Its not really a loss. It is current that would otherwise be on the other
side of the dipole. The problem with it is:
1. it takes away from the desired pattern
2. is often the wrong polarization
3. adds lobes where there shouldn't be in the pattern
4. and worst of all, brings rf closer to or even back into the shack or
house and since it is much closer than the rest of the antenna can easily
cause tvi and other nasty problems like rf burns.
5. and just the opposite on rx, it picks up rf from surrounding devices it
is close to and couples that into the antenna adding to rx noise.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Al Williams
> Sent: Friday, July 22, 2005 16:15
> To: TOWERTALK@contesting.com
> Subject: Re: [TowerTalk] dipole question
>
>
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@contesting.com>
> Sent: Wednesday, July 20, 2005 3:56 AM
> >
> > Since you attach the
> > grounded shield to one half, the outside of the cable
> > elevates in voltage in an attempt to get current flowing in
> > the grounded half of the dipole. That causes some current to
> > flow down the outside of the shield, and the cable to
> > radiate. The outside of the shield actually becomes part of
> > the antenna when the feedpoint isn't properly designed!
> >
> > > It actually is a bit more complex than this, but this is a
> > good rough idea of what is happening.
> >
> I appreciate the explanation given, but (I and maybe others) would like a
> follow-on of what is happening to this outside-of-the-shield current and
> how
> significant is it?
>
> 2. The current flowing on the outside of the shield must have an opposite
> current (charge) flowing somewhere. Is it the portion of the antenna that
> is connected to the center conductor.
>
> 1. If for example, 100 watts is output but 10 watts is diverted to the
> outside, that still leaves 90 watts to the intended antenna--an
> insignificant (?) loss.
>
> 3.
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|