Re: Guy Atkin's Independent Testing of PL-380 7.5" Loopstick


sdwillingham
 

Gary,

Your results are interesting, but I think it would be good to
take the experiment to the next level. A problem with your
slider method is that it is well-known that loopstick Q is
degraded when the coil is not centered. At the extreme ends,
the effect is severe. I am afraid that your 81-turn coil
may be handicapping the low-inductance values too much.

A good experiment would be to slide your 81-turn coil to the
middle of the ferrite and measure its inductance. Then compute
two winding values that would produce coils with values of
say 330 uH and 550 uH, _when_centered_. Test each centered
coil over a range of stations over the band, strong and weak,
carefully comparing RSSI and SNR. It is also useful to spot-
check the general noise-floor at several points "between"
channels. It is probably necessary to build the coils with
connectors that are easy to swap in order to compare results
without too much error due to time variation of propagation.
(Be sure to retune the radio's frequency after each swap!)

I think a comparison of centered coils with no difference
besides number of turns will be educational and eliminate
possible confounding factors.

-Scott-

--- In ultralightdx@..., D1028Gary@... wrote:

Hi Scott,

Thanks for the clarification. I think if everyone had been clear on this
point originally, it might have saved a few "potshots" directed my way :-)

For those interested, Guy Atkins' modified PL-380 model is now the fifth
such unit to have received a 7.5" Amidon type 61 bar loopstick transplant,
and all of the models have been found (using an 81-turn 40/44 Litz wire
Slider coil) to have maximum AM sensitivity provided when the coil inductance is
somewhere within the 530- 560 uh range.

73, Gary DeBock
(in Puyallup, WA)

In a message dated 2/1/2010 11:23:22 P.M. Pacific Standard Time,
sdwillingham@... writes:






I think there is a bit of a misunderstanding here. The chip's datasheet
indicates a range of inductance over which the chip's built-in varactor
can successfully bring to resonance. This is neccessary, but not
sufficient for calling the antenna "optimal". The choice of which
inductance works "best" overall probably depends on several physical
factors and usage patterns. But once chosen, the chip will
automatically resonate the inductor, which eliminates the factory
alignment procedure which has historically been needed. I don't think
the inductance spec should be read as indicating that all inductances
will perform equivalently; only that all within range can be resonated.

As for the chip tuning inductors outside the specified range, please
recognize that a datasheet is a guarantee of performance. In the case
of integrated circuits, it is critical that a vanishingly small number
of them fall outside the datasheet parameters when deployed in the
field. No manufacturer can afford to scrap complex devices when one of
many chips inside fail to perform as specified. So the numbers in a
datasheet are all conservative, some are very much so.

-Scott-

---------- Forwarded message ----------
From: Guy Atkins <_dx@_ (mailto:dx@_dx@>
Date: Mon, Feb 1, 2010 at 4:56 PM
Subject: Modding the Tecsun PL-380 with an External Ferrite Antenna - My
Experience
To: __ultralightdx@_ultralightd_ul_
(mailto:_ultralightdx@...) _

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.

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