Re: Early results of 7.5" Amidion rod

Gary DeBock

Hi Roy,
Thanks for the excellent report of your coil winding results with the 7.5" Amidon rod.
Concerning the modification of the Tecsun PL-300WT/ Grundig G8 models, Steve Ratzlaff and I did multiple AM "alignments" of the radios by loosening the stock loopstick coils, and shifting them slightly, while listening for increasing or decreasing AM signal levels. Most of the PL-300WT/ G8 models had some AM signal improvement by this procedure, which we used to optimize reception for various DXers.
After aligning the coils for maximum AM sensitivity, we would always check the final inductance of the coils after shifting them. In the case of the PL-300WT/ G8 models, this final coil inductance was always within the 180- 450 uh specification called for in the Si473x spec sheet. Once the optimum coil inductance was found (most of them were around 300 uh, or fairly close to the stock coil position), the same coil position provided maximum AM sensitivity throughout the AM spectrum. Steve and I did about 15 of these alignments, for ourselves and others. In this regard, the optimum loopstick coil inductances observed in the PL-300WT/ G8 models never deviated from the 180- 450 uh specification.
Of course, there were multiple requests for 7.5" Amidon loopstick-modified PL-300WT/ G8 models at the time, and I made several of these up for myself, and others. The procedure was to first "align" the stock coil to determine the optimum inductance value for maximum AM sensitivity (as described above), then to remove the stock loopstick from the radio, replacing it with an Amidon 7.5" type 61 ferrite bar wound with a fixed 40/44 Litz wire coil of the same optimal inductance value. The procedure always provided a maximum level of AM sensitivity from 530- 1700 kHz, with no Si4734 varactor issues on the high band. About five of these 7.5" loopstick PL-300WT/ G8 models were constructed here (two going to the Massachusetts DXers Mark Connelly and Chris Black), and Steve also made several of them-- all of which worked very well in DXing. The issue was that because the DSP selectivity was fixed at the 3 kHz setting in these models, these 7.5" loopstick PL-300WT/ G8 models never were competitive with the CFJ455K5 filter-modified E100 7.5" Slider models in transoceanic DXing. The fanatical ULR hot-rodders (like me) then decided to hold off on more modifications, until we could receive some new Si4734 radios offering the desired 1 kHz DSP selectivity.
I hope this information will be useful.
73 and Good DX,
Gary DeBock

In a message dated 1/13/2010 4:50:53 P.M. Pacific Standard Time, roy.dyball@... writes:

Hi All

The first coil was wound on the 7.5 inch rod with 165/46 spaced using 0.7mm solder as the spacer which is about the same size as the Litz, wound over 110mm centred on the rod. The capacitance reported by the G8 showed it had bottomed out at 1710Khz. I removed 2mm from each end and the capacitance showed 18 Pf at 1710Khz (7pf is the lowest). This resulted in aprox 15dbu increase on the G8 when compared to a stock PL-310 set a 3Khz bandwidth, holding this to the end of the band.

I was able to hear a station in another state on 1323Mhz (Cruise AM in Adelaide) last night that I have tried on and off and have never been able to receive at a distance of over 1600 KM (great circle) while listening with the radio on my bed side table.

I then decided to emulate Gary's 81 turn 40/44 coil starting at 43mm. The G8 reported it was bottoming out of capacitance at 1540Khz. I ran my standard tests and found this configuration was performing the same as the 165/46 coil at the bottom of the band but was dropping off by around 1200Khz (2-3 dbu).

I had to unwind 18 turns to get the G8 to report a value of 12pf at 1710Khz. This only dropped the very end of the (low) band by 2 dbu but had started to pick up by 1200Khz. I won't know  how well it performs until tonight when the high end of the band opens.  For now it is performing aprox 13dbu better than the stock PL-310.

Gary has reported that his coil shows a better improvement when fitted to the 380 than the 310. Probably this coil would perform better in the PL-380 than the G8. I will try that later. I am also going to rewind the first coil but use 0.5mm spacing.



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