Topband: W8ji ATR-10 design 160M?

[Paul Christensen]

>*L network tuners (like the Ten-Tec, Nye Viking, etc) handle more*
>*power into impedances near 50 ohms, but often do a poor job *
>*matching reactive or very low impedance loads on low frequencies.

That's a true statement only if we severely limit the C value in an L tuner. 
But if we use a very high value of C in an L type -- or high C values in a 
T, losses are significantly minimized when either tuner is terminated into 
low Z loads on 160m.

I just computed tuner losses between a high-pass L, low-pass L,and high-pass 
T.  For the comparison, I kept coil Q at 200 although a roller inductor's Q 
is greatly affected by its mechanical design.   See this link that shows 
typical Q variance in a common roller inductor: 
http://www.karinya.net/g3txq/temp/rollercoaster_q.png

I also assumed a resistive 5-ohm load at the output terminals of the tuner 
models.  Q for the C values was held at 1000.  Frequency is 1810 kHz.

Example #1 - High Pass L

L =1.47 uH
C = 5800 pF
Loss = 0.08 dB (1.8%)

Example #2 - Low Pass L

L = 1.33 uH
C = 5200 pF
Loss = 0.08 dB (1.8%)

Example #3 - High Pass T (500 pF Output C)

L= 11.4 uH
Cin = 180 pF
Loss = 1.09 dB (22%)

Example #4 - High Pass T (1000 pF Output C)

L =  5.8 uH
Cin = 343 pF
Loss = 0.57 dB (12%)

Example #5 = High Pass T (5000 pF Output C)

L = 1.44 uH
Cin = 3100 pF
Loss = 0.11 dB (2.4%)

See a pattern here?  To get minimum loss in a high-pass T with low-Z 
terminations, it takes C values approaching the high values required in 
either L type.  This should be of no surprise.  There's no clear winner here 
except the T does offer an attribute not yet mentioned:  We can easily 
control the phase shift through the T for use in various phasing projects 
like directional antenna systems.  We can't easily do that with just an L 
tuner.  In that case, controlled phase shift needs to be attained by another 
method, like changing line length.   With adequate C size and reasonable 
coil Q we can get low loss on low bands -- and don't need the XMatch to get 
it .  However, based on my limited knowledge of that device, it employs a 
lot of switched C on the output and should work very well into low Z loads.

So even with the high-pass T, we need a ton of output C (and nearly 
commensurate input C) to get low loss into low Z terminations -- way more 
than what you get when you buy a T tuner off the shelf.

For the examples, I used a very low load Z value of 5 ohms.  Apart from a 
mobile installation, these are not antennas I want to use.  Even on 160m. 
If the input end of a line is anywhere near that value, most antenna systems 
will be very short.  Thanks, but no thanks.  I will do just about anything 
to ensure an antenna length that's long enough such that the Z seen at the 
input end of the line, no matter the line length and without any other 
external components -- is at least 50 ohms and don't care if it rises well 
into the K-ohm area.  Almost any simple T or L tuner will perform the 
matching function in this case.  When using multiband wire antennas where 
the lowest operating frequency is a half-wave radiator length -- or base-fed 
verticals that are not unreasonably short, then no matter the line length, 
the Z at the input end stays well into the double-digits and tuner loss is 
reasonably low.

On my QRZ.com page, you will see a motorized balanced L tuner that uses 
Eimac vacuum relays to switch a Jennings vacuum variable cap either in front 
of, or behind the balanced coil pair.  If I was to build the tuner today, I 
would eliminate that expensive piece.  That part of the circuit was designed 
when the input end of the line is less than about 50 ohms.  Again, unless 
it's a mobile installation, I really don't want to operate with short 
antennas that result in low Z at the line input.

Moral of my story: If your 160m tuner will terminate into some really low Z 
values: (1) try and keep coil Q high; and (2), make damn sure you can 
switch-in some big-value caps for the job.

Paul, W9AC