ehsjr <ehsjr@verizon.net> wrote:

On 3/16/2025 6:39 AM, Liz Tuddenham wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the quarter-wave shorting stub, which is made of co-ax with the inner disconnected, must be taken into account. Other sources say that the velocity factor is that of an open wire, not co-ax, because the stub is only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it
is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that the length of feeder co-ax shorted by the stub needs to be a different length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

The only possible explanation I can think of is that the current in the braid of the shorted section of the feeder is cancelled by the current
in the stub, so that section of the feeder is not acting as co-ax and
the velocity factor doea not apply to it. Nowhere can I find anything which says that - so it there another explanation?

It's an interesting question, but there is an inherent problem.
Published velocity factor may contain an error as great as 10%,
so what does that mean to the calculation of stub length? Seems
to me that you are forced into empirical measurements either way
to determine the "proper" stub length - where "proper" is
whatever your design specs are. In other words, it sure would be
nice to be able to compute "the" answer, but I don't see how that
is possible without measuring the velocity factor - or measuring
the stub performance at the design frequency and over the design
frequency range. :-(

I have measured it by simpy measuring a physical length of cable with my
trusty dressmaking tape and then comparing that with the Time Domain Reflectometer measurement using the VNA. The VNA can be set to any
velocity factor, so I chose the one that gave the result to the best approximation of the physical length; It came out as 67%.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

--- SoupGate-Win32 v1.05
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john larkin <jlArbor.com> wrote:

On Sun, 16 Mar 2025 12:57:20 -0400, ehsjr <ehsjr@verizon.net> wrote:

On 3/16/2025 6:39 AM, Liz Tuddenham wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner
disconnected, must be taken into account. Other sources say that the
velocity factor is that of an open wire, not co-ax, because the stub is
only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it >> is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that >> the length of feeder co-ax shorted by the stub needs to be a different
length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

The only possible explanation I can think of is that the current in the
braid of the shorted section of the feeder is cancelled by the current
in the stub, so that section of the feeder is not acting as co-ax and
the velocity factor doea not apply to it. Nowhere can I find anything
which says that - so it there another explanation?

It's an interesting question, but there is an inherent problem.
Published velocity factor may contain an error as great as 10%,
so what does that mean to the calculation of stub length? Seems
to me that you are forced into empirical measurements either way
to determine the "proper" stub length - where "proper" is
whatever your design specs are. In other words, it sure would be
nice to be able to compute "the" answer, but I don't see how that
is possible without measuring the velocity factor - or measuring
the stub performance at the design frequency and over the design
frequency range. :-(

Ed

Why not use a transformer?

I have one on order but I am looking at other solutions too. Balancing
the centre point of the vertical diople is only part of the problem,
getting the co-ax to it from underneath is much more difficult if you
don't want standing waves on the braid. A choke is a distinct
possibility and very cheap to make.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

In article <1r9aa77.1vp4ggx1eqrulmN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner >disconnected, must be taken into account. Other sources say that the >velocity factor is that of an open wire, not co-ax, because the stub is
only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it
is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that
the length of feeder co-ax shorted by the stub needs to be a different
length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

As I understand it:

- The current flow on the _inside_ of the feeder coax is subject
to the cable's velocity factor, because the electrical fields
are applied across the cable dielectric.

- The current flow back down the _outside_ of the feeder coax
(which is what you want to choke off, in order to force
balance in the antenna) is not subject to the cable's
velocity factor, because the electrical field on the
outside isn't going through the cable dielectric. It's
going only through the outer insulation and then out
into space.

Again, if I understand it correctly, the presence of the outer
insulation (on both the feeder, and the choke section) does cause
current flow here to have a velocity factor of somewhat less than 1.0
(as you would see in a bare wire). However, the velocity change is
much less than what occurs inside the cable (the VF here might be .98
rather than .67 as it might be inside the coax), and most opinions
I've read say that it can generally be neglected when figuring out the
length of the choke section (and thus the point at which the bottom of
the choke is soldered to the feeder).

I don't believe it matters significantly whether you remove
the center conductor from the choke section, or simply
trim it off flush at both ends and don't connect it.

--- SoupGate-Win32 v1.05
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piglet <erichpwagner@hotmail.com> wrote:

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r9aa77.1vp4ggx1eqrulmN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner
disconnected, must be taken into account. Other sources say that the
velocity factor is that of an open wire, not co-ax, because the stub is
only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it >> is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that >> the length of feeder co-ax shorted by the stub needs to be a different
length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

As I understand it:

- The current flow on the _inside_ of the feeder coax is subject
to the cable's velocity factor, because the electrical fields
are applied across the cable dielectric.

- The current flow back down the _outside_ of the feeder coax
(which is what you want to choke off, in order to force
balance in the antenna) is not subject to the cable's
velocity factor, because the electrical field on the
outside isn't going through the cable dielectric. It's
going only through the outer insulation and then out
into space.

