*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
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Topband Reflector
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