Re: Resonance

Roy <roy.dyball@...>

Hi Jim

Thank you for the links to the Rider article, I have just had a skim through but can see it is well written and I will enjoy digesting it.

I first learnt about resonance 45 years ago during a 6 year traineeship in electronic telecommunications which I started straight out of school. Resonance fascinated me as much then as it does today (funny how the formulas haven't changed).

I have experienced very similar results with loopstick coils with my Tecsun radios as you have noted. Yesterday I wound a large inductance sliding coil to check the upper limit of the loopstick coil in the G8 radio by using the Si4734 AM_TUNE_STATUS function to return the value of varactor capacitance being used to tune the loopstick coil at different inductances.

The lowest value of capacitance the Si4734 can tune to (just like turning a mechanical variable capacitor to one end) is when the varactor is set to (decimal) 1 this represents a value of 0.095Pf, however SiLabs says we must add 7Pf to this value giving us 7.095Pf.

The upper limit of the varactor in decimal is 6143 and that equals 590.5Pf.

The chip in my G8 reported it was at 7.095Pf (rock bottom) with a value of 430µH adjusted on the loopstick. At around 350µH the varactor was reading around 12Pf which is where I found my best results.

When I first started experimenting with the original G8 loopstick moths ago I found it did not make much difference where I put the stock coil on the rod because as soon as I retuned the station the signal strength readings came back to about where they were before I moved it. I could not just simply slide the coil and look for a peak. Not then knowing how this little radio worked it made me want to find out what was happening. After obtaining a programming manual and seeing the range of commands available I was hooked and then spent hundreds of hours leaning how to talk to the Si4734 chip by building a USB interface and writing software to use it.

Now that we are all in love with our PL-380 PL-310 I recommend you put your old G8 to good use and convert it to a USB interface. You can do this in an afternoon just by removing three SMD resistors and attaching three wires plus earth. The USB interface PCB already assembled and only cost a little over twenty dollars. When it is finished you can use it as a tool for winding loopsticks and see for yourself what the radio is telling you (no guessing). The G8 380 310 have similar front ends in that the loopstick goes straight to SI4734 AM input via a coupling capacitor. The results you get on your G8 will be valid for the 310 and the 380. The project is on this site and it is free

Chreers Roy.
--- In ultralightdx@..., "jim_kr1s" >
> Roy Dyball's notes on winding an antenna coil mentioned the need to
> re-tune the radio after each change in the number of turns on the
> antenna. As Roy mentioned, confirmed by my experiments, the Tecsuns (I
> have a G8 and a PL-380) don't like inductances higher than about 350 uH.
> What's that all about?
> The antenna forms half of a tuned circuit, the other half being on the
> integrated circuit. Is resonance important? You betcha! It's been shown
> that inductances greater than 350 uH give improved signal strengths at
> frequencies below the broadcast band. The integrated circuit has limited
> tuning range, so what you gain at the bottom you necessarily give up at
> the top. 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!
> The other problem comes when you don't have local stations at the high
> end of the band, or they are far enough away to vary in signal strength.
> That's why I built a shielded enclosure, into which I can inject signals
> of repeatable strength. Starting at about 500 uH and removing turns, I
> do see X-band signal strengths starting to improve at around 430 uH, but
> they improve further at lower inductances. Signal strengths at LW are
> reduced with lower inductance, so you have to decide what's more
> important. Tecsun made their PL-380 coils about 330 uH, which does a
> good job from 530-1710 kHz. Again: You can't have good LW and good
> high-band MW performance with one coil.
> Looking around for a lay-person's explanation of resonance and its
> effects, I found an old Rider publication from the early days of radio.
> It's in DJU format; you can get the free reader here:
> The file is here:
> Like Roy, I'm not seeing evidence that using a spacer under the coil
> improves reception; test-bench measurements by Ben Tongue, a well-known
> engineer (co-founder of Blonder-Tongue) and crystal-set experimenter
> make me think it's not worth doing. See Table 2 here:
> His best results
> across the band were with 1/16-inch polyethylene sheet, but not greatly
> different than with no spacer. Steve Ratzlaff, who has an expensive
> Hewlett Packard "Q" meter has also found no advantage to using a spacer.
> My own Q-measurement setup (from -- Wes
> Hayward, W7ZOI was co-author of "Solid State Design" and "Experimental
> Methods in RF Design," and author of "Introduction to Radio Frequency
> Design," as well as many articles on related subjects) is not as
> accurate as Steve's meter, but nothing I've seen contradicts Tongue's
> and Steve's results. If you can get some 1/16-inch polyethylene, it will
> smooth out the Q (see the Rider book) across the band, and perk it up as
> much as 10-percent at the high end.
> What is critical is using an inductance the integrated circuit can
> resonate at the frequencies of interest. The tuning varactor has a
> design minimum capacitance, and there is stray capacitance in the
> circuit between the board and antenna (the connection between the rf
> board and the display board is a separate circuit). If the inductance is
> too large the varactor will be adjusted to its minimum capacitance, like
> a mechanical variable capacitor turned fully clockwise. You might still
> hear signals, just as you can still hear signals when a tunable loop
> antenna is mistuned. I can still hear Radio Enciclopedia on 530 kHz when
> my Terk is tuned to 1600 kHz, but the ladies are much stronger when the
> Terk is tuned to 530 kHz. Here's an article about varactors:
> While I cite others' work in this post, I speak only for myself. My
> results, which I feel are corroborated by others, are at variance with
> some other results reported here. You are free to make up your own mind.
> It is suggested that, if you have enough Litz wire and time, it might be
> worth experimenting with the coil size on your antenna modifications.
> When you can remove one or two turns without seeing any difference in
> high-band signal strength, you've hit the mark. This is hard to do
> accurately with off-air signals. Anyone who has a stable, accurate
> signal generator with an output meter, a big cardboard box, and some
> aluminum screen material can duplicate my tests. If you'd like to
> discuss the tests, email me at jkearman at att dot net. Thanks to Roy,
> Steve and other correspondents for sharing data!
> 73,
> Jim, KR1S

Join to automatically receive all group messages.