[TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

[Paul Christensen]

>I think a tuner only fools people to think things are working right (unless 
>you use a Matchbox type tuner and twinlead).

Depends on the entire transmission system, not whether a Matchbox or 
twinlead is used.  Let's look at two different examples:

Example #1:

In this antenna system, let's use 100 ft of RG-58 at 28 MHz and an RF 
generator output power of 100W.  For the radiator, let's pick an 80m dipole 
whose overall length is 135 ft. and 50 ft in height above average ground . 
Under this condition, NEC4 says the feedpoint Z is 1525+j1143.   If you 
place an SWR meter at the input to the antenna it will read 47:1 (assuming 
the reading is not adversely affected by direct radiation, nor common mode 
current).  However, if the meter is moved to the transmitter end, it will 
read 3:1.

 - SWR *does* change along a line when line loss is present, but not on a 
lossless line.

Again, the Z at the antenna is 1525+j1143 but the Z at the transmitter end 
is 24+j34.  If we use tuner here to transform 24+j34 to 50+j0, we will get 
our objective SWR reading of 1:1 at the transmitter but there's so much 
matched line loss + mismatch loss, that any mirrored reflection at the tuner 
is also attenuated.  The round trip matched and mismatch loss here is much 
too high and in this case -- we can truly say that the tuner is "just making 
the transmitter happy," or nearly so.  We have a whopping 12 dB of 
systematic loss.  Power dissipated at the load is only 6 watts.

Example #2:

In this antenna system, let's use 100 ft of 600-ohm balanced open feeders. 
We're still transmitting 100W at 28 MHz into the same 80m dipole and as 
such, the antenna input Z is still 1525+j1143.  The SWR at the antenna end 
of the line is 4:1.  the SWR at the transmitter end is now much closer and 
is 3.9:1.

- Once again, SWR does change along a line when line loss is present, but 
*not* on a lossless line.

The Z at the transmitter end is 725-j969.  That represents a 4:1 600-ohm 
line SWR of 4:1.  The 50-ohm SWR is a gigantic 47:1 at the antenna and 40:1 
at the transmitter.  Since we're using a line with a characteristic Z of 600 
ohms, we don't care about the 50-ohm SWR on the line -- only the 600-ohm SWR 
of 4:1.  The systematic loss is now only 0.2dB and 95 watts is dissipated at 
the load.

We're not done yet.  The SWR at the transmitter end of the 600-ohm line is 
3.9:1 but has a whopping Z of 725-j969.  We need to transform that into 
50+j0 with a tuner, a fixed L/C network, or a change in line length. 
Changing the line length will have: (1)  a dramatic effect on input Z; (2) 
minimal effect on 600-ohm SWR; but (3) a huge 50-ohm SWR effect on a nearly 
lossless line like the type used in this example.

- When we have a low loss line and moderate SWR, system losses are low and 
we can take advantage of the tuner's reflection properties.  In this case, 
the tuner is NOT ONLY just making the transmitter happy.  It's doing more by 
redirecting the reflected wave into radiated energy.

At this point, some folks are scratching their heads about "50-ohm SWR" and 
"600-ohm SWR."  Isn't SWR the same?  Yes, and no.  Study this difference. 
It's important when trying to understand why a change in line length affects 
one characteristic SWR but not the other.

Paul, W9AC