Antenna tuner/Smith chart question
 

Still having fun playing with my 8.5 foot whip on the metal patio roof, with
coax feedline to an antenna tuner in the shack. Operating on 15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and move
around the SWR circle on the Smith chart. Apparently this provides a set of
matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess is
that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to be
more difficult to match, and should be avoided?

Antenna tuner/Smith chart question
 

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Try rearranging your network, try pi series C, pi shunt C, etc.

Also make friends with someone with a vswr analyzer ;-)

Antenna tuner/Smith chart question
 

On Jan 2, 3:39*am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com

Antenna tuner/Smith chart question
 

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Have you ever compared the field strengths above and below the steel
cover?

Antenna tuner/Smith chart question
 

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS at
x feet from the antenna at y height above the cover, vs below the cover at
what point?

Antenna tuner/Smith chart question
 

"dave" wrote in message
...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:

Still having fun playing with my 8.5 foot whip on the metal patio roof,
with coax feedline to an antenna tuner in the shack. Operating on
15,12, and 10.

The antenna tuner is a typical t network with series capacitors and an
inductor to ground. I was unable to get a match on 15 meters with the
antenna tuner. So, I thought I would add more coax to the feedline and
move around the SWR circle on the Smith chart. Apparently this provides
a set of matching parameters that the antenna tuner can now cope with.

I don't have instruments to measure many parameters, but a rough guess
is that the 15 meter SWR is at least 20:1.

So the question is: are there parts of the Smith chart that are going to
be more difficult to match, and should be avoided?

Try rearranging your network, try pi series C, pi shunt C, etc.

Also make friends with someone with a vswr analyzer ;-)
-
Or, I could leave in the additional feedline ;-)
But a good point is that I have an old homebrew 2 element tuner with
alligator clip configuration. Might give that a try just for fun.

I have one of the older MFJ analyzers, but I wouldn't trust the accuracy at
high VSWRs.

Antenna tuner/Smith chart question
 

"W5DXP" wrote in message
...

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use for
ground, so I used Real/Minimec. I'm showing about 18-j142 as the feedpoint
impedance at 21.1 Mhz, which is not totally out of the ballpark with your
number.

At any rate, I have bought enough telescoping tubing sections to build
about a 20 foot vertical. Final height is subject to approval by "the
Admiral". Fabrication of a more substantial base support will be necessary.

In the meantime, this AM I replaced the 8.6 ft whip with about 13 feet of
telescoping tubing, using the same mobile whip mount clamped to conduit.
Resonance is around
18.7 MHz and it loads up nicely on 18 through 10, with 10 meters having the
highest calculated swr of 17.

With the antenna removed, the feedline shows a 1/4 wave null at 4.33MHz.
This gives an estimated 1/2 wave electrical length of 57 feet at 8.66 MHz,
or with 0.66 VF a physical length of 37.6 ft. That seems about right.

I'll get a Smith chart and see where the existing feedline moves the
parameters. Per your comments I'll try to avoid low r values.

Wish the bands would open back up like they were in Oct and Nov.
-
Wayne
W5GIE

Antenna tuner/Smith chart question
 

On Mon, 02 Jan 2012 08:47:29 -0800, Wayne wrote:

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:


Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS
at x feet from the antenna at y height above the cover, vs below the
cover at what point?

I was thinking, it would be a rough indication what proportion of
power is radiated from the feedline.

Antenna tuner/Smith chart question
 

"Wond" wrote in message ...

On Mon, 02 Jan 2012 08:47:29 -0800, Wayne wrote:

"Wond" wrote in message ...

On Mon, 02 Jan 2012 01:39:48 -0800, Wayne wrote:


Have you ever compared the field strengths above and below the steel
cover?
-
No I haven't. What would be the practical tests? Example, measuring FS
at x feet from the antenna at y height above the cover, vs below the
cover at what point?

I was thinking, it would be a rough indication what proportion of
power is radiated from the feedline.
-
Good point, and I'm not sure that the feedline is currently in optimal
configuration.

The feedline lies on the metal roof to the edge, where there is a small 8
turn loop. Then it goes another 6-8 feet to the tuner.

