Re: Guy Atkin's Independent Testing of PL-380 7.5" Loopstick
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Looking at the video of the coil being moved indicates that the all that is happening is that you are moving the coil away from a peak that has already been established from a previous tuning operation that has set the varactors capacitance to tune the loopstick at the black mark for the coils inductance at that position on the rod. It appears to me that the coil was originally tuned at the black mark then moved off peak before the video started.
You cannot expect the Si4734 to track while moving the coil because it does not know the coil has moved and keeps the capacitance unchanged, but you have changed the inductance by moving the coil and of course the signal will rises and fall when you move the loopstick.
Had the radio been retuned each time the coil was moved the Si4734 would have returned its varactor capacitance for the new inductance brought about by the movement and really the signal would not have changed by much more than about 3dbu across the rod.
The only time the Si4734 changes its capacitance to match the loopsticks inductance is during a tuning operation and this was not done in the video. The capacitance does not change automatically when you slide the coil. The whole idea of the Si4734 is it is self aligning. And that is why you must step off and step back on frequency every time you move the loopstick.
It would be very interesting to see the video redone showing the method described above because every time the coil was moved and the frequency retuned the audio and the signal levels (within about 3dbu) would have come back up.
We must understand how this chip works and use this chip as it is designed and not copy old methods from completely different technology without applying them to this new technology correctly.
I think there has been a misunderstanding as to the correct operation of how the Si4734 works and I now that criticism can be taken the wrong way especially when someone has put a lot of work into their results and presented them sincerely.
I really believe that the video showing a peak signal on the rod is incorrectly presented and had the radio been retuned between movements it would have only shown how cleaver the Si4734 really is at realigning itself and not a signal dropping off from an established peak.
--- In ultralightdx@..., D1028Gary@... wrote:
> Hello All,
> This is forwarded from Guy Atkins, who apparently had a problem getting
> this message posted to the Ultralightdx Yahoo site:
> ---------- Forwarded message ----------
> From: Guy Atkins <_dx@..._ (mailto:dx@...) >
> Date: Mon, Feb 1, 2010 at 4:56 PM
> Subject: Modding the Tecsun PL-380 with an External Ferrite Antenna - My
> To: _ultralightdx@... (mailto:ultralightdx@...)
> I've followed some of the recent discussion about modifying the PL-380
> with the Amidon type 61, 7.5" ferrite rod that is commonly used with the
> hotrodded Eton E100 receivers. Having modified Kchibo D96Ls and Eton E100s with
> this useful Amidon item, I decided that it was time to try my hand at the
> Tecsun PL-380.
> I wasn't sure what to expect regarding the optimum coil inductance and
> position of the coil on the ferrite, since there's been a good amount of
> debate on this topic. Gary DeBock claims an inductance around 554 uH is the best
> for all MW frequencies, and that there's a "sweet spot" on the rod where
> an 81 turn coil of 40/44 Litz wire provides this inductance, giving the
> strongest signals across the band with this fixed position coil.
> The SiLab Si4734 chip's datasheet indicates an acceptable 180-450 uH
> inductance range for the antenna, and the chip tunes a varactor capacitor for
> best performance on each MW frequency (resonating the LC circuit)... no
> manual adjustments at the factory are needed. For the ULR experimenter this
> means there should be NO "best" position (sweet spot) of a coil on the external
> Amidon ferrite rod, as long as the inductance is within 180-450 uH.
> I dove into the disassembly of my PL-380, very curious as to what I would
> find gives the best signal gain with an Amidon rod modified receiver. Maybe
> there's be no difference 180 to 450 uH, just as SiLabs' data implies.
> I removed the small, flat ferrite antenna from the PL-380 and found it
> measured 239 uH, clearly within the specified range.
> An 81 turn Litz wire coil (the recommended 40/44 style) with Amidon 7.5"
> type 61 rod was tried next, mounted externally with the "piece of a
> carpenters level" trick that Gary pioneered as a simple frame to support a slider
> coil antenna.
> It was quickly evident that Gary is correct--there *is* a "best"
> inductance for the PL-380, and it is far outside the range given in the SiLabs
> chip's documentation. I listened to low, mid, and high-band MW stations for an
> audible peak and kept a close watch on the RSSI signal numbers on the
> radio's display while sliding the coil left and right (and taking average
> readings over a minute or so). With the coil positioned at the ends, signals were
> barely audible, but rose quickly to maximum levels at a location about
> 1-3/4 inches from the end of the rod to the first edge of the coil. Beyond that
> distance, the RSSI values started dropping (but not as quickly as they
> rose within the 1-3/4 inches from the end of the rod.) No matter what MW
> frequency I tuned, there was one small place on the Amidon rod where signals
> were strongest.
> I taped the coil in this "best" position, disconnected the antenna leads
> from the receiver, and measured the resulting value. It was 533 uH! This
> number is very close to Gary's claimed 554 uH optimum, and obviously beyond
> the 450 uH upper limit stated by SiLabs for an AM broadcast antenna. With the
> coil moved toward the end of the Amidon rod, I measured around 300 uH (all
> but the strongest signals were inaudible with 300 uH inductance on the
> Amidon rod).
> See below for a short video (1 Mb WMV file) I made of the peaking of my
> local TIS station on 530 kHz using this PL-380 setup. It's a bit blurry, but
> the RSSI and SNR numbers can be seen changing. I placed a triangle of black
> tape on the support frame pointing to the "optimum" position of the left
> edge of the coil to help you observe and hear the signal changes left &
> right of the best spot. It is this position marked with the black arrow that
> results in a 533 uH inductance on my setup.
> Here are some photos and captions of different steps of this PL-380
> project: _http://tinyurl.com/ycajpex_ (http://tinyurl.com/ycajpex) Click on the
> thumbnail images and use the PicasaWeb magnifying glass tool for up-close
> This modified PL-380 is as sensitive as any slider coil E100 hotrod ULR
> radio I've built and used. I look forward to snagging some TP DX with this
> little wonder! The multiple DSP bandwidths are as useful as those on the
> Kchibo D96L, but the minimal soft mute is an excellent benefit of the PL-380. I
> also appreciate the easy frequency entry and the overall build quality of
> the Tecsun receiver.
> So what do we conclude about the "correct" inductance for the PL-380
> antenna? First of all, Gary is correct--an inductance of approximately 530-550
> uH when used with the 7.5", 61 type ferrite material from Amidon and 40/44
> Litz wire clearly provides the most sensitivity. Why this is well out of the
> SiLab-recommended range of inductance, I have no idea. Maybe it involves
> the physical size and ferrite mix of the rod, along with the Litz coil
> parameters, although this contradicts what others have said about the Si4734
> chip automatically matching any 180-450 uH coil, period.
> Whatever yet-to-be-discovered reason for the modded PL-380's behavior, I'm
> happy that fellow DXer and Puyallup, WA resident Gary DeBock has shared
> his discoveries freely with the rest of us. I'm sure he knew he'd get some
> "heat" for his unorthodox claim about modding the PL-380. I commend him for
> not keeping quiet, and for taking a risk to publicize his findings.
> As a reminder, Gary's article that details this modification to the PL-380
> is found here:
> I'm sure others will continue to debate theory vs. hands-on results with
> the PL-380 receiver, but if you'll excuse me I have some DX to snag with
> this little hotrod from Tecsun...
> Guy Atkins
> Puyallup, WA
> _http://fivebelow.squarespace.com_ (http://fivebelow.squarespace.com/)