[Elecraft] Question about antenna matching

Wed Jul 14 14:43:11 EDT 2021

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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