[TowerTalk] 30m Dipole

[Jim Lux]

At 12:50 PM 8/3/2006, Scott Stembaugh wrote:
>On 8/3/06, Bill Turner <dezrat at copper.net> wrote:
> > ORIGINAL MESSAGE:
> >
> > On Thu, 3 Aug 2006 09:41:08 -0700, you wrote:
> >
> >
> > > i forget we live in a world without antenna tuners
> > >
> > >mike w7dra
> >
> > ------------ REPLY SEPARATOR ------------
> >
> > Without antenna tuners?
> >
> > Nearly all modern rigs have them built in, don't they?
> >
> > Bill, W6WRT
> > _______________________________________________
>
>As I understand it that just makes the radio happy.  What is the ERP
>delta between a resonate antenna and a non-resonant tuner-tuned
>antenna?

Essentially identical.



>  How much signal is lost on the feedline?

Depends on the loss of your feedline.  It's a dipole, so clearly you aren't 
obsessing about eking out the last tenth of a dB of gain.

Say you're pretty far off 50+50j instead of 50.. a VSWR of 2.4:1..  100 ft 
of RG-8X and you'll lose 1.44 dB, compared to 1.1 dB if it's perfectly 
matched.  If you use a Low pass L network at the rig end (as if you had an 
autotuner) you'd actually be about the same place: 1.44 dB down.

But let's take an interesting case.. 72+j0 ohms... 1.4:1 VSWR
Put in 100 ft of RG-8X, and you're at 38.8+2j at the transmitter, anout 
1.3:1...  The 1 dB loss in the feedline helps a bit.


I deliberately chose a fairly lossy TL for this, because you asked about 
feedline losses with mismatch.

If you had a lower loss feedline, the mismatch at the TX end would be 
worse, so you might have more loss from the tuner, but I suspect it all 
comes out about the same.  Going to RG-213 drops the matched loss down to 
about 0.75 dB, and the mismatched loss (in the feedline) to about 1 dB


Jim Lux

(BTW, the spreadsheet XLZIZL, free for the downloading from AC6LA's 
website, makes this kind of trial and error easy.)