June 2015
I am interested in building the shortwave radio shown in the schematic https://www.nutsvolts.com/uploads/wygwam/TF_2154_SW_Radio.jpg from Michael Williams Tech Forum question #2153 on page 78 of the February 2015 issue. However, I need more information, specifically the dimensions of L1 — length, diameter, etc., and the frequency range of the receiver.
I’m also seeking information on the coil data, size, number of turns, etc., for the shortwave and broadcast band coils for the Allied Space Spanner regenerative receiver, as well as modifications (plug-in coils VFO) that would increase the frequency coverage above and below the stock range of 6 to 12 MHz on the shortwave section of the receiver.
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First I cannot help you with the information you are looking for on the Allied Space Spanner receiver. However, the other part of your question is about calculating the information for coil L1. The formula for calculating inductance needed for a resonate frequency as the value of the capacitor is shown.
To calculate the inductance of L1 for the standard broadcast band (535KHz to 1700KHz), L= the inductance in µH, f= the frequency in MHz, and capacitance is in pF. Since the capacitance is given as 10 to 365pF, and we know the minimum frequency is 535KHz (or .535MHz), L=25330/(.535 x .535 x 365). L=242µH. In the above formula, we replace the C an L can be interchanged. So if we know the value of the capacitance we can calculate the value of the inductance and visa versa.
With the calculated inductance (242µH) and the maximum frequency of the broadcast band (1700KHz or 1.7MHz), C=25330/(1.7 x1.7x242). C=32pF. So, with a coil wound very close to 242µH, the receiver would cover the standard broadcast band, and a little higher.
Let’s now calculate the number of turns of #30 and diameter of the coil needed to get 242µH. N=√(L(9r+10l) )/r is the formula for calculating the number of turns on a coil, but we have to know the values of radius, and the length of the coil. With a coil, the larger the diameter and the closer the turns are together increases the inductance. r = the mean radius, l = the length of the coil.
I recently did a project very similar to this so I have first hand knowledge of the approximate dimensions of the coil. Looking in a wire table, I find that #30 wire will make approximately 90 turns for a linear inch. I then choose the l of the coil of 1 inch long and the diameter to be 2 inches, so r=1. After plugging the values into the formula, 67 turns is the result. So close wind 67 turns of #30 wire on a 2 inch diameter form and then loosen the turns slightly, and spread them out so the length of the coil is 1 inch. With this information you can calculate the other resonate circuits.
L1 value us very close to 250µH. You could wind one on a form, but buying on would be much easier as the magnet wire may be much more expensive.
