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72MHZ Model Aircraft Antenna Design Question - RF Cafe Forums
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| monzie
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Post subject: 72MHZ Model Aircraft Antenna Design Question
Posted: Thu Jan 26, 2006 12:49 am
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Joined: Thu Jan 26, 2006
12:37 am
Posts: 2 |
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I am working on the front end of a 72MHZ receiver
project for model aircraft.
Most model airplane
receivers use a monopole antenna consisting of a
single 1/4 wavelength long wire . However I have
yet to find an antenna design book or reference
that does not analyze a monopole antenna without
referring to a "good ground". Obviously a large
metal sheet acting as a counterpoise is not possible
for this project, and I do not want to use a dipole
antenna.
What is the radiation impedance
of a 1/4 wavelength wire far from any metal ?
I am trying to determine the impedance matching
for this type of antenna.
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Whitebird |
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Post subject
Posted: Sun Mar 05, 2006 6:05 pm
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Joined: Tue Dec 27, 2005
1:51 pm
Posts: 8 |
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When you say far from any metal you think "without
any ground plane"??
I think the ground plane
even if very small is mandatory because without
it there isn't any return and then the impedance
becomes very high. Of course ground proximity can
act as a return but in a plane it may be very far
from ground.
At 72 MHz your 1/4 wave length
antenna will be 105cm long. Is it not too much?
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monzie |
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Post subject
Posted: Sun Mar 05, 2006 9:21 pm
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Joined: Thu Jan 26, 2006
12:37 am
Posts: 2 |
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Yes, when I say far from metal I mean without a
ground plane.
The pcb does have a ground
plane but the area of the entire pcb is about 2cm
x 5 cm. My understanding is that since these dimensions
are much smaller than the wavelength of the received
signal, the small ground plane on the pcb will be
essentially invisible to the wave, and the standard
monopole method of antenna impedance calculation
will be invalid.
The simplest way to approach
this problem might be just to build the circuit
and measure the antenna impedance.
I am not
sure what you mean when you write "return" as in
"there won't be any return".
105cm is indeed
too long, but what I am looking for is a method
to analyze a monopole antenna with a very small
(or non-existent) ground plane. Then I can determine
what the impedance of a shorter antenna will be.
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fred47 |
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Post subject: 72 MHz antenna
Posted: Thu Mar 23, 2006 8:31 pm
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| General |
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Joined: Wed Feb 22, 2006
3:51 pm
Posts: 104 |
Hi! One antenna type that does not need a
"good ground plane" is the "slim jim". Here's a
link:
http://www.hamuniverse.com/slimjim.html
It's an antenna that in effect provides its
own return. Although probably too big for
you, a "J-Pole" is also a no-ground-plane antenna.
Google is your friend. Good Luck!
Fred |
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Posted 11/12/2012
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