[Lowfer] FW: 600MRG> Helically wound vertical on a city lot>NO WAY!!!

[Tom Rauch]

>  The optimum shape for minimum losses/maximum Q is so broad that it's
> hardly worth bothering about. 

In system requiring low values of reactance, that is true within 
reasonable limits of form factor. I rarely worry about form factor in 
amplifier tanks operating at low impedances. I certainly do pay 
attention in short loaded antennas without large hats, or in low-C 
high-L series-resonant circuits like traps!

In cases requiring high values of inductive reactance, the optimum 
inductor must be stretched out to minimize distributed and end-to-
end capacitance. Otherwise, the self-resonant frequency 
approaches the operating frequency, and inductor losses increase 
dramatically.

Many times that effect is missed because inductor Q is measured 
below the actual operating frequency or Q is measured by 
measuring bandwidth (both of which can be misleading). Any 
shunting capacitance has the effect of decreasing system 
bandwidth while actually increasing inductor loss and effective 
inductance.

Of course as you and I agree this matters less if ground losses are 
high, because the ground losses swap out inductor losses.

As for the antenna experiment, it makes total sense. The 
frequency makes no difference, it is the radiating structure design 
that counts. That result should NOT be applied as a general rule to 
other unrelated system designs, such as thinking form factor 
"never matters" at VLF LF or any other frequency!

When a large loading hat is involved things change dramatically. If 
the hat capacitance is large compared to capacitance in the 
vertical conductor, it matters little where we place the coil. Current 
is basically uniform throughout the structure because nearly all 
current flows into the hat. That means radiation resistance is the 
same no matter where we place the inductor. Current distribution is 
set by current in the hat. Rrad/Rrad+Rground remains basically 
unchanged no matter where the coil is located. 

Also, any errors in constructing the coil are minimized because the 
required reactance is smaller. Stray C is less important, because 
required reactance is less.

My mobile antenna loading coils only used with whips are long and 
slender and mounted higher up, my mobile antennas with big end-
hats are almost square in design and mounted anywhere I like 
(usually low) for these very reasons.

It works that way on 160 meters. It works that way on 1600 meters.
73, Tom W8JI
W8JI@contesting.com