Topband: Coax Length

[Terry Conboy]

At 11:24 AM 2005-09-23, AA6VB wrote:
>The resonant frequency at my elevated vertical (5/8 wave on 30; 1/4 wave
>on 80; and coil loaded for 160, mounted 3 feet off the ground, with
>radials sloping up from the feed point to the roof) is about 100khz
>different at the shack than when measured at the antenna.  This is not
>true on 30 or 80 - only on 160.  I have not been able to figure out why
>this is happening, and wondered if the length of coax could be a
>contributing factor (I am not sure of the length, but think it might be
>about 150 feet).

If the feed impedance of an antenna isn't 50 ohms and the feedline 
length isn't a multiple of 1/4 wavelength, the resonance (zero 
reactance) at the antenna and the resonance looking into the feedline 
will always occur at different frequencies.  Odd multiples of 1/8 
wavelength (0.125 wl, 0.375 wl, etc.) are worst in this regard.  Your 
150 foot feeder would be about 0.42 wl with VF=0.66 or 0.35 wl with 
VF=0.8, so there probably will be input reactance with a resistive 
load that isn't 50 ohms.

It's worth noting that resonance and lowest SWR are not always at the 
same frequency if the impedance isn't 50 ohms at resonance.  In some 
systems, actual resonance never even occurs (of course, this antenna 
should resonate, given the right inductance).

If your antenna does indeed look like 50 ohms, either suspect your 
test gear (AM Broadcast overload, etc.) and make sure that the shield 
of the coax is connected to the ground system during measurement at 
the antenna, since it will look like another radial if a current 
balun (choke) is not being used.

73, Terry N6RY