Re: Building a FSL antenna


Gary DeBock
 

On Fri, Dec 11, 2020 at 01:23 AM, Jorge Garzón wrote:
Sorry, I wanted to say 5 inches FSL (not 3,5" as I did)
 
Hello Jorge,

Attached is a design photo of an effective 5 inch (12cm) diameter FSL antenna, which provides powerful gain in a compact size. One of these FSL models was recently sent to Paul Walker in Alaska, and he is tracking down lots of long range DX with it.

<<<   1. The core of the FSL, has to be foamed or can be just a non conductive cylinder-shaped object? What about PVC?   >>>

The core of the FSL needs to be a dielectric (insulating) material, and should also be soft enough to provide some protection for fragile ferrite rods or bars. My own FSL models use a PVC frame, with a PVC pipe through the center of the ferrite sleeve, through a rubber plumbing coupler packed with soft pipe insulation and a swimming floatation aid. This kind of FSL design is waterproof, tough and survivable in risky environments like ocean side cliffs. But if you just want to use your FSL in a safe indoor shack, you can use almost any kind of inner core, as long as it is a good insulator.

<<<   2. I am interested in NDB and MW band, so my interest would be from 150 to 1750 (more or less). Which kind of variable capacitor do I need? I know it depend on the diameter of the aerial. Maybe a 3" baby loop or even a 5" are suitable for me as a first project. My portables are DEGEN DE1103 and Tecsun PL-330.    >>>

If you want to keep your FSL antenna compact and relatively lightweight there are some ideal 384 pF variable caps, which provide vernier tuning so that it's easier to peak the antenna's gain boost. But a 384 pF variable cap will not cover the frequency range from 150 kHz to 1750 kHz unless you have separate, switched Longwave and Medium Wave coils. My advice would be to have separate Longwave and Medium Wave FSL antennas with 384 pF variable caps, since there are different types of ideal Litz wire for the two different bands. But there are some larger FSL designs with the LW-MW coverage you desire, although the variable caps will be much larger, and trickier to tune with such a wide frequency range.

<<<   
3. Is it possible avoid the variable capacitor adding a pre-amp to get a broadband FSL?   >>>

This type of design was tried back in 2013, but the results were poor. The FSL antenna's effectiveness is related to its High-Q tuned circuit, which optimizes one single frequency very sharply, providing excellent gain for its small size. When you take away this important advantage you are basically left with a mediocre replacement antenna that receives all frequencies poorly.

Gary DeBock (in Puyallup, WA, USA) 

    
       

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