bidirectional 440 yagi antenna
 

Does anyone know of a good design for a reasonably high gain
bidirectional antenna for a 440mhz video TX? I am putting a 4 watt A/V
TX in an RC plane that will fly on auto pilot directly away from the RX,
make a 180 degree turn and fly directly back. So to get the best range
I was thinking some kind of bidirectional yagi. Depending on element
spacing and how big of a plane I get, I can probably only have 3
elements maybe 5.

--
Chris W
KE5GIX

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bidirectional 440 yagi antenna
 

A simple dipole will do what you want. That is a lot of power in the
airborne TX. 400 mw is still over kill and 40 mw will probably do the
job. At some point you will need to regain control with conventional
RC equipment and getting a signal into the airplane with that much
radiated power may not be easy.

If you are short on the RC airplane experience I suggest you consider
a Telemaster from Hobby Lobby. Smaller versions are available for
experience and the Senior (8 foot span) with carry many pounds of pay
load.

What you are considering is called an RPV and is a hot item in
military use now days.
John Ferrell W8CCW

On Sat, 03 Dec 2005 01:06:53 -0600, Chris W wrote:

Does anyone know of a good design for a reasonably high gain
bidirectional antenna for a 440mhz video TX? I am putting a 4 watt A/V
TX in an RC plane that will fly on auto pilot directly away from the RX,
make a 180 degree turn and fly directly back. So to get the best range
I was thinking some kind of bidirectional yagi. Depending on element
spacing and how big of a plane I get, I can probably only have 3
elements maybe 5.
John Ferrell W8CCW

bidirectional 440 yagi antenna
 

John Ferrell wrote:

A simple dipole will do what you want. That is a lot of power in the
airborne TX. 400 mw is still over kill and 40 mw will probably do the
job. At some point you will need to regain control with conventional
RC equipment and getting a signal into the airplane with that much
radiated power may not be easy.

If you are short on the RC airplane experience I suggest you consider
a Telemaster from Hobby Lobby. Smaller versions are available for
experience and the Senior (8 foot span) with carry many pounds of pay
load.



I guess you didn't notice the part about an autopilot, I plan on having
the plane fly out about 10 or 15 miles out and then back, not just where
I can see it. I have several years and a few hundred hours experience
flying RC planes but still plan on using a plane that is reasonably easy
to fly so the autopilot has an easier time controlling it. I have a few
tests I plan to do to check if the video TX will cause a problem with
the RC RX. I will rig up a servo to turn the video TX on and off and
will set a fail safe so that if the RC RX can't get it's signal it will
shut off the video TX. I will first set the video TX to very low power
(the TX I am looking at is adjustable from 0 to 4+ watts), then I will
make sure my video shut off switch is working good then make small
increases in the video TX power and see how far away I can fly the plane
before the fail safe shuts down the video TX.

If the high power video TX does prevent me from retaking control of the
plane from the autopilot/GPS navigation system, I have a plan B that I
am working on. A system that will make adjustments in video TX output
power at various way points, so it adjusts the power up as it gets
farther away, and back down as it gets closer, this will have the added
benefit of increasing the life of my video TX battery. I'm not sure how
I will do that yet but I have a few ideas I am looking into.

--
Chris W
KE5GIX

Gift Giving Made Easy
Get the gifts you want &
give the gifts they want
One stop wish list for any gift,
from anywhere, for any occasion!
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bidirectional 440 yagi antenna
 

Great! It sounds like you have it well planned. I was afraid you were
one of those fellows who thought it is a trivial problem.

I have a little experience(old) in ATV on 439 and current experience
in RC.
You have probably already planned for these things but let me share
observations:

All antennas with gain have reduced bandwidth.. a TV signal without
sound requires at least 4mhz without degradation.

All antennas with gain have reduced beam width. A little yaw with
anything resembling a yagi is going to be very noticeable.

It is easier to get gain at the receiving end than the transmitting
end. My 48 element collinear was very wide band and measured 21 db
gain. It is 3X5 feet.

Good luck, John Ferrell W8CCW

PS, AMA insurance specially excludes autonomous flight.


On Sat, 03 Dec 2005 11:00:06 -0600, Chris W wrote:

John Ferrell wrote:

A simple dipole will do what you want. That is a lot of power in the
John Ferrell W8CCW