My thoughts are that your antenna theory has holes in it.
If you want it to tune on 15 meters, it needs to be a fraction of a wavelength long on 15 meters - which a antenna tuned for 10 meters has a real hard time sometimes tuning up on 12 or 15 meters.

Instead of trying to redesign the mouse trap, why not invest your money towards a used 11 meters chicken band antenna.
A antenna - regardless of who makes it or what they market it for - is a antenna.

By shortening a 11 meter antenna a couple of inches, it will work wonderfully for 10 meters.

The suggested length of coax with solid dielectric .66 velocity factor usually comes out somewhere around 18 feet long for a mobile, so working with the Smith Chart will get your SWR - line length to a better SWR, but to make it resonant - it needs to be a certain size.

You would be better off designing the antenna for 15 meters and then using the transmatch to tune it down to 10 meters then to try to use a 10 meters antenna and try to tune it up for 15. Then again, your antenna really isn't any good for either band in my opinion, due to the fact that a 5/8ths wave antenna probably works best in most situations for a base station radio for 10 meters and your antenna is way too short.

Even a 1/4 wave antenna at 15 meters would probably need to be at least 11.5 feet long, with a nominal length of about 31 feet using a vertical antenna.

Antenna tuner/Smith chart question
 

"Wayne" wrote in
:



"W5DXP" wrote in message
news:1fe694d0-ff12-48dd-9abe-
.
..

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use
for ground, so I used Real/Minimec. I'm showing about 18-j142 as the
feedpoint impedance at 21.1 Mhz, which is not totally out of the
ballpark with your number.

At any rate, I have bought enough telescoping tubing sections to build
about a 20 foot vertical. Final height is subject to approval by "the
Admiral". Fabrication of a more substantial base support will be
necessary.

In the meantime, this AM I replaced the 8.6 ft whip with about 13 feet
of telescoping tubing, using the same mobile whip mount clamped to
conduit. Resonance is around
18.7 MHz and it loads up nicely on 18 through 10, with 10 meters
having the highest calculated swr of 17.

With the antenna removed, the feedline shows a 1/4 wave null at
4.33MHz. This gives an estimated 1/2 wave electrical length of 57 feet
at 8.66 MHz, or with 0.66 VF a physical length of 37.6 ft. That seems
about right.

I'll get a Smith chart and see where the existing feedline moves the
parameters. Per your comments I'll try to avoid low r values.

Wish the bands would open back up like they were in Oct and Nov.
-
Wayne
W5GIE


So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to lose
70% of the power out of the ATU as heat. Of course, it is likely that
the power out of the ATU is somewhat less than the transmitter output as
the ATU load is some 7+j10, relatively low R (but typical ATUs tend to
be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Owen

Antenna tuner/Smith chart question
 

On Jan 8, 10:21*pm, Owen Duffy wrote:
Did the Smith chart reveal any of that?

There are loss scales at the bottom of the Smith chart that allow
losses to be included. That's why I mentioned "tuner efficiency"
and "considerable feedline losses" in my posting.
--
73, Cecil, w5dxp.com

Antenna tuner/Smith chart question
 

Owen Duffy wrote in
:

....
So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to
lose 70% of the power out of the ATU as heat. Of course, it is likely
that the power out of the ATU is somewhat less than the transmitter
output as the ATU load is some 7+j10, relatively low R (but typical
ATUs tend to be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Well, subject to how carefully you plotted the data, you might get a
reasonable answer for the line input impedance (as seen by the ATU), but
the Smith chart will not directly read the line loss with a mismatched
losd.

With a lot of care in plotting, you might scale off the values of G at
line input and output, and find the ratio Vload/Vin from the chart to
calculate the ratio of Pload/Pin and so find the loss under mismatched
load.

The line loss in this case is just over 5dB, more than 10 times the
0.5dB loss you would observe with a matched load.

An exercise for the reader is to use a Smith chart to find the loss
under the mismatched load with RG213 where the matched line loss is
about 0.31dB.

Owen

PS: if you had in mind using the handy dandy "Additional loss due to
VSWR" graph in the ARRL, the line input VSWR is 7.7, load VSWR is 23.7.
If you get the same loss figure are mentioned above, it is by accident
of the scenario rather than soundness of the method.

