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


Gary DeBock
 

Hi Jerry,

Thanks for your comments on the FSL designs.

The "Affordable" 7" diameter medium-wave FSL design does use the 140mm
x 8mm Russian surplus ferrite rods currently available on eBay from a
couple of sellers, and the entire requirement of 63 ferrite rods can
currently be obtained for $104 (US dollars), including airmail shipping
to North America. The price I quoted was from the seller "Alexer1" (who
may soon go into early retirement, after all of the multiple ferrite
orders that I have placed with him).

Of course the asking price of these Russian surplus ferrite rods on
eBay can always change for the worse, as sellers try their luck at
getting more rubles for the rod. There has been a curious supply and
demand situation for the past four months with these ferrite rod
sellers on eBay, as sellers who try to raise their prices suddenly find
themselves shunned by North American purchasers. The ferrite sellers
obviously know about the increased demand for their rods because of the
FSL experimentation, but so far the incresed demand hasn't been enough
for them to make the higher prices stick. That could change at any
time, however, so if a DXer is serious about building an FSL, it's best
to stock up on the Russian surplus ferrite rods while the prices are
still low (currently under $1 per 140mm x 8mm ferrite rod, before
shipping).

73, Gary DeBock (in Puyallup, WA, USA)

-----Original Message-----
From: Jerry Popiel <jerry_popiel@...>
To: ultralightdx <ultralightdx@...>
Sent: Tue, Aug 30, 2011 11:18 pm
Subject: Re: [ultralightdx] Can an FSL Antenna get Interference from a
Strong Local Station?






Gary, thanks for the great detailed info. Its very easy to notice the
great care and planning that went into your FSL's. Do you know yet (re
your testing) if you will be recommending the 5.5 Inch Length   - 140
mm Ferrite Rods for your  "Affordable FSL  Design"  shown in your
detailed picture sent yesterday, or will it be the much more expensive
7.87  or  8 inch  -  200 mm length Rods? Or both!  (The way this
calculation seems to work is 140 mm / 25.4 mm = 5.5 inches). 
 
Jerry






From: "D1028Gary@..." <D1028Gary@...>
To: ultralightdx@...
Sent: Tuesday, August 30, 2011 8:06:32 PM
Subject: Re: [ultralightdx] Can an FSL Antenna get Interference from a
Strong Local Station?


 

<<< 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.

Thanks
Jerry >>>

Hi Jerry,

Thanks for your question about the loop support systems. Steve's 10"
diameter 4" wide styrofoam cake forms are certainly fine to support his
14" Litz wire air core loop for usage inside the shack, considering
that an air core loop has much less weight than a ferrite-based FSL
loop. But I think that you need to take a serious look at the weight of
your chosen antenna (and its intended mission), before choosing a
suitable support structure.

When you add the serious weight of multiple ferrite bars onto a support
structure (and throw in a few possible bumps and drops from Murphy's
Law in a DXpedition situation), you then have quite a different design
challenge than that of supporting a lightweight air core loop inside
the shack. From the beginning of the FSL experimentation it seemed to
me that the main advantage of the FSL antenna would be its compact
portability, and portable antennas need to be ready for anything that
Murphy's Law can (and will) throw at them. Ferrite rods are naturally
fragile components (some more than others), and my own FSL support
designs always have multiple layers of soft, protective padding to
avoid any ferrite rod fractures. Multiple ferrite rod assemblies also
have serious weight challenges, so an FSL frame needs to both handle
the weight and cushion the ferrite rods from sudden shocks.

To show how various layers of FSL antenna cushioning material can
protect multiple ferrite rods from fractures, I've uploaded a side
photo of the new 7" Medium Wave FSL ("Affordable" model) at
http://www.mediafire.com/i/?47iunjk9ylafh5z . I've
found that Schedule 40 PVC water pipe of 3/4" diameter is strong
and shock-resistant enough to provide a nice support frame for any FSL
antenna, and it's an excellent insulating material. In the inner core
of my FSL's there is a soft, resilient 3" diameter section of the
Funnoodle "Monster" swimming aide for kids around the PVC pipe (this
comes with a convenient 3/4 hole through the middle, to accommodate the
PVC pipe). Next are multiple, cut up layers of soft pipe insulation,
followed by a 4" diameter rubber plumbing pipe coupler. The ferrite
rods are attached by J & J Waterproof tape (sticky side out) to the
soft rubber coupler, then wrapped with one layer of Oatey closet flange
foam spacing material (3/8" thickness) before the Litz wire is wrapped.

The use of so many layers of padding material in an FSL may seem
redundant, but it actually proved essential during my first FSL-based
DXpedition this summer. The 6.5" Longwave FSL actually fell off of its
5' PVC base in the predawn darkness at Lincoln City, Oregon and crashed
to the ground, subjecting the 35 ferrite rods to quite a shock. Because
of the multiple layers of padding the ferrite rods were unharmed,
though, and the antenna was receiving its best DX of the entire trip
(270-FA in Samoa, a 1 kw beacon at 5,200 miles) about 15 minutes later.

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

-----Original Message-----
From: Jerry Popiel <jerry_popiel@...>
To: ultralightdx <ultralightdx@...>
Sent: Mon, Aug 29, 2011 10:20 pm
Subject: Re: [ultralightdx] 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.
 
Thanks
Jerry 

From: "D1028Gary@..." <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
http://www.mediafire.com/i/?lq2ummbunzz5p3s ) 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
Station?
. . .
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.

Thanks
Jerry
----------------------------------------------------------
------------

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
have
built FSLs will be able to respond with more authority and their
experience.

I would be intruigued to know whether a secondary ferrite or FSL in
proximity
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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