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

[Paul Christensen]

"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."

Clarification on my statement above:  Yes, in instances of high system loss, 
the tuner is providing a condition where the "transmitter is happy," [God, I 
hate that term] but the tuner is also providing a secondary benefit in that 
maximum power is transferred into the line despite high system loss.  I 
thought it was obvious, maybe it wasn't.

Paul, W9AC


----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: <towertalk at contesting.com>
Sent: Saturday, December 14, 2013 1:18 PM
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?


> >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
>
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