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R: QFH modification



Hi Mak,SV1BSX and Dave,WB6LLO

Wondering if has significant difference in the behavior of a QFH antenna
if we replace the UT-141 section or the "hard line" by wich is made the
half-turn bifilar loop changing it with a soft copper tube wich has inside
a piece of coaxial cable having smaller diameter of the copper tube diameter
it is necessary to understand how the infinite balun feed works as described
by Walter Maxell W2DU in the ARRL Antenna Book 15 th Edition Chapter
20 Page 20-5  "The Infinite or Inherent Balun"

In operation, current flowing on the inner conductor of the first half
bifilar loop made with coax emerges at the feed point to flow onto
the second half of the bifilar loop wich is made of solid wire.
For current flowing on the inside surface of the outer conductor of the
coax, on its arrival at the end, the only path for current flow is around
the end of the braid and than fold onto the outside of the outer conductor
of the coax and such external feed line current is the desired antenna
current so that the outside portion of coax extending from the feed point to
the antipodal point became part of radiator.

If we replace the UT-141 or the "hard line" using a piece of coaxial cable
in to a soft copper tubing it works provided that the braid end of the
coax cable is carefully soldered all around to the open end of the copper
tubing.

Fail to solder the end of the braid with the end of the copper tube the
current will tend to flow over the outside surface of the braid and not
over the outside surface of the tube as requested by it to work as a
radiator.

73" de

i8CVS Domenico


----- Original Message -----
From: Dave Guimont <dguimon1@san.rr.com>
To: SV1BSX <sv1bsx@yahoo.gr>
Cc: <amsat-bb@amsat.org>
Sent: Wednesday, March 16, 2005 10:20 PM
Subject: Re: [amsat-bb] QFH modification


> Mak,
>
> I've done exactly as you've suggested...hardline is hard to find in small
> quantities, and I did many, many experiments on two meters using RG58
> stripped of the outer insulation, and placed in copper tubing...After
> satisfaction with the result I duplicated it in hardline with no
detectable
> difference...  Go for it..
>
>
> >    I wondering if has significant difference in the behavior of QFH
antenna,
> >if we replaced the UT-141 section on Domenico's design or the "hard line"
> >in the US-model, with a soft copper tube which has inside  a piece of
> >Coaxial cable
> >with smaller diameter of tube's diameter (RG-174?) till the N-type
connector
> >(pre-amp input).
> >
> >  Actually, for many of us is very difficult in some cases to find a
piece of
> >"hard-line".
> >The QFH antenna is simple and effective, I know that because I've tested
a
> >QFH, but
> >this piece of coaxial is a big trouble !
> >
> >  On the other hand, I saw several models around Internet, without this
> >technique.
> >They have a simplest feeding method: the coaxial pass through the
> >plastic-tube (and QFH)
> >and is connected in the upper-point of QFH.
> >
> >Question:  is it accurate this method? if not, the modification with
> >copper-tube including a smaller coaxial cable is it acceptable?
>
>
>
>             73, Dave wb6llo@amsat.org
>                     Disagree: I learn....
>
>                Pulling for P3E...
>
>
>
> --
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> ----
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