Antenna tuner/Smith chart question
 

On Jan 9, 3:12*pm, Owen Duffy wrote:
Well, subject to how carefully you plotted the data, you might get a
reasonable answer for the line input impedance (as seen by the ATU), but
the Smith chart will not directly read the line loss with a mismatched
losd.

Owen, I respect your obvious technical ability and the only time I
ever disagree with you is when you go off the deep end and accuse me
of saying things that I never said and/or try to pick a fight like you
are trying to do on this thread. What in the world do you expect to
gain from such antisocial behavior?
--
73, Cecil, w5dxp.com

Antenna tuner/Smith chart question
 

"Owen Duffy" wrote in message
...

"Wayne" wrote in
:



"W5DXP" wrote in message
news:1fe694d0-ff12-48dd-9abe-
.
..

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use
for ground, so I used Real/Minimec. I'm showing about 18-j142 as the
feedpoint impedance at 21.1 Mhz, which is not totally out of the
ballpark with your number.

snip
***********

Update. Current configuration is a 14.6 ft element fed with 40 ft RG-8.

Wayne
W5GIE


So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to lose
70% of the power out of the ATU as heat. Of course, it is likely that
the power out of the ATU is somewhat less than the transmitter output as
the ATU load is some 7+j10, relatively low R (but typical ATUs tend to
be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Owen
-
I think that losses were discussed on a previous iteration of the thread,
and I have no quarrel with the loss numbers.

Many years ago, I might have been able to somehow deduce cable losses from a
Smith chart, but nowadays I just use the published curves.

The current configuration (14.6 ft vertical) is promising for 14-28 MHz
operation, as three of the bands have SWRs of less than about 10:1, and the
other two are below about 20:1. The antenna is not checked out on all of
those bands, but it does an acceptable job on 12 and 10 meters where the SWR
is highest. That statement should not be construed as meaning that it is
the final configuration.

Antenna tuner/Smith chart question
 

On Jan 9, 6:29*pm, W5DXP wrote:
Owen, I respect your obvious technical ability ...

Owen, I apologize for my previous bad mood outburst.
--
73, Cecil, w5dxp.com

Antenna tuner/Smith chart question
 

"Wayne" wrote in
:

Many years ago, I might have been able to somehow deduce cable losses
from a Smith chart, but nowadays I just use the published curves.

An email correspondent suggested the radially scaled "SW LOSS COEFF"
gives a direct reading loss under mismatch from the chart.

There are assumptions that underly that scale, pre-requisites if you
like for it to be accurate. They are explained in Smith's book: "If a
waveguide is one or more wavelengths long, the average increase in
dissipative loss due to standing waves in a region extending plus or
minus one-half wavelength from the point of observation may be expressed
as a coefficient or factor of the one-way transmission loss per unit
length".

So, it does not apply to a feed line of less than a full wave, or any
residual of less than a full a wave after working out the additional
loss in each of the full wave segments of a line. It is tedious and
accurate only for lines of integral full waves length.

The potential error applying it to arbitrary line length ranges from
insignificant to huge.

The graphs in the ARRL suffer the same problem, and IIRC they do not
state the assumptions.

The Smith chart solution I described in my earlier post works for
arbitrary line lengths, and properly reveals the cases where loss under
standing waves is less than matched line loss. TLLC
(http://www.vk1od.net/calc/tl/tllc.php) also calculates the loss
correctly for arbitrary line lengths.

Owen

Antenna tuner/Smith chart question
 

"Owen Duffy" wrote in message
...

"Wayne" wrote in
:

Many years ago, I might have been able to somehow deduce cable losses
from a Smith chart, but nowadays I just use the published curves.

***

The Smith chart solution I described in my earlier post works for
arbitrary line lengths, and properly reveals the cases where loss under
standing waves is less than matched line loss. TLLC
(http://www.vk1od.net/calc/tl/tllc.php) also calculates the loss
correctly for arbitrary line lengths.

Owen
-
Great link, Owen. I have it bookmarked and will do some playing around with
it.
Wayne
W5GIE