[Elecraft] K3 - Numeric SWR Display

[Jim Brown]

On Thu, 18 Sep 2008 09:35:45 -0600, Bill W5WVO wrote:

>as feedline losses due to SWR can be
>significant on both transmit and receive

It depends on your definition of "significant" and the degree 
of mismatch. This is another one of those "old wives' tales 
that is a wild exageration of reality. A graph of the loss due 
to mismatch has appeared in every version of the ARRL Handbook 
and the ARRL Antenna Book for many years. It shows that the 
WORST CASE additional loss for a 1.5:1 VSWR is 0.18 dB, no 
matter how much the matched loss of the line! That worst case 
number is 0.5 dB for a 2:1 VSWR and 1.2 dB for 3:1 VSWR. 

If the loss in the line for a perfect match were 1 dB, the 
additional loss due to SWR is about 0.05dB for 1.5 VSWR, 0.2 dB 
for 2:1, and 0.46dB for 3:1. You've got to have a VSWR of 5:1 
for there to be 1 dB of additional loss due to mismatch!  

Yes, the loss in any transmission line causes the VSWR to come 
closer to 1:1 as we move along the line from the antenna to the 
transmitter. The lossier the line, the greater this effect. BUT 
-- coax of reasonable size and quality is NOT very lossy on the 
HF bands. Decent RG8X (Belden 9258 or LMR240) has only about 1 
dB/100 ft on 20M, and less than 0.5dB/100ft on 80M. Loss rises 
t about 1.6dB on 10M, and 2 dB/100 ft on 6M. The best RG8s have 
roughly one-third that loss. This graph is in Chapter 19 of my 
2002 ARRL Handbook, and Chapter 24 of the ARRL Antenna Book. 
Both publications also show the equation from which the graphs 
are derived, and the ARRL Antenna Book comes with a calculator 
(by N6BV) that will compute the loss in the line for any length 
of line, frequency, and value of termination impedance that you 
plug in, based on its database of loss for many popular coax 
cables (Belden, LMR, and a few others). You can also plug in 
the VSWR or complex impedance you've measured at the 
transmitter and the length of the line, and it will compute the 
line loss and the VSWR at the antenna.  

The bottom line is that unless your antenna is badly mismatched 
(far off resonance or broken), the additional loss due to VSWR 
on the HF bands is truly insignificant. 

Now, I DO use big coax (RG8 and RG11) on my high wire dipoles 
that are up about 110 ft (and the lines are about 140 ft). I do 
that because I am contesting (high duty cycle) with legal power 
and over rather wide bandwidths (a single dipole for all of 
80/75 and for 160M), AND because I use the 80/40 fan dipoles on 
30M, 17M, and 12M. The fact that I'm using the big coax saves 
me a dB or so in these far off resonance conditions, and allows 
them to work pretty well!  

73,

Jim Brown K9YC