Larry Clark wrote:
Mike Coslo wrote:

On my HF6V, it calls for a "tuning stub" of a 1/4 wavelength to tune
the antenna at 20 meters. The stub is made of 75 ohm coax, and is
11'4" long, after which the rest of the coax to the shack is 50 ohm.

How exactly does this work? There is an explanation of sorts in
the manual, but nothing I can sink my teeth into.

One of the first things I trip over is that an 11'4" piece of 75
ohm coax is not a quarter wave in the 20 meter band - more like 17+ MHz.

The book notes that the 20 meter portion is more like 100 ohms,
and the stub allows your rig to see 50.

If the antenna works, that's cool, but I'd sure like to know a
little more detail.

- Mike KB3EIA -


are you accounting for velocity factor?


They do have different lengths for different dielectrics. solid
dielectric is the 11 foot +, and foam dielectric uses a 13 foot + length.

It's likely that velocity factor is involved in that part.


beginner alert!


Lessee if I have this correct here. Pardon if I seem a little slow, as
I'm new at this.

Since I calculated 16' 6" inch for the middle of the 20 meter band.

And they are using 11' 4" of solid dielectric coax for a stub.

11'4" is around 70 percent of the length of 16'6".

So the solid dielectric velocity factor is around .70 correct?

This would make the foam dielectric around .80 if I'm not all wet.

looking at velocity factors for various cable types, I'm seeing from .66
to .85.

Of course I don't know where they calculated the exact frequency for,
and this antenna probably has some other interactions, but it seems to
be in the ballpark.

Is this solid, or nonsense?

- Mike KB3EIA -

Now I wonder how that all ties in with changing that 100 Ohm impedance
to 50 Ohms?

beginner alert off!