 ```I find that rather impossible to believe - 300m of thick cable being a dummy load at 137kHz ! Go back to the fundamental equations and calculate properly rather than rely on tables and software used for the wrong purpose . This may be a better way to estimate the performance.... Inductive reactance of a shorted length of line Xl = Zo.TAN(2 . PI . L / ```vf / Wavelength) With Wavelength = 2188m, velocity factor = 0.67, Zo = 50 ```this gives 171 ohms (= 200uH at 137k) you must have slipped a digit somewhere to get 2mH. This is the reactance looking into the coax, shorted at the far end and ```neglecting any losses. ```To get 2mH Xl = 1722 ohms, L = 0.245 wavelength (in air) = 373m of coax. Not very much of an increase on 300m and shows how critical the length is and how fast Xl will change with frequency. (In fact,since a lot of the numbers above have been rounded and we are very close to a shorted quarter wave, a back calculation using the rounded values to check gave Xl = 1600 rather than the 1720 ohms used in the forward calculation - that's how twitchy this technique will be) For an estimate of losses : Skin depth of copper at 137kHz is approximately 0.18mm From D = 503 SQRT(Resistivity / Freq / uo) For Cu Resistivity = 1.7E-8 Ohms / m, and uo (magnetic permeability) = 1 Diameter of centre conductor = 2.5mm (near enough anyway) ```so cross sectional area of conducting path is 0.18mm * 2.5mm = 0.45E-6 m^2 ```RF Resist = Resisivity * Length / Area = 1.7E-8 * 370m / ```0.45E-6m^2 = 14 ohms. ```For a quick estimate assume the braid losses are a lot less than the centre conductor as they have a much larger surface area, so can be ignored (although that may not necessaily be the case) and we can also ignore dielectric losses (a reasonable assumption at these freqs) so Q = Xl / R ```= 1722 / 14 = 123 Which is about what I got on my 5mH conventional coil of 1.5mm wire, 300mm ```diameter and 400mm long. In other words, a very expensive, very large and heavy 'coil' - making it from coax However, if you have a lot of large coax available think about this .... Make a transmitting loop out of the coax, using the outer braid as the loop element. Use the inner / outer capacitance to resonate the loop by connecting the inner to the OPPOSITE end of the outer at ONE end only. ```Feed by personal preference as for any mag loop antenna. ```For topband a loop made this way from LDF350 (roughly similar dimensions to UR67/RG213 but solid copper sheath and foam dielectric) is self-resonant when at 1.9m diameter. This tested out in practice. A quick calculation for 137kHz suggests a loop of 29m diameter of the same material will be self resonant, or at least possibly 90m of cable forming a loop of some shape other than a circle might be. A very rough and ready calculation but it does suggest that a 100m reel of UR67 would contain all the conductor and capacitance needed for a decent loop at 137kHz. I wrote a spreadsheet prog for designing these self resonating mag loop antennas, and one of 28m diameter using LDF-350 for 137kHz suggests a gain of -28dB is feasible (neglecting ground proximity losses). A bit pointless when that is in the same region as the gain from a 12m high tee antenna. If anyone wants a copy (Excel 97), contact my other EMail account [email protected] Andy G4JNT ``````Resistive part of impedance at load: 0.0001 (I typed 0 Ohms, but the program apparently changes that into 0.0001 - PA0SE) Reactive part of impedance: 0 SWR at load: 4793489.50 SWR at line input: 16.67 Additional line loss due to SWR: 60.281 dB Total line loss: 60.803 dB (100.0%) At line input, Zin = 49.42 + j 172.52 At 1500 W, max. rms voltage on line: 988.6 V Distance from load for peak voltage = 984 ft `````` ```