Re: Recent FSL Antenna Experimentation

kevin asato

[there are embedded links for illustration purposes rather than having everyone suffer with my artwork]

Schematically, the FSL and Terk resemble a tuned LC circuit.; the difference is that the Terk would be an air core inductor and the FSL would be ferrite. It is the tuned circuit which resonates to a particular frequency at the expense of "hearing" other frequencies. This is what makes it not feasible for use with an SDR and panoramic display which is a wideband function but excellent for DX work.

A more generalized loop that would work better in a wide band scenario would look like a coil like this and is normally the throwaway antenna supplied with stereo receivers. i would attach one conductor to the wire antenna and the other to ground/earth. Some alternate arrangement will have to be made to support the antenna connection to an SDR dongle. The coil in this usage is to couple the signal from the antenna wire onto the radio tuned front end/loopstick antenna. The particular resonance of the loop is of no real concern here although one can play with the number of turns or diameter of the loop for best coupling to the radio. 

No matter which approach you take, have fun and play with it to see what works for your situation. 


On Mon, May 31, 2021 at 6:35 PM kevin asato via <> wrote:
Well. yes. A FSL is supposed to increase selectivity over a very small frequency range to aid in finding that one station buried adjacent in frequency of a higher powered station. Not just the FSL but also air core loop antennas as Terk,Tecsun, and homebrew implementations as well. A wideband implementation defeats the purpose of enhancing selectivity. For SDR in which you are looking at a panorama display, you probably just want to couple to a basic wire antenna which is broadbanded and can be readily coupled (multi-turn wrapped?) to a radio receiver. After that you can couple to a loop antenna to peak the frequency of interest.

On Mon, May 31, 2021 at 2:30 PM <robconboy@...> wrote:


I have determined that broadband FSL’s don’t work very well for broadband recording with an SDR to sift thru later, unless wound on a large amount of ferrite. A broadband winding on a small FSL might be useful for live dx using an SDR, to help identify a frequency to tune to. Furthermore, I have not yet explored broadband enhancement for a inductively coupled small portable, relying exclusively on the the portable’s selectivity. It would entail using a bias tee, power supply or battery, and an inductive coupler probe.


My testing used a Wellbrook ALA1530LN as a reference antenna and a Perseus receiver. Perseus has a true 50 Ohm resistive input.

The FSL broadband amplifier is a Wellbrook FLX1530LN which I think is the same amplifier as the reference but in a different package.


I used copper tape to prototype, using various widths, and tapped the amplifier in at various points to determine the best broadband windings.


4 turns of 1-1/2 inch wide flashing wound on a 49 rod 200 mm long FSL yielded signal amplitudes 10 dB below the reference antenna, while the QRN daytime noise floor was about 11 dB below the reference.


2 turns of 2-1/2 inch wide flashing wound on a 204 rod 600 mm long FSL (68 rods x3) yielded amplitudes 3 dB below reference, and the daytime noise floor was about 6 dB below reference, (while still above the radio’s own noise floor). I don’t know why the large FSL consistently is yielding better performance than the reference antenna. My guess is that directional nulls of the FSL are broader and deeper than the Wellbrook loop’s and therefore the FSL is intercepting less QRN from some directions.


This antenna has broadband and tuned outputs. The tuned portion has a variable selectivity control. 


It’s grossly impractical, heavy and the cost of materials was more expensive than I will admit, even here, but its performance is nothing short of amazing.  First off, the field around this antenna is immense. It begins enhancing reception of a portable within about 6 feet.    Selectivity is adjustable. The narrower the selectivity, the greater the sensitivity. Tuning for dx is a balance between bandwidth, sensitivity and offset. Those of us with experience dx’ing with an old Hammarlund that has a regenerative selectivity control should remember what this is like.


The ferrite sleeve is 24 inches long and 9 inches diameter and made from 204 eight inch ferrite rods mounted on the outside of a 8-inch i.d. PVC pipe.  There is a 2-rod-width gap along the length of the sleeve that serves a dual purpose. It’s where the coil connections are made, as the electronics are inside the pipe. It also creates a gap to cut down magnetic coupling between conductors that run along the length of the sleeve. Without the gap, the sleeve becomes a toroidal core. The gap runs along the bottom of the sleeve for symmetry.


BTW, the 200mm by 10mm ferrite rods were still available on eBay last I looked, from a supplier in China. The specs aren’t given which is probably why they they haven’t been discovered by the dx community (as far as I know), but they appear to be mix 61. 


I bought 5, to check their suitability. After some testing and measurement, I was able to determine their permeability is above 100. So I bought 20 more, then 50, then 200.  Mix 61 is commonly used for AM loopstick antennas.


Tuned coil: 12 turns 1100/48 Litz, 230 uH, Centered on ferrite sleeve.


