Re: Resonance
Hi Roy,
Thanks for your comments, which are appreciated.
I think everybody is in agreement that the loopstick coil inductance providing maximum AM sensitivity in the PL-300WT/ G8 models is typically from 300 uh- 350 uh, and always within the 180- 450 limits. Steve Ratzlaff and I did many AM alignments on these models for various DXers, although our procedure was to loosen up the loopstick coil form and slide the coil while listening for an audio signal peak on a weak station, instead of using the RSSI readings to determine a peak. Trying to find a signal strength peak by using RSSI readings (and/or S/N readings) alone is frustrating, since as Scott has confirmed there is a delay in the display response to sudden increases (or decreases) in signal strength.
The audio signal peaking method used by Steve and me was successful in about 15 alignments (and is described at http://www.mediafire.com/?jyjg5hmkyqm ), since the coil position could be optimized for maximum AM sensitivity during actual weak signal audio reception. We always recorded the final coil inductances after the alignments, which usually were from mabout 300-350 uh (although there were a few oddballs outside of this range).
When Tecsun and Kchibo switched to loopstick coils without movable forms (as in the D96L, PL-310 and PL-380 models), it was no longer possible to use this sliding-coil method to find the optimal coil position providing maximum AM sensitivity. Rather than accept all these DSP models as already having the maximum level of AM sensitivity thoughtfully provided by the Chinese factories (a ludicrous thought, for someone who has performed as many AM alignments as me), I decided to set up an actual dynamic testing method that would perform the exact same function as the sliding-coil tests in the PL-300WT/ G8 models, allowing the use of a sliding a coil to peak a weak AM audio signal under actual reception conditions.
I turned to one of our ULR group innovations, the Slider coil, which provides a variable inductance by means of a special 40/44 Litz wire coil that can be easily shifted across a 7.5" Amidon ferrite bar. When transplanted into these newer DSP models in place of the stock loopstick, this Slider coil provides the exact same AM-sensitivity testing function as the stock sliding coils in the PL-300WT/ G8 models (that Steve and I aligned).
When this 81-turn Slider coil is shifted along the ferrite bar after being transplanted into a PL-310 or PL-380 model, it can always quickly determine the actual coil inductance providing the maximum AM sensitivity in the model, by showing a sharp audio peak whenever the coil is close to the optimal position during reception of a weak AM station. This diagnostic test immediately corrects for any circuitry changes (from previous models) in the Si4734 radio, or in the DSP chip itself, since it is being performed under actual DXing conditions, with the best weak-signal AM reception being the bottom line. As such, it essentially creates a test where the radio tells you exactly which loopstick coil inductance it wants, to provide the best possible AM weak-signal reception. The sharp audio peak never lies, and is not confusing, as are the delayed RSSI and S/N readings. This was the diagnostic method by which the 554 uh optimal inductance was found for best AM sensitivity, in the PL-380 7.5" external loopstick project.
The Slider coils can be somewhat intimidating to construct without experience (the process is described at http://www.mediafire.com/?nqggfm2jymc ), but they are routinely made here, and I would be happy to send one to you (complete with an Amidon type 61 ferrite bar) free of charge, if you wish to try out this method in your PL-380.
Thanks for listening, and of course I respect any differences of opinion. Thanks also for your detailed descriptions of the Si4734 chip functions, which are greatly appreciated.
73 and Good DX,
Gary DeBock (in Puyallup, WA USA)
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.
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@yahoogroups. com, "jim_kr1s" wrote:
>
> 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:
> http://djvu.org/resources/
>
> The file is here: http://www.tuberadio.it/download/ rrider.djvu
>
> 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:
> http://www.bentongue.com/xtalset/ 29MxQFL/29MxQFL. html 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 http://w7zoi.net/coilq. pdf -- 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:
> http://en.wikipedia.org/wiki/ Varactor
>
> 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
> http://qrp.kearman.com/
>