[Elecraft] Balanced Circuits and Loss Due to Mismatch In Transmission Lines

[Jim Brown]

On 9/2/2013 4:32 PM, Don Wilhelm wrote:
> One can feed the 43 foot vertical with ladder line, and do the tuning 
> in the shack (yes, you can feed a vertical with balanced line), 

I strongly object to the word "balanced" with respect to transmission 
line. A better description is 2-wire or parallel wire line. The balance 
of a circuit is determined by the impedances with respect to the 
reference plane (usually the earth).

> but if feeding with coax, the best place for the matching network is 
> at the base of the antenna.  Coax is good for an swr of 2.0 or less 
> (depending on the frequency).  If the SWR is greater than that, some 
> kind of matching device at the base of the antenna is in order.

That depends entirely on the length of the coax, how lossy the coax is 
at the frequency(ies) of interest, and the power level.
> Ladder line feed OTOH is quite OK with an SWR of 20:1 if it is 
> properly routed - away from conducting surfaces by at least 3 times 
> its spacing and similarly away from the earth.
>
> Again, yes, balanced feedline is an alternative for vertical antennas 
> - the antenna may be *unbalanced*, but the feedline does not care, You 
> still need balanced current and return current for the antenna to 
> functon correctly. 

Calling a feedline "balanced" does not make the currents equal.  If the 
system is unbalanced, the currents will be UNequal, and the difference 
will be a common mode current. There are several ways to make the 
currents equal, or more nearly equal -- we can use an ordinary 2-winding 
transformer at each end, or an impedance-transforming balun, or even an 
ordinary common mode choke.

I don't disagree that higher impedance 2-wire line can be a reasonable 
way to connect a badly mismatched antenna to an antenna tuner.  But if 
like many hams, the vertical is close enough to the shack that it can be 
fed with 50-75 ft of coax, I'd seriously consider running a good quality 
RG213 or hard line to a tuner in the shack. Yes, line loss increases 
with high SWR, but one of the most misunderstood things about 
transmission lines is how SMALL that increase is for short runs of big 
coax. This is clearly shown by a family of curves that has been 
published in every edition of the ARRL Handbook since I've been a ham. 
It's Fig 20.4 in the 2010 Handbook.  Plugging in loss data for LMR400 
(0.3dB for 75 ft at 10 MHz), the loss would be only 1dB with 10:1, and 
2.3 dB with 20:1 SWR. For 1/2-in hard line, divide those loss numbers by 
2.  Think that coax is expensive?  Try pricing a decent weatherproof 
enclosure and other costs associated with remoting the tuner.

Where you get in trouble with loss in coax is when you use long runs of 
small stuff -- because you're only running 5 watts. :)  Our Field Day 
group holds the all time record for 1A battery, and we run RG213 to all 
of our antennas.

73, Jim K9YC