Gary DeBock
Hi George and Ralph,
Thanks again to George for explaining many of the theoretical concepts
behind loopstick design, which I have also found quite fascinating.
Ralph, the 7.5" loopstick ULR designs commonly used in the ULR group
(Slider loopsticks, 7.5" loopstick PL380's, etc.) were all developed
through extensive livesignal A/B testing and tinkering, with the only
standard of success being DXing performance. Although I was fortunate
to receive Navy electronics training as a sonar repair technican, loopstick
design theory was not part of the training curriculum. As such,
George's ability to explain loopstick design science is
appreciated.
Regarding the practical use of very long loopsticks, during the
development of the variableinductance E100 Slider loopstick (in the
summer of 2008), John Bryant and Guy Atkins went in the direction of longer
loopsticks, while I concentrated on refining the 7.5" Slider loopstick as a
reasonable compromise of DXing performance and portability. The 20" and larger
loopsticks did provide slightly more gain than the 7.5" Slider models, but
not as much as we had hoped. On the other hand, Guy's 18" Stormwise
ferritebar PL380 loopstick design does provide significantly more DXing
gain than the 7.5" loopstick PL380, although there certainly is a tradeoff
between portability and performance in such a case.
73, Gary DeBock
(in Puyallup, WA)
toggle quoted messageShow quoted text
In a message dated 11/15/2010 8:40:34 A.M. Pacific Standard Time,
rpollock@... writes:
Dear George:
Your 17 swallows analogy got me to really laughing! In any event, I will await
your practical applications in that the vast majority of our fellow hobbyists,
myself included, lack much indepth knowledge about the underlying
electronics, let alone the theoretical modeling that you are so generously
providing to us. It will be a real service to the hobby to be able to come
forth with an optimized and practical application, including stepbystep
instructions as well as supply sources. It may very well be that a 7.5”
ferrite bar, such as what is being used for so many of the boosted Tecsun
ULRs, is the best option, end of discussion. I continue to be very pleased
with the 19” ferrite bar booster that I built, “designed” by reading through
sources from the various Yahoo clubs, the web, Gerry’s Qstick + which is my
original inspiration, etc. All that is well and good, but is it the best that
can be done??? If I am to interpret the graphs below correctly then a 19”L x
1”D 125 mu ferrite bar wrapped with a primary coil of 38 turns should yield a
SNR of about +6dB and a 27” L x 1” D would be about +8dB. As a practical
matter both of these antennas generate just about the same amount of apparent
gain when inductively coupled to any of several radios that I own that have
Smeters—slightly more gain with the 27”er but not enough to have ever made a
difference in being able to receive a weak signal, plus the nulls are sharper
with the 19”er.
I am really
enjoying your insights, and look forward to more. BTW, basic demographic
info? I’m about to turn 60, have been playing w/ radios since second
grade, am a surgeon and work/live in the middle of Houston, TX. If I had
been better at calculus I might have gone into EE; instead, I stand on the
sidelines and play around with ferrite bars and wires and PVC piping while
being taught by smarter folks like yourself!
73s
Raph
Pollock
From: ultralightdx@...
[mailto:ultralightdx@...] On Behalf Of george
magiros Sent: Saturday, November 13, 2010 9:20 PM To:
ultralightdx@... Subject: Re: [ultralightdx] Re: ferrite
vs sensitivity
I'm still on my quest to simulate ferrite loop antennas, this time for
their signal to noise ratios. I hope I'm not proving that 17 swallows
are as strong as a horse with all my graphs. Experimentation and actual
DXing comes next I promise.
Let me point out a few simple design
observations that I gleamed from the graphs.
First, use as many turns
as possible without making the Q too high. My Sony ICFS10MK2 apparently
uses a small loopstick with an inductance of 650 uH. That is more than
enough turns I would think. 300 uH is probably sufficient too, maybe
lower. Having a good number of turns puts you into a kind of linear
region where increasing the length of the rod improves the SNR the most.
While increasing the number of turns on the loop does improve the SNR, best
results come from increasing the length of the rod.
Second,
increasing the diameter of rod does not improve the SNR that much, unless of
course your rod has a very small diameter to begin with. It appears to
be better SNRwise, and less costly, to simply increase the length of the rod
rather than its diameter. In addition there appears to be, for a given
rod diameter, a maximum rod length beyond which no increase in the SNR
occures.
Now here are the graphs. First for a ferrite
loopstick antenna with an initial permeability of 125. One graph plots
rod length against turns and the other plots rod diameter against turns.
For a loopstick with a diameter of 12.7mm the best length seems to be between
200mm and 300mm FYI.
http://img225.imageshack.us/img225/2042/mu125len.png
http://img833.imageshack.us/img833/1018/mu125dia.png
Next,
here are the graphs for a loopstick antenna with a mu of 800. The best
length for a rod with a 12.7mm diameter seems to be between 300mm to
450mm.
http://img585.imageshack.us/img585/3674/mu800len.png
http://img577.imageshack.us/img577/2258/mu800dia.png
George
Ps.
Just to be sure I haven't made any glaring mistakes here is the Matlab/Octave
function I used to calculate the signal to noise ratio.
% mui: initial
relative permeability of the rod % n: number of turns % lr: length of
rod in mm % dw: diameter of wire in mm % f: frequency in Hz % E:
Efield in V/m function [ snr Rac Rdc ] = loopstick (mui, n, d, lr, f, dw,
E) Kb =
1.38e23;
% boltzmann constant T =
293;
% room temperature 20C (293K) muo = 4 * pi *
1e7; %
permeability of free space sigma =
5.96e7;
% conductivity of copper at 20C (293K) B =
3e3;
% bandwidth (3 khz)
lw = n .* pi .*
d;
% length of wire need delta = 1 ./ sqrt(pi * muo * sigma *
f); % skin depth of wire Rdc = lw * .001 ./ (pi /
4 .* (dw * .001).^2 * sigma); Rac = lw * .001 ./ (pi * sigma * delta
.* dw * .001); en = sqrt(4 * Kb * T * B *
Rac); %
noise voltage
mu = 1./(1./mui+(d./lr).^2.*(log(lr./(d/2))1)); %
rod relative permeability a = pi / 4 * (d *
.001).^2;
% rod winding area es = 2 * pi * n .* a .* mu * E / (3e8 / f); %
signal voltage snr = 20 * log10(es ./
en);
end
