[Elecraft] 75ohm Impedance question K3S and Antenna side.

[Don Wilhelm]

Bob and all,

One problem is that you are using '468/fMhz' to compute the length of a 
half wavelength.  That is the common 'cutting formula' which includes an 
end-effect factor of about 5% which reduces the length of a half wave 
radiator.  An actual half wavelength is longer.

The actual length (in feet) of a wavelength is 983.5712/fMhz which is 
considerably longer than that computed using 468/fMhz times 2.

When dealing with a length of transmission line, the use of the '468' 
factor should not be used - compute the actual wavelength and then apply 
the velocity factor.

For those who work in meters instead of feet, the factor is 299.7925/fMHz.

See any antenna handbook which has essential characteristics of antennas 
for validation of my numbers.

73,
Don W3FPR

On 4/1/2020 9:14 AM, Bob McGraw K4TAX wrote:

> 
> NOTE:  I just noticed that the numbers below are somewhat in error.
> 
> 468/3.540 = 132.2 which is 1/2 wavelength.  Thus 132.2 X 2 = 264 ft 
> which is 1 wavelength, not 2 wavelengths as stated.   I didn't run the 
> math on the others.
>