Capacitor:  20-400 pF, semi- logarithmic taper, 3-turn vernier shaft. This is mounted in the center of the ferrite sleeve (inside it) at the same depth as the coil.  This location was chosen after experimentation. I noticed that a small portable (Tecsun PL-330) placed inside the ferrite sleeve (before winding any coils) had greatly attenuated reception at some spots inside the pipe. So I wound a sniffer probe on a rod and connected it to my Perseus. There is a deep broadband null along the center axis that is greatest halfway in. I chose this spot for the tuning capacitor. The shaft is connected to an acrylic extension shaft with an insulated coupler. The coil/cap combination tunes from 525 to about 1850 kHz. Details on mounting come later.


The tuning knob is a dual concentric 1:1 and 40:1 vernier from Ukraine I bought on eBay. When combined with the vernier drive on the capacitor, it takes 120 turns of the inner knob to fully open/close the capacitor. The outer knob takes 3 turns. This makes pinpoint tuning at narrow bandwidths easy. Unfortunately there is some slop in this vernier when reversing rotation. It takes about 1/10 rotation before the rotor plates move. It’s inconsequential with such an extreme vernier ratio, but with a 1:1 shaft capacitor (40:1 vernier instead of 120:1) the slop was intolerable.


The pickup coil has 7 uH, is two widely spaced turns of 1100/48 Litz. The pickup coil feeds a step down transformer. This pickup coil is located 4 inches from one end of the ferrite sleeve. 


Step Down transformer: BN-73-202 core, 

Primary: 15 turns 64/46 Litz

Secondary: 3 turns 64/46 Litz.


The purpose of this transformer ratio is so that the pickup coil places a very light load on the tuned circuit. It also serves as a balun between the pickup and the amplifier. This is critical.


From the transformer it goes to a Mini-Circuits low noise amplifier model ZFL-500LN with a 50 Ohm input. This amplifier, due to the mismatch introduced by the transformer, places a 1250 Ohm load on the pickup coil and the amplifier’s effective gain is reduced to about 16 dB from 30 dB


Amplifier output goes to a Mini-Circuits signal splitter which reduces the gain by another 3 dB while isolating the feedback circuit from any connected radios.


Port one of the splitter goes through a 6 dB attenuator to a Mini-Circuits bias tee for powering the amplifier. This attenuator reduces the possibility of overloading the front end of a radio.


The RF+DC port of the bias tee goes to a BNC jack to connect a radio (via an external bias tee). The amplifier gain here is now about 6 dB.  


Port 2 of the signal splitter goes to a 10 dB attenuator.  This makes about 3 dB of gain available for feedback.


The output of the attenuator goes to a Bourns 10-turn 500 Ohm potentiometer. 


The output from the potentiometer goes through a switch to the feedback coil.


Between the switch and the feedback coil is a common mode choke balun: 5 bifilar turns around a small mix 75 toroid


In series with the feedback coil is 100 Ohms in parallel with 1330 pF. This makes the feedback slightly favor higher frequencies. This high pass filter makes potentiometer feedback adjustment less touchy when changing frequencies.


Feedback coil: 1-turn of 1100/48 Litz wire located 4 inches from the end of the ferrite sleeve opposite the pickup coil. Choosing Litz wire here might be complementary to the high pass filter but may yield no advantage over common wire..


Broadband  pickup coil:  Two turns of 2-1/2 inch wide copper flashing. They are wound on opposite sides of the tuning coil. 


This broadband coil goes through a DPST switch to a Wellbrook FLX1530LN low impedance loop antenna amplifier. The switch is necessary or else the broadband loop behaves like a partially shorted turn when using the the tuned portion of the antenna.


The output of the Wellbrook amplifier goes to a BNC jack.


The entire assembly is housed in a Rubbermaid trashcan liner.

I used 12 inch and 8 inch diameter wooden discs to center the antenna in the liner at the control panel end. The 12 inch disc fits into the liner, the 8 inch disc fits into the pipe, and the discs are glued together. The electronics are mounted on the outside of a 4 inch diameter, 11 inch tall plastic jar. The jar is mounted in the 8 inch wooden disc. The jar’s threads are lined with teflon tape. The 8 inch disc is cut with a 4 inch hole saw. The jar is mounted in the 4 inch hole using paste epoxy. This jar extends halfway down the center of the pipe. The teflon tape makes it so the jar can be unscrewed. The capacitor is mounted on the outside bottom of the jar, level with the tuned coil. Its shaft runs trough the inside of the jar to the vernier knob outside of the assembly. 


I built up the opposite end of the antenna with 2 inch wide strips of neoprene rubber to center it into the liner. 


Three modes of operation: 


Active tuned antenna with variable selectivity control, powered through a bias tee such as a Wellbrook antenna interface. Tune-able range 525 kHz to 1.85 MHz.  Measured narrowest possible (stable) -3 dB bandwidth @ 1750 kHz is about 80 Hz, making maximum possible Q about 22000 before oscillation.  In use, Q is dialed lower than this because a radio is not a subwoofer. 


The other mode is a Broadband antenna with it’s own separate coil and Wellbrook loop amplifier. This is usable up into the shortwave bands but performance gradually begins to degrade above about 2 MHz.


Passive mode, amplifiers unpowered. A tuned FSL that is proximity coupled to any nearby radio that has an internal loop antenna. 



















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