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[aprssig] Scaling APRS antenna

nc8q nc8q-1 at gelm.net
Thu Sep 23 17:13:54 UTC 2004

----- Original Message ----- 
From: "Andrew Rich" <vk4tec at tech-software.net>
To: "Glenn Little WB4UIV" <glennmaillist at bellsouth.net>; "TAPR APRS Mailing
List" <aprssig at lists.tapr.org>
Sent: Saturday, September 18, 2004 5:48 PM
Subject: RE: [aprssig] Scaling APRS antenna

> Thats the thing the elements and boom are stailness steel and welded.
> dough
> -----Original Message-----
> From: Glenn Little WB4UIV [mailto:glennmaillist at bellsouth.net]
> Sent: Sunday, 19 September 2004 5:17 AM
> To: vk4tec at tech-software.net; TAPR APRS Mailing List
> Subject: Re: [aprssig] Scaling APRS antenna
> You will need to change the spacing as well as the element lengths.
> This web site has some data and pointers to other web sites:
> http://www.hamuniverse.com/antennas.html
> There is a pointer to technical info on Cushcraft antennas. Some of the
> manuals give dimensions.
> Hope this helps
> 73
> Glenn
> At 08:36 PM 09/17/04, you wrote:
> >Hi,
> >
> >I picked up a beam stainless steel 400-420 MHz
> >
> >How can I work out how much to cut off to bring it up to 438 MHz ?
> >
> >Cheers

Spacing is not as critical element length.
Typical spacing of a reflector is .15 to .25 wavelength. Typical
spacing of the first director is .1 to .15 wavelength.  In this case
you are increasing the element spacing and boom length (electrically),
and from what I have seen in commercial antennas,
this is probably a good thing. ;-)
i.e. a 6% change in the element spacing is insignificant.  However a 6%
change in element length is.  How well does the antenna work, now,
'out of resonance' and, if it is acceptable, how tolerant is your
with the SWR of the unmodified antenna?

If you cannot cut the elements, perhaps you could bend them back.
The reflector should resonate lower than the operating frequency,
which it already does.  So, the reflector could be left alone.  The
director(s) must resonate higher than the operating frequency,
they 'should be' (electrically) shortened.

HTH, Chuck

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