# 72MHZ Model Aircraft Antenna Design Question - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

monzie
 Post subject: 72MHZ Model Aircraft Antenna Design Question Posted: Thu Jan 26, 2006 12:49 am
 Lieutenant

Joined: Thu Jan 26, 2006 12:37 am
Posts: 2
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.

Whitebird
 Post subject Posted: Sun Mar 05, 2006 6:05 pm
 Captain

Joined: Tue Dec 27, 2005 1:51 pm
Posts: 8
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?

monzie
 Post subject Posted: Sun Mar 05, 2006 9:21 pm
 Lieutenant

Joined: Thu Jan 26, 2006 12:37 am
Posts: 2
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.

fred47
 Post subject: 72 MHz antenna Posted: Thu Mar 23, 2006 8:31 pm
 General

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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