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'[OT] Re: Programmer sold, RF freq's'
1998\03\26@125013 by

-----Original Message-----
From: Justin Crooks
<jcrooksWYOMING.COM>
To: PICLISTMITVMA.MIT.EDU
<PICLISTMITVMA.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.
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
rohtsjiglenayre.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.

{Quote hidden}

> {Original Message removed}
At 10:27 AM 3/26/98 -0800, "Randie Ohtsji [4555]"  wrote:
>I thought formula for 1/2 wave dipole...

You are indeed correct.

>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.

I'd be surprised if you did.  The return loss will be almost the same with
either length on either frequency.  If you're looking to improve
performance remember that a quarter-wave radiator must work against a good
ground.  Depending upon the amount of metal (or lack thereof) attached to
electrical ground, you could see quite a range of performance.  Try adding
one or more one-quarter wavelength ground-connected radials.

You could also try a 5/8 wave radiator but you'll need a tiny bit of series
inductance to tune out the capacitive reactance of the antenna.

Have fun!

Andrew Gerald
Custom Communications Systems

***

This e-mail address is a 'facsimile receiver' as defined
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to this address is a violation of US Federal Law.
>You could also try a 5/8 wave radiator but you'll need a tiny bit of series
>inductance to tune out the capacitive reactance of the antenna.
>
>Andrew Gerald
>Custom Communications Systems

Has anyone tried to connect something like a J-pole antenna to one of
these modules ?? How could I do the coupling of such an antenna ? The 2.15
Db gain would be really nice for reception and the antenna could be made
with very simple 300 ohms paralell antena wire.

Best regards,
Alexandre Guimaraes
Microset Eletronica Ltda
alexgiis.com.br

Alexandre Guimaraes wrote:

>  Has anyone tried to connect something like a J-pole antenna to one of
> these modules??  How could I do the coupling of such an antenna?  The
> 2.15 Db gain would be really nice for reception and the antenna could
> be made with very simple 300 ohms parallel antena wire.

The so-called "J-pole" and its popularity, or rather the enthusiasm it
engenders, really gets under my skin!

A "J-pole" is in fact a half-wave dipole, nothing more, nothing less,
end-fed by a non-radiating stub.  Its gain probably is 2.15DbI, the "I"
standing for "isotropic" whcih is the radiation pattern of some
theoretical antenna radiating equally in all directions, something that
is in practice *very* difficult to do.  Naturally, *any* antenna that
radiates more energy in some directions and less in others will have
"gain" over isotropic, unless it is resistive (converts power to heat).

When a quarter-wave sits on a ground plane, the element is supposed
to be "reflected" in the ground plane; that is, it behaves as if the
other half of a half-wave dipole is present; it effectively *is* a
dipole.  The only differences between this and a half-wave dipole are a
slight distortion of pattern by the ground plane (and this depends on
the slant {conical} of the ground plane), and the feed impedance which
depends on a lot of things.

Conclusion:  Half-wave dipole or J-pole has no gain compared to
quarter-wave ground-plane.  Its advantages are that it is more compact
(no ground plane) and that the active element is held a quarter-wave
distant from the mounting point (good for cars!).  Tuning consists of
adjusting the length of the active element *or* the stub and the tapping
point of the feed up the stub (or use a loop or "gamma" match with a
trimmer).  But at these frequencies, end-feeding a half-wave with a
simple wire-loop inductor and trimmer cap may be just as easy.

Tune-up is probably best done using a signal strength meter at these
frequencies.

Cheers,
Paul B.

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