I thought formula for 1/2 wave dipole when the antenna is close to the
earth's surface (5% reduction in length) is:

468/Freq (in MHz) = length in feet


"Free Space" formula (many wavelengths above the earth's surface) is:

492/Freq (in MHz) = length in feet



To obtain length of 1/4 wave antenna, divide answer by 2.
To obtain length in inches, multiply answer by 12


418MHz 1/4 wave antenna = 468/418/2*12
                        = 6.72 inches (17.06 cm)

433.92MHz 1/4 wave antenna = 468/433.92/2*12
                           = 6.47 inches (16.44 cm)

I confirmed the above formulas and lengths with Lynx Technologies.

Note the specified lengths in the Lynx application notes are
incorrect (for 433.92MHz module).  I have also made Lynx aware of
their error in the specified length and they gave me verbal confirmation
that they did indeed make an error in the application notes.

-Randie
rohtsji@glenayre.com

P.S.  I tried both the incorrect length specifed by Lynx (6.7 inch for
433.92MHz module) and the correct length (6.47 inch).  I couldn't see
a noticeable difference in range or performance.



> From zc@INTEKOM.CO.ZA Thu Mar 26 09:50:53 1998
> MIME-Version: 1.0
> Content-Transfer-Encoding: 7bit
> X-Priority: 3
> X-MSMail-Priority: Normal
> X-MimeOLE: Produced By Microsoft MimeOLE V4.72.2106.4
> Date:         Thu, 26 Mar 1998 19:50:13 +0200
> From: Zack Cilliers <zc@INTEKOM.CO.ZA>
> Subject:      Re: [OT]     Re: Programmer sold, RF freq's
> To: PICLIST@MITVMA.MIT.EDU
>
> -----Original Message-----
> From: Justin Crooks
> <jcrooks@WYOMING.COM>
> To: PICLIST@MITVMA.MIT.EDU
> <PICLIST@MITVMA.MIT.EDU>
> Date: Thursday, March 26, 1998 8:16 PM
> Subject: Re: Programmer sold, RF freq's
>
>
> snip
> >I remember right, 16.5 cm for 418MHz,
> 15.5 cm for 433MHz (1/4 wave, you can
> >do the math).  Use a thick, straight
> piece of wire or something.  We even
> >went so far as to use a digital caliper
> when cutting the antenna (with good
> >results.  Of course, we were probably
> overly anal).  Good luck!!!
> >
> the formula is Length = speed of
> light/freq - 5% for end losses, that
> will give you
> full wave. If you want 1/4 wave devide
> lenght by 4, but try and use full wave
> if possible.
>