Again, if I understand it correctly, the presence of the outer
insulation (on both the feeder, and the choke section) does cause
current flow here to have a velocity factor of somewhat less than 1.0
(as you would see in a bare wire). However, the velocity change is
much less than what occurs inside the cable (the VF here might be .98 rather than .67 as it might be inside the coax), and most opinions
I've read say that it can generally be neglected when figuring out the length of the choke section (and thus the point at which the bottom of
the choke is soldered to the feeder).

I don't believe it matters significantly whether you remove
the center conductor from the choke section, or simply
trim it off flush at both ends and don't connect it.

Yes I think that’s right. The quarter wave transmission line we want is formed between the stub shield and the feedline shield. These fields are in the thin outer jacket insulation and air so velocity factor will be high
and near one. Spacing between the two shields should be minimised?

I agree, that seems like the correct explanation - mystery solved!


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

piglet <erichpwagner@hotmail.com> wrote:

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r9aa77.1vp4ggx1eqrulmN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner
disconnected, must be taken into account. Other sources say that the
velocity factor is that of an open wire, not co-ax, because the stub is
only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it >>> is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that >>> the length of feeder co-ax shorted by the stub needs to be a different
length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

As I understand it:

- The current flow on the _inside_ of the feeder coax is subject
to the cable's velocity factor, because the electrical fields
are applied across the cable dielectric.

- The current flow back down the _outside_ of the feeder coax
(which is what you want to choke off, in order to force
balance in the antenna) is not subject to the cable's
velocity factor, because the electrical field on the
outside isn't going through the cable dielectric. It's
going only through the outer insulation and then out
into space.

Again, if I understand it correctly, the presence of the outer
insulation (on both the feeder, and the choke section) does cause
current flow here to have a velocity factor of somewhat less than 1.0
(as you would see in a bare wire). However, the velocity change is
much less than what occurs inside the cable (the VF here might be .98
rather than .67 as it might be inside the coax), and most opinions
I've read say that it can generally be neglected when figuring out the
length of the choke section (and thus the point at which the bottom of
the choke is soldered to the feeder).

I don't believe it matters significantly whether you remove
the center conductor from the choke section, or simply
trim it off flush at both ends and don't connect it.

Yes I think that’s right. The quarter wave transmission line we want is formed between the stub shield and the feedline shield. These fields are in the thin outer jacket insulation and air so velocity factor will be high
and near one. Spacing between the two shields should be minimised?

I guess for precision you could make a test piece of two lengths of the intended coax taped together and measure the velocity factor of the shield
to shield transmission line thus created. I imagine the spacing is
critical.

--
piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r9aa77.1vp4ggx1eqrulmN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner
disconnected, must be taken into account. Other sources say that the
velocity factor is that of an open wire, not co-ax, because the stub is
only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it
is just acting as wire (and the fact that it is made from the braiding
of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that
the length of feeder co-ax shorted by the stub needs to be a different
length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

As I understand it:

- The current flow on the _inside_ of the feeder coax is subject
to the cable's velocity factor, because the electrical fields
are applied across the cable dielectric.

- The current flow back down the _outside_ of the feeder coax
(which is what you want to choke off, in order to force
balance in the antenna) is not subject to the cable's
velocity factor, because the electrical field on the
outside isn't going through the cable dielectric. It's
going only through the outer insulation and then out
into space.

Again, if I understand it correctly, the presence of the outer
insulation (on both the feeder, and the choke section) does cause
current flow here to have a velocity factor of somewhat less than 1.0
(as you would see in a bare wire). However, the velocity change is
much less than what occurs inside the cable (the VF here might be .98
rather than .67 as it might be inside the coax), and most opinions
I've read say that it can generally be neglected when figuring out the
length of the choke section (and thus the point at which the bottom of
the choke is soldered to the feeder).

I don't believe it matters significantly whether you remove
the center conductor from the choke section, or simply
trim it off flush at both ends and don't connect it.

Yes I think that’s right. The quarter wave transmission line we want is formed between the stub shield and the feedline shield. These fields are in
the thin outer jacket insulation and air so velocity factor will be high
and near one. Spacing between the two shields should be minimised?

--
piglet

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

piglet <erichpwagner@hotmail.com> wrote:

piglet <erichpwagner@hotmail.com> wrote:

Dave Platt <dplatt@coop.radagast.org> wrote:

In article <1r9aa77.1vp4ggx1eqrulmN%liz@poppyrecords.invalid.invalid>,
Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:

I've been mucking about with a design which includes a Pawsey stub.
Some sources say the velocity factor of the feeder co-ax and the
quarter-wave shorting stub, which is made of co-ax with the inner
disconnected, must be taken into account. Other sources say that the
velocity factor is that of an open wire, not co-ax, because the stub is >>> only the braid acting as a piece of wire.

I can see that the stub does not need to be treated as co-ax, because it >>> is just acting as wire (and the fact that it is made from the braiding >>> of co-ax is irrelevant). I can also see that the feeder co-ax
apparently *is* being used as co-ax which means its velocity factor
should be taken into account. This leads to the logical conclusion that >>> the length of feeder co-ax shorted by the stub needs to be a different >>> length from the length of the stub itself - which none of the
descriptions mentions or illustrates (the kinks would be obvious).

