[NJARC] Antenna Feedline question

[Jim Whartenby]

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
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