Re: ferrite vs sensitivity

Pollock,Raphael E <rpollock@...>

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 in-depth 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 step-by-step 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 Q-stick + 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 S-meters—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!




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 ICF-S10MK2 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 SNR-wise, 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.

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.


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: E-field in V/m
function [ snr Rac Rdc ] = loopstick (mui, n, d, lr, f, dw, E)
  Kb = 1.38e-23;                  % boltzmann constant
  T = 293;                        % room temperature 20C (293K)
  muo = 4 * pi * 1e-7;            % 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);          

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