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I'm an amateur amateur radio enthusiast finally building my own antenna, and have decided on a double cross antenna, with the goal of pulling data from NOAA satellites. Diagram of 50ohm coaxial connections to the four dipoles of the double cross antenna

Four coax cables, two of length x, and two of length x+λ/4. For 137 MHz, the wavelength is ~218.8cm, and a quarter of that is 54.7cm.

However, everywhere I go online I see a quarter of the wavelength being ~36cm.

Eg, 2x79cm lengths and 2x115cm lengths, or 2x14.25in lengths and 2x28.5in lengths - they always come out to be ~36cm.

Am I overlooking something, or is this a case of everyone copying from an incorrect source? I'm inclined to believe I'm wrong here because I'm quite new, but I don't see how.

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The difference is the "Velocity factor". A 36cm long physical coax wire of this type is electrical 54.4cm long. Different types of wire have different velocity factors. https://en.wikipedia.org/wiki/Velocity_factor

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A wavelength/4 in a vacuum at 137 mhz is about 300 / 137 / 4 = 54.7cm. However in coaxial cable the velocity of the radio wave is significantly less.In the case of RG 58 for example 66% of the speed of light in a vacuum. So! .66 x 54.7 = 36.1 cm. There fore to get the free space wave and the wave from the cable in phase the length of the cable has to be reduced by the velocity factor of the cable. Often you will have a situation where you are trying to space two elements at say a quarter of a wavelength apart. The cable between them will be too short because in your example the space between the elements will be 54 cm and the cable is only 36 cm long! Just add a half wavelength to the cable. The additional length just repeats the input and output impedances and allows you to space the elements correctly. This is a neat trick with antennas as you can match an antenna to a load with any impedance cable as long as it is a multiple of a half wavelength. For instance you can have a 50 ohm antenna connected to a 50 ohm transceiver with 75 ohm cable that is a multiple of half wavelengths long. Great if you have a load of 75 ohm tv cable lying around!

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