Re: Can an FSL Antenna get Interference from a Strong Local Station?


Thanks for the detailed replies Gary and Michael. 
I think that Gary's 3/4 inch PVC Supporting Legs are a great idea for supporting an FSL. Gary when you have time, do you know if I use one of those 10 inch diameter 4 inch wide solid Styrofoam  Cake Forms (for the Ferrite Bars & Litz wire)  that Steve Ratzlaff is using, and put the PVC Pipe right in the middle, would I need to somehow insulate or line the middle of the Styrofoam Form before inserting the PVC pipe?  Its a bit difficult  to tell on your detailed 64 Russian Rods picture, what the middle section of your PVC Pipe is resting on.

From: "D1028Gary@..."
To: ultralightdx@...
Sent: Monday, August 29, 2011 2:26:37 PM
Subject: Re: [ultralightdx] Can an FSL Antenna get Interference from a Strong Local Station?

Hello Jerry and Michael,

Thanks for your questions regarding the FSL antenna's risk of
overloading a receiver, and its nulling ability.

After the construction of 7 of these models for both Medium Wave and
Longwave frequencies, it's very clear to me that because of the
extremely sharp tuning system a well-designed FSL antenna is very
unlikely to overload a receiver. A DXer using the Tecsun PL-380 model
quickly learns that stations on frequencies 2 or 3 kHz away from the
FSL's optimized (tuned in) frequency are reduced greatly in strength,
and stations farther away than that are almost never audible. This has
provided a lot of help to me during a couple of summer DXpeditions,
when I was trying to chase some weak South Pacific stations with
domestic pests only 2 or 3 kHz away. In fact, the FSL tuning system is
so sharp that unless you are exactly on the same frequency (within
about 500 Hz) as the DX station, the FSL will not provide its maximum
gain. This is a much different situation than with the large air core
tuned passive loops, where you can get a fairly nice gain boost on the
DX station even if your variable capacitor is tuned 2 or 3 kHz away
from the DX station frequency. The FSL antenna does require more
precise tuning, but the razor-sharp rejection of unwanted frequencies
is more than worth it.

As for nulling capability, as long as the collection of ferrite rods is
in an accurate cylindrical shape and the the tuning capacitor is
located as close as possible to the loop itself, the FSL should provide
nulling capability that will outperform any other tuned, high-gain
antenna (with the possible exception of the single-loopstick Quantum
Loops made by Gerry Thomas). It's helpful to think of an FSL as a large
collection of individual loopsticks, all working together to combine
their gain (on only one tuned frequency), and also all working together
to null out stations that are off the ends of the ferrite rods. As long
as the cylindrical shape of the ferrite sleeve is accurate and the
ferrite rods all point in exactly the same direction, both of these
capabilities can be optimized. The Longwave FSL's here can completely
null out any pest beacon on their own frequencies, and the MW FSL's
here can null the local pests down to about 2 kHz of spectrum on their
own frequencies. This nulling performance is almost identical to that
of a single, razor-sharp loopstick in the Ultralight models like the
SRF-59 or SRF-T615, but once again, it's very critical that care is
taken to ensure that all the ferrite rods are arranged in an accurate
cylindrical shape, and that the tuning capacitor is as close as
possible to the loop itself.

I'm aware that there is intense interest in the FSL antenna among
various DXers in the Ultralightdx group who would like to try one out,
but who feel perplexed about how to get started. With all the intrigue
about high component prices, Russian Roulette ferrite orders and
different physical designs, I know it's tough to get started. Several
models of the "affordable" 7" MW FSL design (shown at ) are being built here for
local DXers to test out, in the hope that this $150 FSL antenna design
will provide a reasonable starting point for other DXers to jump into
the fun. It has already outperformed a 4' sided tuned air core loop
here (the August 2010 South Pacific DXpedition antenna, no less), so it
seems like a nice basic design to get started.

73 and Good DX,
Gary DeBock (in Puyallup, WA, USA)

-----Original Message-----
From: Michael <michael.setaazul@...>
To: ultralightdx <ultralightdx@...>
Sent: Mon, Aug 29, 2011 12:06 am
Subject: Re: [ultralightdx] Can an FSL Antenna get Interference from a
Strong Local Station?

----- Original Message -----

From: jerry_popiel
Subject: Can an FSL Antenna get Interference from a Strong Local
. . .
My question is does anyone know that if I build a new FSL, will I be
subjected to
this same Interference from my local 50,000 watt station?
Any comments would be very much appreciated.


Jerry, my guess is that a FSL would be subject to your 50kW neighbour.
However, if well-built, rotating it to null the local would help -
unless a
wanted station was in line with the unwanted one. Gary and others who
built FSLs will be able to respond with more authority and their

I would be intruigued to know whether a secondary ferrite or FSL in
to the main one could be tuned and positioned to usefully "absorb" an
intruding signal.
Relative positioning would be critical - if it works at all. If the FSL
were directly wired
into the receiver, inserting a notch filter or phasing network would
probably be better
- but then we would be wandering away from simplicity and uldx maxims!

Michael UK

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