Re: Resonance

jim_kr1s <jkearman@...>

--- In ultralightdx@..., D1028Gary@... wrote:
> Hello Jim,
> Thanks for your amusing comments. Since you have already apologized once
> for doubting the 554 uh inductance value in the PL-380 7.5" external
> loopstick project (as pasted below), I find it amusing that you are now rehashing
> the same arguments for which you apologized :-)
> Since you are obviously contradicting yourself, I'll let everyone decide
> whether to take you seriously, or not. I have already made my decision, by
> the way :-)
> 73, Gary


I'm sorry you're making this about you. I hope that others besides you have the right to talk about ULR experiments, even when their findings differ from yours. I have gone out of my way to explain my equipment and methods, even providing online references where possible. I won't respond to your condescending sarcasm, but will continue to stick to provable theory and experimental results. For those who just tuned in, Gary's ire stems from my repeated attempts to convince him that the antenna circuit is tuned, which is critical to understand if you're winding antenna coils. I don't know if Gary accepts that, so let's get it out of the way right now. We heard it from Scott Willingham, an engineer at Silicon Labs, the chip manufacturer:

"First, the AM front-end of the Si4734 is a tuned-tank circuit with a fairly conventional LNA and quadrature mixer as shown in the block diagram....

"As Roy has pointed out, the loopstick (or air-loop) inductance is resonated with an on-chip varactor, which tunes in small discrete steps. This tuning is done each time the frequency is changed." 

Now, on to tonight's episode: After Scott Willingham of Silicon Labs made his initial comment about the chip possibly tuning a 550 uH at 1700 kHz, which prompted my apology, he backed away from it. (I guess I should have then backed away from my apology, but I was trying to maintain the peace.) I quote (emphasis mine):

"I will slightly amend what I said about the stock inductance range topping out
at about 550 uH. My assumption there is that parasitic capacitance stays
constant. It occurs to me now that as the inductance is increased, the
loopstick's self-capacitance will increase as well, limiting the ultimate
achievable inductance. On the other hand, at 10% too high inductance, you will
only give up at the top 5% of the band. And if the Q at the top of the band is
only 20..."

The "top 5% of the band" is 1624-1710 kHz. I count 51 active stations in the U.S. and Canada from 1630-1700 kHz. I'm not willing to give them up. YMMV.

In a private email to me, Steve Ratzlaff reported these Q values for a 562-uH coil wound on the center of a 7.5" -61 rod with foam-core spacer:

1300 kHz, Q 400
900 kHz Q 442
700 kHz Q 460
530 kHz Q 449
530 kHz Q 442
400 kHz Q 420
300 kHz Q 361
200 kHz Q 303

Steve was unable to measure above 1300 kHz, but you can discern the trend. Q is starting to decline at 900 kHz. Q declined about 10-percent from 900-1300 kHz. Extending the curve, we might expect Q=357 at 1700 kHz. I think Steve may have measured the Q of a Tecsun antenna and posted the results to this forum, but I don't feel like looking for them. Even if the rewound Tecsun ferrite's Q is half of Amidon's, and even if the varactor is low Q (the similar, discrete MVAM108 varactor has a specified minimum Q of 150. Data sheet:, circuit Q probably exceeds 20 at 1710 kHz, especially when better-quality Litz wire replaces the original stuff.

Therefore, my statement,

"The problem with removing turns until you just see an increase at the top is two-fold. First, you may stop before you actually achieve resonance. It isn't either-or. The integrated circuit has lots of amplification, and you may see stronger signals, but they could be even stronger if you keep going!"

is valid. If you find it amusing, perhaps you don't understand resonant circuits and Q. That Rider booklet may help. Or dust off that ARRL Handbook I know you have on the shelf.

NB, though, I never once mentioned "554 uH" in this thread. Since you brought it up, however, let me quote now from an email from Roy Dyball, from three days ago (January 20): "I wound that 81 [-turn] coil on the 7.5" rod and it needed 18 turns off it* before the varactor came off the bottom." * Removing 18 turns works out to about 338 uH. In other words, the chip was unable to resonate the 81-turn coil at the high end of the MW BC band. Roy's isn't an empirical, subjective observation; he's seeing varactor (tuning) capacitance reported by the chip.

I, too, could not get an 81-turn coil on a 7.5-inch -61 rod to resonate. (Hint: There's too much stray capacitance (Scott Willingham calls it "parasitic capacitance," but we are referring to the same thing) in the radio, the wires going up to the rod, and in the coil winding.)

You have previously stated that you have no nearby X-band stations to use for references, so given your experimental methods the error is understandable. At the time you didn't understand how the chip tuned the antenna, either, though I tried more than once to explain it. You may not take me seriously, but I took you seriously enough to try to help you understand something. According to John Bryant, that's the great thing about this forum. Until somebody disagrees with a founder. That I disagree with your methods and results is no reflection on you as a person, though I'm afraid that's how you perceive it.

Last weekend you told me to repeat your experiments, and I did. Then I reported what I found. To ensure my observations were not skewed by fading signals I made a Faraday box, and used a stable signal source in lieu of off-air signals. Based on my results, some of which have been independently verified by Roy Dyball, I suggested people try removing more turns from the internal re-wound loopstick, and the consequences of boosting LW reception: a reduction in high-band sensitivity.  Where's the harm in that suggestion? (BTW, your observation of increased LW sensitivity provided a clue, which you overlooked. The chip cannot tune the same inductance from 1710 kHz far down into the LW band. If it's perking up on LW, it has to be sucking wind higher up. As I mentioned at the beginning of this thread, just because you're hearing stations doesn't mean the antenna is resonated.)

It is hard to discuss technical subjects without using formal language, lest we fall into the tar pits of ambiguity and imprecision. My regrets to those who find my writing too instructive or professorial. This isn't rocket science, but it is science, and I take it seriously. If you want to discuss radios, let's do it. If you want to attack me, let's take it off-line.


Jim Kearman, KR1S 

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