As I understand it:

- The current flow on the _inside_ of the feeder coax is subject
to the cable's velocity factor, because the electrical fields
are applied across the cable dielectric.

- The current flow back down the _outside_ of the feeder coax
(which is what you want to choke off, in order to force
balance in the antenna) is not subject to the cable's
velocity factor, because the electrical field on the
outside isn't going through the cable dielectric. It's
going only through the outer insulation and then out
into space.

Again, if I understand it correctly, the presence of the outer
insulation (on both the feeder, and the choke section) does cause
current flow here to have a velocity factor of somewhat less than 1.0
(as you would see in a bare wire). However, the velocity change is
much less than what occurs inside the cable (the VF here might be .98
rather than .67 as it might be inside the coax), and most opinions
I've read say that it can generally be neglected when figuring out the
length of the choke section (and thus the point at which the bottom of
the choke is soldered to the feeder).

I don't believe it matters significantly whether you remove
the center conductor from the choke section, or simply
trim it off flush at both ends and don't connect it.

Yes I think that’s right. The quarter wave transmission line we want is formed between the stub shield and the feedline shield. These fields are in the thin outer jacket insulation and air so velocity factor will be high and near one. Spacing between the two shields should be minimised?

I guess for precision you could make a test piece of two lengths of the intended coax taped together and measure the velocity factor of the shield
to shield transmission line thus created. I imagine the spacing is
critical.

I have attempted to thread the co-ax inside the bottom element of a
dipole and use the tubular element as the Pawsey stub, but the spacing
between the co-ax outer and the inside of the tube is uncontrolled and
the SWR is unstable.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

In article <vr8m9j$3oe4k$1@dont-email.me>,
piglet <erichpwagner@hotmail.com> wrote:

Yes I think that’s right. The quarter wave transmission line we want is >formed between the stub shield and the feedline shield. These fields are in >the thin outer jacket insulation and air so velocity factor will be high
and near one. Spacing between the two shields should be minimised?

That's the usual approach for this sort of stub, for several reasons.

It will help minimize radiation from the transmission line (by keeping
the distance between two sides of the TL down to a tiny fraction of
one wavelength). It keeps the length of the connections _between_ the
coaxes (at the top and bottom) to a minimum, and minimizes any excess inductance introduced by the connection. It's also physically easy
and convenient - just use plastic zip-ties to fasten the two pieces of
coax together, and glob some good weather-sealant over the connections
at each end, and you're done.

Spacing the two sides of the TL apart will probably make a tiny
difference in the velocity factor, but I believe it'll be
negligible (since all you're doing is adding air dielectric).

It'll also change the characteristic impedance of this TL, but again,
that really won't make a significant difference in behavior. No
matter what the impedance of a quarter-wave TL is, it'll transform a
short at the bottom into an open circuit at the top.

Now, this TL will only be a "perfect" quarter-wave length at a
single frequency. On either side of that frequency its
transformation will be imperfect, and the impedance looking down
into it from the top won't be "infinite". Over a limited
bandwidth (say, 2 MHz of the 2-meter band) that's only a percent
or two and it'll look enough like an open circuit to provide
good choking performance over the whole band. An error in
the cable length of a percent or so, or getting the VF wrong
by a percent or so, will have a similar (and probably
neglible) impact on the actual choking performance.

So, I'd say the pragmatic thing to do is pick your center operating
frequency, assume a VF of .98 or so, calculate the length, cut, strip
ends of jacket, solder, weatherproof, and be happy!

The fancy thing to do is actually measure the actual VF, by making
a test line out of two pieces of the coax closely strapped together and
shorted at the far end. A longer test cable set, and a lower test
frequency (one at which this cable set is theoretically 1/4 wavelength
long) might make this easier.

This could be done using a NanoVNA, and a coax jumper a few feet long
ending in a choke (run the coax through a ferrite core or toroid a few
times) and short clip leads. Calibrate the NanoVNA with this jumper
in place (open, short, and a good 50-ohm resistor) at your chosen test frequency. Then, sweep the cable set around that frequency, looking
for the point at which the S11 impedance is as high as possible
(resistive) and has no reactive component. At that frequency, the
test cable is a quarter-wavelength... so, knowing its length you can
compute the VF, and you can cut a similar stub which will be exactly
1/4 wavelength at your chosen center operating frequency.

The NanoVNA firmware (or its host software) may have a "distance to
fault" feature - a pseudo-TDR which sweeps the frequency and looks for
this high-resistive-impedance peak. Or, there may be a "compute cable
velocity factor" feature... same idea, different user interface.

If you have access to an actual TDR (one which sends a sharp
pulse down the TL and lets you look for the actual
reflection) you can get a very precise VF measurement that way.

All of this is, very probably, just gilding the lily and going for
"perfect precision" that doesn't matter at all in actual operating
practice. That's something I've never been guilty of myself... no
never!... well, hardly ever :-)

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)