Actually, I never suggested a Q for the coil. Al must have been
thinking about somebody else when he said that part, although the rest
of what he attributed to me is accurate. I usually use a Q of 200 for
an air core coil if I'm trying to be conservative, but a Q of 400 is
reasonable if you have room for a coil of decent size and as you say,
700-800 is achievable if you have the ability to optimize it. I have
no idea what the Q of a ferrite core inductor in a typical antenna tuner is.
Your description of the MN-2700 makes me want to go look for one. ;)
73,
Dave AB7E
On 7/14/2021 3:49 AM, Alan Bloom wrote:
The Drake tuners used a Pi-L circuit topology in which the circulating
current in the inductor is independent of the load impedance. Assuming
almost all the loss is in the inductor, that means that the loss is
independent of the load impedance.
(Another advantage of that topology is you get good harmonic
suppression for all load impedances.)
So when I was designing the Drake MN-2700 I just measured the loss
into a 50 ohm load and made sure it was less than the 0.5 dB spec with
some margin. That won't work when using most topologies (such as the
L networks used in the Elecraft tuners) because the loss does change
drastically depending on the load impedance. For those, you can use
two identical tuners back to back, both adjusted for the same load
impedance. The loss for each tuner is approximately half the measured
loss. (I think I did do a few tests like that on the MN-2700 just as
a sanity check.)
I found that the hardest band to get to meet all specs (5:1 SWR, 0.5
dB loss, 1000W average, 2000W PEP) was 160 meters. That's partly
because it is hard to get a high-inductance, super high-Q coil small
enough to fit in the cabinet and partly because of the large
capacitances required. (The MN-2700 has 3-position switches to add
fixed capacitance to each tuning capacitor.)
To measure the matching capability at different phase angles, I just
connected a 50-ohm load to the input and an HP impedance analyzer to
the output. By adjusting each tuning capacitor throughout its range
and plotting the results on a Smith chart you can see the (complex
conjugate of the) matching range. Actually the output impedance of
the tuner and the antenna impedance it matches are not exactly
conjugates, but are close as long as the tuner insertion loss is low.
As suggested by Dave, I chose typical Q values of 100 for the inductor
The coils in the MN-2700 have much higher Q than that. To such an
extent that it was difficult to get accurate readings on an HP
Q-meter. But by tightening the connecting bolts down as hard as
possible and making sure there were no absorbing objects (like human
hands) in the near field of the inductor I was getting values in the
700-800 range on some bands as I recall. (These were all air-wound
solenoidal inductors.)
Alan N1AL
On 7/13/2021 10:32 AM, Al Lorona wrote:
Thanks to Al N1AL, Jack W6FB, and Dave AB7E for great information
that helped me a lot.
I'm in the circuit simulation business, after all, and I confess that
I was just being lazy, so I ran some simulations that confirmed what
Dave, in particular, had said.
As suggested by Dave, I chose typical Q values of 100 for the
inductor and 1000 for the capacitor. Then I simulated as many points
as I could on the entire Smith Chart to see 1/ if the tuner could
tune each point to 50 ohms, and 2/ what the power loss was in the
tuner at each of those points. Then, I discovered that K6JCA had
already done this on his excellent blog
at: https://k6jca.blogspot.com/2015/03/notes-on-antenna-tuners-l-network-and.html . The
guy is totally professional and exhaustive in his discussions. I
really admire his work.
Anyway, it turns out you can make a graph of power lost in the tuner
versus phase angle of the load. As you might suspect, 'easy' loads of
5 or 500 ohms resistive (SWR = 10:1) don't tax a tuner as much as
reactive loads do. In fact, they're near (but interestingly, not at)
the areas of *minimum* power loss.
Whenever an antenna tuner is reviewed in QST, resistive mismatched
loads are usually used. I'd like to see tuners tested with reactive
loads, but the number of loads required to do this from 160 to 10
meters would be enormous. I see why resistive loads are preferred,
because you can re-use the loads on every band.
I'm frustrated by imprecise statements like, "This tuner will tune an
8:1 mismatch." What does that mean? There has to be a better way for
manufacturers to spec the exact impedance ranges that their tuners
will tune. I like the method that I used, which shades a Smith Chart
in color based on the two criteria I listed above. One picture would
tell you all about a tuner's effectiveness. No real tuner can tune
the entire Smith Chart, but the more of the chart that is covered,
the better the tuner. And if you can shade the areas of higher tuner
loss in red, then that would also tell you an important piece of
information. (However, to generate such a plot through measurement
you'd probably need a very expensive load-pull setup, which is a
totally separate discussion.)
For the L-network I simulated, a particularly difficult 10:1 load was
near the 7 - j30 ohm point, which is toward the bottom edge of the
Smith Chart at a phase angle of 282 degrees (or -77 degrees), and a
similar point near the top edge. The lower impedances with capacitive
reactance were definitely the most difficult (using power loss as the
measure of 'difficulty') for the tuner to handle, which Dave stated
in his post, while the high impedances with inductive reactance were
generally more difficult. If your antenna must be mismatched, and
you're using an L-network tuner, you want it to be > 50 ohms with a
little bit of capacitive reactance, or below 50 and inductive.
By the way, K6JCA actually put the Elecraft KAT500 through this
simulated evaluation and it tested so well that he ended up buying one.
Al W6LX/4
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