Making a ½λ or ¼λ Coax Cable
To make a ½λ or ¼λ coax cable for a specific frequency, I use a (grid)dip meter. Taking the nominal velocity factor into account, (it is 0.66 for cables with a solid polythene dielectric such as RG58, RG217 or RG174) I unroll a little over a half wave. I then connect a little loop to the free end (fig»). At the expected ½λ point, I pierce the cable with a pin, shorting the braid to the inner conductor. If the dip meter, coupled to the loop, dips at the desired frequency, the actual ½λ point has been found. If the dip frequency is lower, try again a little closer to the loop, and vice versa, till the correct point is found. If your dip meter calibration is not accurate enough, listen for it on your station receiver. The ¼λ length is exactly one-half of the ½λ length found. If the cable is to be used outdoors, do not forget to seal the pinholes.