[NJARC] Antenna Feedline question

[Scott Roberts]

Thanks! Ok, so I will go with the RG-6 CATV stuff... What about the balun it wants? Should I use the RG-6 there as well?

Thanks Again,
Scott


--- On Sat, 1/3/09, Jim Whartenby <antqradio at sbcglobal.net> wrote:

> From: Jim Whartenby <antqradio at sbcglobal.net>
> Subject: Re: [NJARC] Antenna Feedline question
> To: njarc at mailman.qth.net
> Date: Saturday, January 3, 2009, 12:50 PM
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Hi Scott
> I would not worry about the 50 ohm - 75 ohm issue.  If you
> calculate the VSWR for this mismatch it is 1.2:1 or a return
> loss of approximately 20dB.  This means that for every 100
> watts of output power, one watt is reflected back to the
> transmitter.
> 
> VSWR becomes a problem when it reaches about 2:1.  Then the
> reflected power is about 10 watts for every 100 watts out of
> the transmitter.
> 
> As for the type of coax, I would be cautious about using
> Radio Shack coax.  Like life in general, you get what you
> pay for.  RS coax is inexpensive because there is less
> copper in the shield.  The shield coverage is 70% at best. 
> On the other hand, RG-6 used for CATV is double shielded
> with an aluminum braid over aluminum foil for 100%
> shielding.
> 
> If you can find 75 ohm BNC crimp connectors (and borrow the
> crimper) to fit your RG-6 then you should be able to run
> several hundreds of watts without a problem.  On the face
> value for both the BNC connector and RG-6 coax, both are
> rated for about 500 volts RMS maximum (about 3KW at 75
> ohms)so 300 or so watts will not be a problem.
> 
> For VSWR information look at:
> http://www.microwaves101.com/encyclopedia/vswr.cfm
> 
> For RG-6 information look at:
> http://www.vk1od.net/RG6/index.htm
> Regards,
> Jim
> 
> 
> 
> --- On Sat, 1/3/09, Scott Roberts
> <ng19delta at yahoo.com> wrote:
> 
> > From: Scott Roberts <ng19delta at yahoo.com>
> > Subject: [NJARC] Antenna Feedline question
> > To: "NJARC Radio Club"
> <njarc at mailman.qth.net>, "Al Klase"
> <al at ar88.net>
> > Date: Saturday, January 3, 2009, 10:06 AM
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > Gentlemen(and ladies), I direct your attention to the
> ARRL
> > 2008 "Handbook for Radio Communications",
> page
> > 22.19
> > 
> > I am currently building the W8NX Multiband, Coax-Trap
> > Dipole"  for 10, 15, 20, 40 & 80M, as this
> seems to
> > be one which will work for my needs, given the limited
> > (attic) space available, and no trees nearby. I have
> most of
> > the parts together, and am finishing building the
> traps.
> > 
> > My question is about the feedline- My Hallicrafters
> > equipment is specifying RG-8, and 50 ohm output: the
> antenna
> > specs call for 75 ohm through a balun.
> > 
> > I am planning to use a coiled coax balun, (P.21.16,
> Tab.
> > 21.4) going in, but am still not sure what cable I can
> use.
> > I was planning to use Radio Shack RG-8 (13AWG Type CL2
> > E111378A RS#278-1312) which I have a reasonable amount
> of
> > for this exercise.
> > 
> >  My other options would be RG-6(I don't have the
> spec#
> > handy) CATV wire for the aerial drops(I have almost a
> full
> > spool of this stuff in storage, complete with support
> wire
> > attached) which has dual shielding & is also gel
> > filled(for moisture prevention), or Radio Shack RG-58
> coax.
> > 
> > I think the RG-8 would be ok, as that is what the
> equipment
> > calls for-
> > What would I be suffering should I use that? The RG-6
> seems
> > to have the 75ohm the antenna calls for, however...
> > 
> > I am going to get back to the build and set-up, but am
> > hoping someone can help with this question. I am
> trying to
> > get the antenna in this weekend, since I have had time
> to
> > concentrate on radio stuff for a change, and want to
> get the
> > HF gear ready to go! My max power available (once I
> find
> > tubes for the HT-41) will be 1500w, although the HT-37
> is
> > 300 without it.
> > 
> > As a side note, I built the simple groundplane(for 2M
> &
> > 70CM) from pages 22.56 & 22.57, and it seems to
> work
> > quite nicely. :)
> > 
> > Thanks!
> > Scott
> > _______________________________________________
> > NJARC mailing list
> > NJARC at mailman.qth.net
> > http://mailman.qth.net/mailman/listinfo/njarc
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