Hard-Wired FSL Antennas-- How Big is Too Big?


Gary DeBock
 

For those interested, the first signs of some limitation in the PL-380's Si4734 DSP chip's ability to handle powerful RF inputs from a hard-wired FSL have appeared.
 
In a step-by-step approach to test this out, the 3" Bar FSL composed of 8 Russian surplus 100mm x 20mm x 3mm ferrite bars (the construction article PL-380 version) was thoroughly proof-tested in this aspect, and had superb performance from 521-1700 kHz. A similar experiment with the same size of  3" FSL hard-wired into a CC Skywave had excellent performance on the frequencies up to about 1400 kHz, but showed erratic operation on the X-band frequencies (1600-1700 kHz). This erratic reception sounded similar to signal flutter, with the choppy-sounding signal dropping in and out several times a second. As such, I do not recommend any transplant of the 3" bar FSL into a CC Skywave model.
 
Tonight's experiment was to transplant a 3.25", 10-bar FSL into a PL-380. As with the CC Skywave, the low band performance was excellent, but the X-band frequencies had the same choppy-sounding reception as the CC Skywave did with the 3" FSL. Side-by-side comparisons of a 3" FSL PL-380 (the article version) and this new 3.25" FSL PL-380 on the X-band showed the former to be clearly superior, with no issue in the reception or nulling of any station up to 1700 kHz.
 
For experimenters with access to these Russian surplus 100mm ferrite bars, it is not recommended to hard-wire FSL's composed of more than 9 bars into a PL-380. I know that Steve R. has made a 9-bar FSL model, and he reported good reception across the band.
 
73 and Good DX,
Gary DeBock (in Puyallup, WA, USA)
 
  .


Rik
 

My understanding of RF theory is pretty limited. Do you think the 'fluttering' is from overloading the DSP chip? If so, would a variable resistor used to cut the signal when needed on some frequencies fix that, or am I misunderstanding what is happening?

Do either radios you worked with use the same  chip as the GP5/SSB or the Pl-360?

I think I read you to say the FSL antennas have about the same figure 8  pick up pattern as an air core loop, rather than a tighter pattern like some radios have. Is that correct? For example, my PL-600 seems very directional with a pick up pattern much tighter than 90 degrees. I looked inside my radios as much as I could, and saw no pattern to how the internal ferrite antennas looked to explain why some radios have a much narrower pick up pattern than others. For DXing it could be a huge advantage, but you could miss stations in the wide nulls for ordinary listening.

While certainly not compact, lightweight or inexpensive, my few experiments with Polydorof windings on a toroid stack show a sharp peak with much better S/N, but ordinary nulling. I have not attempted having a  FSL  built yet, and have not been following all the advances. It sounds like you are coming up with improved designs tailored  for DSP radios. FARMERIK.


Alex P
 

Sounds like AGC circuit 'pumping'.   Common when too much signal is presented to the circuit beyond what was designed for.   Could also be mixer or IF amp overload.

Alex


Michael <michael.setaazul@...>
 

This explanation was the one which first came to mind.
A lot of radios struggle with too much or too little signal :-)

Michael


Original Message ----- From: Alex winston376

Sounds like AGC circuit 'pumping'. Common when too much signal
is presented to the circuit beyond what was designed for.
Could also be mixer or IF amp overload.

Alex


Gary DeBock
 

Hello Farmerik (and Alex and Michael),
 
Thanks for your messages.
 
It's probably best to remember that the experimentation to "smoke test" the Si4734 DSP chip to see how much RF it can handle isn't exactly the mission that Silicon Labs intended for it :-) After the "article version" hard-wired FSL worked very well, of course it was irresistible to find out how much additional RF the component could handle. As it turns out, although the Si4734 chip can handle the "article version" FSL superbly, it can't handle much more RF power. 
 
<<<   My understanding of RF theory is pretty limited. Do you think the 'fluttering' is from overloading the DSP chip? If so, would a variable resistor used to cut the signal when needed on some frequencies fix that, or am I misunderstanding what is happening?   >>>
 
There's no doubt that the "fluttering" is a sign of DSP chip overload, Farmerik, but a variable resistor would be an impractical solution to the issue. A much better idea is to determine the maximum size of hard-wired FSL that the chip can handle without such overload, and then stay within the component's parameters. The primary attraction of the hard-wired FSL design is the chance to enjoy superb DXing performance without tuning a variable cap, or adjusting a variable pot.   

<<<  Do either radios you worked with use the same  chip as the GP5/SSB or the Pl-360?   >>>
 
The PL-360 uses the same Si4734 DSP chip as the PL-380, so if someone made up a plug-in design for a 3" FSL, the PL-360 would have the same enhanced DXing performance as the PL-380. The challenge isn't that tough; it would just require some mechanical modifications to the plug-in antenna frames that were designed for the PL-360 7.5" loopsticks in 2011.
 
<<<    I think I read you to say the FSL antennas have about the same figure 8  pick up pattern as an air core loop, rather than a tighter pattern like some radios have. Is that correct? For example, my PL-600 seems very directional with a pick up pattern much tighter than 90 degrees. I looked inside my radios as much as I could, and saw no pattern to how the internal ferrite antennas looked to explain why some radios have a much narrower pick up pattern than others. For DXing it could be a huge advantage, but you could miss stations in the wide nulls for ordinary listening.   >>>
 
FSL antennas, air core (box) loops and ferrite loopsticks all have figure-8 reception patterns, Farmerik, and none of these are particularly directional (except for nulling pest stations). There are some excellent cardioid-pattern broadband loops and beverage wire systems which can be made quite directional, and have been used very successfully by DXpeditioners on ocean beaches.
 
73, Gary DeBock (in Puyallup, WA, USA)
 
   
   
 
 

-----Original Message-----
From: farmerik@... [ultralightdx]
To: ultralightdx
Sent: Tue, Jan 19, 2016 7:20 am
Subject: [ultralightdx] Re: Hard-Wired FSL Antennas-- How Big is Too Big?

 
My understanding of RF theory is pretty limited. Do you think the 'fluttering' is from overloading the DSP chip? If so, would a variable resistor used to cut the signal when needed on some frequencies fix that, or am I misunderstanding what is happening?

Do either radios you worked with use the same  chip as the GP5/SSB or the Pl-360?

I think I read you to say the FSL antennas have about the same figure 8  pick up pattern as an air core loop, rather than a tighter pattern like some radios have. Is that correct? For example, my PL-600 seems very directional with a pick up pattern much tighter than 90 degrees. I looked inside my radios as much as I could, and saw no pattern to how the internal ferrite antennas looked to explain why some radios have a much narrower pick up pattern than others. For DXing it could be a huge advantage, but you could miss stations in the wide nulls for ordinary listening.

While certainly not compact, lightweight or inexpensive, my few experiments with Polydorof windings on a toroid stack show a sharp peak with much better S/N, but ordinary nulling. I have not attempted having a  FSL  built yet, and have not been following all the advances. It sounds like you are coming up with improved designs tailored  for DSP radios. FARMERIK.