Goodbye winter

30jan2017
The winter is leaving us, higher temperatures, more cloudy days and rain, fewer blue sky days and freezing temperatures. The high pressure area that was over Europe went away.
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Cable winders and accessories

Cable winders: I make them from 5mm plywood. 30 cm tall, 10 cm width, inner gap 16 cm by 6 cm, both top ends 3 cm by 6 cm. Drill two holes to fix the cable ends if needed. Good for 40m of thin wire, it avoids a mess of entangled wires in the field or in the shack.

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Cable winders made from plywood

Guy-wires and accessories: The local hardware store provided: nylon cord, nylon thread, thick needles, metal rings, snaphooks and cable-eyes. You easily convert them into what you need to guy a glas-fiber mast in the field.

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And now we wait for better weather, below 15 degree with rain is no fun to be in the field.

Last update: 19-Feb-2017

Current baluns

Recently I made two current baluns, I used 0.8 mm transformer wire, copper transformer wire and 4c65 ferrite cores that I got from dx-wire in Germany. On the internet I found two different designs that either come with one or with two cores, the question is now which design performs best. In order to measure the performance we terminate each balun with a 200 Ohm resistor. At the input we expect to see an impedance of 50 Ohm with a SWR of 1 so that no signal is returned. The schematic of both baluns is as follows:

balunsmeasurements1
Schematic of the 4:1 current balun

With a single core design we use one toroid were core 1 is wound on the right half and core 2 is on the left half of the ring. With the dual core design two 4c65 toroids are used.

Single core 4:1 balun

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4:1 current balun on one core

This is a 4:1 balun made from 4c65 material, a better description of how it is made can be found on http://owenduffy.net/blog/?p=615 A miniVNA was used to measure the SWR and the resistance |Z| between 1 and 50 MHz:

balunsmeasurements2
VNA 1: SWR and |Z|

Dual core 4:1 current balun

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4:1 current balun on two cores

This is also a 4:1 current balun, the schematic follows from http://vk6ysf.com/balun_guanella_current_1-4.htm The observed SWR and |Z| obtained by the miniVNA are:

balunsmeasurements3
VNA 1: SWR and |Z|

A verification of both baluns was performed with another antenna analyzer measuring the SWR between 1 and 35 MHz. The miniVNA returns more information than the antenna analyzer, in fact the miniVNA returns both the in-phase and the quadrature component of the resistance, the antenna analyzer reports the scalar resistance, although not fully correct we call the antenna analyzer VNA2 in the sequel.

balunsmeasurements4
VNA2 The SWR is observer, the blue graph is for a one core balun, the green one is for the dual core balun, and the red curve is a 1:4 voltage balun made by PH2CV Okko. This balun is made from a different ferrite material, namely a purple toroid whereas our is grey white.

Conclusions

The intended use for current baluns is to match the impedance of Delta loops, folded dipoles, or dipoles with loading coals, etc, to 50 Ohm coax cable in the HF domain, so up to 30 MHz. Current baluns are filters for common mode signals, the problem is relevant for frequencies below 30 MHz where it is increasingly difficult to construct antennas that fit near the house. We confirm that the dual core 4:1 current balun is preferred over the single core version.

Ideally the observed resistance |Z| measured by the VNA should stay close around 50 Ohm when the balun is terminated with a 200 Ohm resistor. The Guanella 4:1 current balun wound on a single core shows high SWRs below 5 MHz and this is expected to be a problem for 160m and 80m band antennas. Both VNA measurements confirm this finding. Between 5 and 30 MHz it does not really matter which balun we use. Beyond 30 MHz the ferromagnetic properties of the 4c65 material will change. Also, the baluns were tested for normal temperatures. Whenever a lot of power is dissipated in the balun it will become hot. When the Curie temperature of the ferrite material is reached the efficiency will change.

VNA1 was a miniVNA, you can find it in the Wimo webstore,  VNA2 is a different one. PE5STE Stefano helped me with the miniVNA measurements, VNA2 is owned by PH2CV Okko. For both VNAs you need a laptop and VNA1 must first be calibrated before you can use it.

Last update: 20-Jan-2017

 

Season’s greetings

Almost 1000 unique WSPR listeners heard me in 2016 with most of the reports coming from Europe and the east coast of the US. Most of the transmissions where done with 200 milli Watt by the beacon, but there are also some sporadic events with 5 to 20W produced by the FT-991.

seasonsgreeting2016all
All stations that saw me in 2016
seasonsgreeting2016eu
Zooming in to Europe

Recently I moved the beacon to the 30m band which I believe is better suited for digital modulation. The beacon has now its own antenna, a short tuned dipole, and the frequency separation to other amateur bands avoids interference.

In any case, thank you for all your WSPR reports.

Last update: 4-Jan-2017

Short tuned dipole

How do you make a short dipole? The answer is, put two loading coils at a distance B from the dipole feed point at its center. You can probably get loading coils from MFJ but it is really more fun to make them yourself. You need to know the position B and the overall length A of the dipole, the wire thickness D and the resonance frequency f.

short-tuned-dipole

In the rest of this blog you find all essential ingredients to design a short tuned dipole. The first part is the function loadingcoil, it is written for  MATLAB and it estimates the inductance that you need. The second part is a MATLAB program to demonstrate the functionality of function loadingcoil, the third part shows NEC validation with a Smith chart of a 4.3 m dipole tuned for 10.1 MHz, this is the 30m radio amateur band that I currently use for WSPR.

