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 -

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?

larry
kd5foy

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!

Mike Coslo wrote in message ...
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?


You have the basics OK Mike. Calculated lengths are fuzzy things, the
trick is getting the length right out in the yard with the specific
coax you have on hand. For 1/4 wave cutting purposes coax matching
setions can be designed the same way coax harmonic filter stubs are
designed.

There's a bunch of info on the topic in the K1TTT website and it
applies to both matching and filter line lengths.

http://www.k1ttt.net/technote/onestub.html

http://www.k1ttt.net/technote/stubs.html

But then comes the realities, measuring the results of your trimming
process to find out if you really do have a good 1/4 wave line when
it's done. The handiest way to do this is with an antenna analyzer.
The manuals for the MFJ 259/269 analyzers illustrate (educate) how it
can be done rather simply with any analyzer and can be downloaded as a
..pdf from the MFJ website.



- Mike KB3EIA -

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

See chapter 26-6 of the ARRL handbook (19th ed., 2000) "Coupling the
Line to the Antenna".

Za = Antenna feedpoint impedance = 100 ohms
Zf = Feedline impedance = 50 ohms
Zm = Required characteristic impedance of a 1/4 wave matching section
= ?

Zm = Square root of (Za · Zf)= ?
Zm = square root of (50 · 100) = 71 ohms.

beginner alert off!

w3rv

Often with coax, the velocity factor is taken into account. RG-58 is 0.66,
so it comes out shorter than a quarter wavelength.

Bill, W4WNT

"Mike Coslo" wrote in message
...
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 -

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

See chapter 26-6 of the ARRL handbook (19th ed., 2000) "Coupling the
Line to the Antenna".

.. . . 26-6 of the ARRL Antenna Book (19th ed., 2000) "Coupling the
Line to the Antenna".

Nutz.

Spank.

w3rv

Brian Kelly wrote:

Mike Coslo wrote in message ...

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?



You have the basics OK Mike. Calculated lengths are fuzzy things, the
trick is getting the length right out in the yard with the specific
coax you have on hand. For 1/4 wave cutting purposes coax matching
setions can be designed the same way coax harmonic filter stubs are
designed.

There's a bunch of info on the topic in the K1TTT website and it
applies to both matching and filter line lengths.

http://www.k1ttt.net/technote/onestub.html

http://www.k1ttt.net/technote/stubs.html

Thanks, Brian. Fascinating material.

But then comes the realities, measuring the results of your trimming
process to find out if you really do have a good 1/4 wave line when
it's done. The handiest way to do this is with an antenna analyzer.
The manuals for the MFJ 259/269 analyzers illustrate (educate) how it
can be done rather simply with any analyzer and can be downloaded as a
.pdf from the MFJ website.



- Mike KB3EIA -

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


See chapter 26-6 of the ARRL handbook (19th ed., 2000) "Coupling the
Line to the Antenna".

Za = Antenna feedpoint impedance = 100 ohms
Zf = Feedline impedance = 50 ohms
Zm = Required characteristic impedance of a 1/4 wave matching section
= ?

Zm = Square root of (Za · Zf)= ?
Zm = square root of (50 · 100) = 71 ohms.

Thanks again, Brian. I did the stub at their reccomended length, and it
works well. 1.5:1 across the whole 20 meter band, and now the upper
bands are in line also. Fascinating, I'm gonna do my homework now! 8^)

- Mike KB3EIA -