click on the British "Union Jack" for a (nearly)
English translation

Flash

  #6  
Old May 25th 06, 07:31 AM
Christopher Christopher is offline
Junior Member
 
First recorded activity by AviationBanter: May 2006
Posts: 19
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I have 30 years experience working with many different types of antennas from 100 KHz through 20 GHz and can answer some questions for you.

If you have a rag and tube aircraft, you can actually couple to the tubing in the frame and use it as a radiator itself but with a fuel tank and instruments in the airframe you could get cross coupling that you might not want. Yet, I believe it might be possible to couple to the tubing in the vertical stabilizer which should be far enough away to be far less coupling than you would have with a standard whip antenna mounted over the cockpit.

If you have antenna questions I'd be happy to give you my opinions.

Quote:
Originally Posted by Ernest Christley
I've been going over my Delta, trying to find a way to hide the
antennaes in what is basically a rag and tube aircraft. (some of the
rags are soaked in epoxy, and some of the tubes ain't round...but,
anyway) I was beginning to think it wasn't possible when I happened
across this guys website:

http://contrails.free.fr/instruments_ant_sonex.php

Looks like it has possibilities. Has anyone ever tried this.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Last edited by Christopher : May 25th 06 at 07:57 AM.
  #7  
Old May 26th 06, 01:04 AM posted to rec.aviation.homebuilt
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Default Pitot tube for the antennae

Christopher wrote:
I have 30 years experience working with many different types of antennas
from 100 KHz through 20 GHz and can answer some questions for you.

If you have a rag and tube aircraft, you can actually couple to the
tubing in the frame and use it as a radiator itself but with a fuel
tank and instruments in the airframe you could get cross coupling that
you might not want. Yet, I believe it might be possible to couple to
the tubing in the vertical stabilizer which should be far enough away
to be far less coupling than you would have with a standard whip
antenna mounted over the cockpit.


The fuselage is one humungous welded cage. If you used it for a
radiator, what would you use for a ground plane? Don't you need a
ground plane?

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."
  #8  
Old May 26th 06, 01:02 PM
Christopher Christopher is offline
Junior Member
 
First recorded activity by AviationBanter: May 2006
Posts: 19
Default

The way it is done is to attach the feed at a point so many inches up from where the coax would be grounded to the fuselage. Although the fuselage is at ground potential where it connects to the battery and negative lead for the avionics if you find the right spot to attach the coax the feed will both be matched and radiation will occur on a vertical member as if it were insulated from ground.

Here is a web page showing how it is done for the lower medium to high frequency portions of the amateur radio band on a grounded tower, of course, with an aircraft you would have more trouble to find a sweet spot for loading into the frame than with a simple tower but I believe it can be done at the tail, if you feed the lower portion of the tail the remaining upper structure might be about the right length for VHF-AM in the aircraft comm band:

http://www.qsl.net/w9rb/webdoc9.htm

The drawing shown on this web page is for a ham radio tower with a beam antenna on top which has nothing to do with what they are trying to show. The drawing shows a parallel wire running up the tower and isn't exactly the method I am thinking of, the one I am talking about is where you ground the coax on the vertical member and then attached the center wire of the coax on the sweet spot where it loads properly, higher up on the tower.

If you are not an RF man with a through-line watt meter I wouldn't expect this to be an easy task for someone new to radio or antennas, but it could be done with a few hours work on a tube and rag aircraft if you did it before it was covered.

Only thing is the radiation pattern would doubtless be other than omni-directional and would probably favor the direction the aircraft is headed into much more than behind it, assuming the forward vertical tube of the tail is chosen to load into. This is very dependent upon the welded structure too, how much length of tube you have to work with, so many things would need to happen to be right that this method might be very difficult to achieve.

You could always use a vertical strip of copper foil to act as a radiator too, glued to the back side of the aircraft fabric as far away from support tubing in the frame as possible, it could be coupled with a matcher so that the length doesn't need to be a full quarter wave length and the frame of the aircraft could act as ground plane, one reason why you wouldn't want the copper strip to line up in parallel to any of the tubing.

All of this stuff would be a compromise, not a good antenna but one which would work in most situations but I wouldn't want to depend on it for communication if I were a long way from where I wanted to talk to.