MATLAB function loadingcoil

function [H] = loadingcoil( f,A,B,D,metric )
%
% What loading coils are required for a short dipole? This problem is
% described by Jerry Hall K1PLP in QST Sep 1974, 28-34. The relevant
% equation and a script can be obtained from
%
% https://m0ukd.com/calculators/loaded-quarter-wave-antenna-inductance-calculator/
%
% To compute the loading coil inductance for a dipole of length A where
% two loading coils are placed at distance B from the center of the dipole,
% you need the wire thickness D, and the frequency f. The variable are:
%
% A is the overall length of a dipole in meter
% B is the place where you put the coil in meter
% D is the thickness (diameter of the wire) in meter
% f is the frequency in MHz
% H is the returned loading coil that you need
% metric=1 when in Europe (all values are in meter), otherwise metric=0 for US/UK
% (feet for A and B and inches for D)
%
% To verify this function you can use the script on the website of m0ukd
% listed before, but bear in mind that he speaks about the height of a
% quarter wave antenna and not the length of a dipole, so there is a factor
% two difference.
%
if (metric == 1),
inch = 0.0254;
foot = inch*12;
A = A/foot;
B = B/foot;
D = D/inch;
end
T00 = (234/f)-B;
T01 = log(24*T00/D)-1;
T02 = (1-f*B/234)^2 – 1;
T03 = (A/2)-B;
T04 = ((f*A/2-f*B)/234)^2 – 1;
T05 = log(24*T03/D)-1;
T06 = 1e6/(68*pi*pi*f*f);
H = T06 * ((T01*T02)/T00-(T04*T05)/T03);
%
% Last update: 1-jan-2017;

MATLAB example for loadingcoil function

% Jerry Hall K1PLP equation
%
% Checking this design:
% https://www.nonstopsystems.com/radio/antenna_80m_vertical-cntr-loaded.htm
% loadingcoil(3.58,20,5,0.0015,1) should result in 42.555 microHenry
%
clear
f = 3.58;
A = 20;
B = 5;
D = 0.0015;
metric = 1;
H = loadingcoil( f,A,B,D,metric );
fprintf(‘F=%8f MHz A=%7.3f B=%7.3f D=%7.5f metric=%1d H=%7.3f microHenry\n’,f,A,B,D,metric,H);

This program should produce:

F=3.580000 MHz A= 20.000 B= 5.000 D=0.00150 metric=1 H= 42.555 microHenry

Verification of a short dipole at 10.14 MHz for WSPR

To validate the results of part 1 and 2 I use the NEC software which is in the public domain. The design criterium is that a short horizontal dipole has to fit between two chimneys under the roof of our house. The solution is: A=4.315m B=0.1m, D=0.8 mm wire and H=13.7 microHenry. It is always a wise idea to independently verify whether the computed values make sense, so this is where the NEC code assumes that there is a 13.7 uH coil at 10cm from the center of the dipole, the value of A was tuned numerically until a minimum SWR relative to 50 Ohm was obtained.

model ( “short tuned 30m dipole” )
{
real length, height, radius, N, X, R, L, d, e;
element driven, left, right, leftmost, rightmost;
frequencySweep( 10.1, 10.15, 10 );
d = 0.05;
e = 0.10;
length =4.315;
radius = 0.0008;
height = 10;
N = 20;
d = 0.05;
e = 0.10;
driven = wire( 0, -d, height, 0, d, height, radius, N);
left = wire( 0, -d, height, 0, -(d+e), height, radius, N );
leftmost = wire( 0, -(d+e), height, 0, -length/2, height, radius, N);
right = wire( 0, d, height, 0, d+e, height, radius, N );
rightmost = wire( 0, (d+e), height, 0, length/2, height, radius, N);
L = 13.7;
X = 2*pi*10.1*L;
R = 0;
impedanceLoad( left, R, X );
impedanceLoad( right, R, X );
voltageFeed( driven, 1.0, 0.0 );
}

This result in the following Smith chart rom which you learn that the SWR relative to 50 Ohm is 8.48.

Screen Shot 2017-01-03 at 12.50.36.png
Smith chart for a 30m short tuned dipole

The conclusion is that the impedance is 5.9Ohm which means that you need an impedance transformer to connect it to a 50 Ohm coax cable, any 1:9 current balun would be fine, I used something I purchased at a radio market, but they are relatively easy to make them yourself. The other option is to use a twin line and an impedance transformer near the beacon. The center section of the dipole consists of 40cm of PVC tube with a diameter of 40mm, at 10 cm from the center there are 17 turns and this results in the required inductance.

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The ‘device’ under the roof, all you need is wire, tape and a PVC tube.

Next I measured with my antenna analyzer the resonance frequency of the antenna. With the analyzer at the feed point I cut the length of both arms of the dipole to the desired length so that the resonance dip occurs at 10.1 MHz. The measurement should be done at the antenna feed point, and not at the end of the coax connector that goes to the WSPR transmitter because cable impedances shouldn’t interfere with the measurement. Another verification is, listen with the SDR to the antenna, there are always plenty of signals, and inspect where the signal envelope is largest. I found it to be at the 30m band.

Last update: 6-jan-2017