The important thing about VHF-AM aircraft comms is that you keep the antenna vertical because if horizontal you loose a large amount of signal when communicating with a vertical antenna on the ground due to being at an opposite polarity if the aircraft antenna where horizontally oriented (transmit wave).

Although, for many situations it would work anyway, if you are close enough it won't matter, the signal scatters into different polarities all over the place as it hits other structures on the ground or the airframe itself anyway, but the majority of the power would be in the polarity the antenna is transmitting it at and should be the same as the polarity of the receivers antenna on the ground for maxiumum signal.

Unless you really need to reduce drag doing something like this isn't going to be the best setup, but people use hand held transceivers inside aircraft with nothing more than a rubber ducky antenna to talk to towers which is probably far more inefficient than a strip of copper glued to the fabric of the airplane to act as a vertical antenna, in that light it isn't too bad a way to go, as long as the pigment in the paint doesn't contain something metallic which would be a reflector to radio waves.

I am sure there are other ways of doing this which would work too. There are flat profile antennas which can be glued to the underside of the wing between ribs somehow. Also, perhaps loading into a landing gear leg would be much better than into the airframe tubing, since it is a single piece and hangs out below the aircraft.


Quote:
Originally Posted by Ernest Christley
Christopher wrote:
I have 30 years experience working with many different types of antennas
from 100 KHz through 20 GHz and can answer some questions for you.

If you have a rag and tube aircraft, you can actually couple to the
tubing in the frame and use it as a radiator itself but with a fuel
tank and instruments in the airframe you could get cross coupling that
you might not want. Yet, I believe it might be possible to couple to
the tubing in the vertical stabilizer which should be far enough away
to be far less coupling than you would have with a standard whip
antenna mounted over the cockpit.


The fuselage is one humungous welded cage. If you used it for a
radiator, what would you use for a ground plane? Don't you need a
ground plane?

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

Last edited by Christopher : May 26th 06 at 01:40 PM.
  #9  
Old May 26th 06, 10:17 PM posted to rec.aviation.homebuilt
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Posts: n/a
Default Pitot tube for the antennae

Christopher wrote:
The way it is done is to attach the feed at a point so many inches up
from where the coax would be grounded to the fuselage. Although the
fuselage is at ground potential where it connects to the battery and
negative lead for the avionics if you find the right spot to attach the
coax the feed will both be matched and radiation will occur on a
vertical member as if it were insulated from ground.

Here is a web page showing how it is done for the lower medium to high
frequency portions of the amateur radio band on a grounded tower, of
course, with an aircraft you would have more trouble to find a sweet
spot for loading into the frame than with a simple tower but I believe
it can be done at the tail, if you feed the lower portion of the tail
the remaining upper structure might be about the right length for
VHF-AM in the aircraft comm band:

http://www.qsl.net/w9rb/webdoc9.htm

If you are not an RF man with a through-line watt meter I wouldn't
expect this to be an easy task for someone new to radio or antennas,
but it could be done with a few hours work on a tube and rag aircraft
if you did it before it was covered.


I don't have any ham equipment, and this page has way more math than I
want to deal with. I think I'll just try wrapping 22" of copper tape
around a cheap aluminum pitot and see how it performs.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."
  #10  
Old May 27th 06, 01:24 PM posted to rec.aviation.homebuilt
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Posts: n/a
Default Pitot tube for the antennae

On Fri, 26 May 2006 13:02:14 +0100, Christopher
wrote:

The way it is done is to attach the feed at a point so many inches up
from where the coax would be grounded to the fuselage. Although the
fuselage is at ground potential where it connects to the battery and
negative lead for the avionics if you find the right spot to attach the
coax the feed will both be matched and radiation will occur on a
vertical member as if it were insulated from ground.


I don't fly, but love airplanes and radio. The only thing I think is
missed here is that the aircraft band is wide (108-136 MHz, I
believe). While a part of that is receive only (for the plane), the
range that the transmitter must be able to talk to is still pretty
wide, 15% I believe. To get a fixed tune system that works well with
reasonably low SWR requires low Q; how well do those systems work?

I know the classic J-Pole is decent, and am not sure this differs in
concept.

Those nifty foil designs of Jim Weir's lower the Q by width, I
believe, as the thickness is slight.
 




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