coax filter dilemma
 

Hi all,

I post this here since probably here lurks a good amount of smith
chart and transmission lines wizards.

I was trying to make a simple two stub filter with coaxial lines,
basicly it would serve as 88-108 MHz notch with a working frequency of
70 MHz.
The filter is like this:

length A
rig--|---------------|--ant
| |
b | | b
| |



The two "b" stubs are 64 cm cellflex 1/2 inch, with the antenna
analyzer they measure R=0 X=21 at 70 MHz, almost as they should per a
quick smith chart check, they should be 1/4 lambda at 98 MHz, open at
the end.
Now with two of these stubs the lenght A that gives a 50 ohm match
should be 0.378 lambda as per the smith chart calculation.

In real life with that value for A the only 50 ohm match (substituting
a dummy load for ant and the analyzer on the "rig" port) is around 36
MHz.

What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

73 de Frank IZ8DWF

coax filter dilemma
 

wrote in message
...
Hi all,

I post this here since probably here lurks a good amount of smith
chart and transmission lines wizards.

I was trying to make a simple two stub filter with coaxial lines,
basicly it would serve as 88-108 MHz notch with a working frequency of
70 MHz.
The filter is like this:

length A
rig--|---------------|--ant
| |
b | | b
| |



The two "b" stubs are 64 cm cellflex 1/2 inch, with the antenna
analyzer they measure R=0 X=21 at 70 MHz, almost as they should per a
quick smith chart check, they should be 1/4 lambda at 98 MHz, open at
the end.
Now with two of these stubs the lenght A that gives a 50 ohm match
should be 0.378 lambda as per the smith chart calculation.

In real life with that value for A the only 50 ohm match (substituting
a dummy load for ant and the analyzer on the "rig" port) is around 36
MHz.

What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

73 de Frank IZ8DWF

Hi Frank

At 70 MHz, when the open stub presents a reactance of -J21 across the 50
ohm "dummy load", the resultant R-Jx is a serious mismatch to the 50 ohm "A"
line. Your Smith Chart calculations for the length of "A" may be incorrect.
Try making "A"
about 1/10th lambda.

Jerry KD6JDJ

coax filter dilemma
 

What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

73 de Frank IZ8DWF

In simulation I am getting 86.8 degrees for the shunt stubs, and
62.4 degrees for the series coax between stubs. Seems about
the best compromise with results as follows:

65 to 75 MHz S11 -10 db
65 to 75 MHz S21 -1 db
88 to 90 MHz S21 -16 db
90 to 116 MHz S21 -20 db.

73, Also Frank (VE6CB)

coax filter dilemma
 

"Frank" wrote in message
news:CDchk.904$nu6.498@edtnps83...
What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

PS the single shunt stub of 86.8 degrees calculates to 11.7 - j 21 ohms,
when in parallel with a 50 ohm load (At 70 MHz).

Frank

coax filter dilemma
 

"Frank" wrote in message
news:_Jchk.905$nu6.310@edtnps83...

"Frank" wrote in message
news:CDchk.904$nu6.498@edtnps83...
What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

PS the single shunt stub of 86.8 degrees calculates to 11.7 - j 21 ohms,
when in parallel with a 50 ohm load (At 70 MHz).

Frank

Hi Frank

My calculations indicate that the length of "A" (50 ohm coax) should be
close to 48.6 degrees between two identical open stubs with a 50 ohm
termiantion on the 50 ohm line when I use your 11.7 -J21 impedance. So, my
recommendation stands. Try a little shorter "A" if impedance match at 70
MHz is the objective.

Jerry

coax filter dilemma
 

In message 8lehk.213$GI.77@trnddc05, Jerry
writes

"Frank" wrote in message
news:_Jchk.905$nu6.310@edtnps83...

"Frank" wrote in message
news:CDchk.904$nu6.498@edtnps83...
What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

PS the single shunt stub of 86.8 degrees calculates to 11.7 - j 21 ohms,
when in parallel with a 50 ohm load (At 70 MHz).

Frank

Hi Frank

My calculations indicate that the length of "A" (50 ohm coax) should be
close to 48.6 degrees between two identical open stubs with a 50 ohm
termiantion on the 50 ohm line when I use your 11.7 -J21 impedance. So, my
recommendation stands. Try a little shorter "A" if impedance match at 70
MHz is the objective.


I never was an expert on Smith Chart calculations, so I would adopt a
somewhat less precise (and definitely more suck-it-and-see) approach.

(1) Make 'A' a quarterwave at the centre of the FM band. This can be
done by temporarily connecting it as a simple shunt stub, and snipping
for a notch centred at around 98MHz.

(2) Make up stub 'B1' by temporarily connecting it as a simple shunt
stub, and snipping for a notch centred at around 93MHz.

Now, at 70MHz, the frequency response will be rolling off into the 93MHz
notch, and the RLR will be getting rapidly worse. It will be a
capacitive mismatch, so it can be corrected by adding shunt inductance
in parallel with the stub. This can be an actual inductor, or a parallel
stub with its end short circuited.

[The advantage of a stub is that you don't need additional screening. It
can be tuned by pushing a shorting pin through the coax. The short can
later be made permanent.]

So,

(3) Tune the shunt inductor/stub for best match and lowest through loss
at 70MHz.

(4) Make up stub 'B2' by repeating (2) and (3), but for (say) 103MHz.

(5) Connect up the complete filter, with the 'matched' stubs separated
by the quarterwave 'A'.

You should now have a filter with minimal loss and a good match at
70MHz, but with two deep notches at 93 and 103MHz. If you are happy with
the results, finalize any temporary short circuits etc. If you are not
happy, you can still make a few tweaks to the tuning. If all else fails,
it costs virtually nothing to replace a bit of coax which is too short.
--
Ian

coax filter dilemma
 

I never was an expert on Smith Chart calculations, so I would adopt a
somewhat less precise (and definitely more suck-it-and-see) approach.

(1) Make 'A' a quarterwave at the centre of the FM band. This can be done
by temporarily connecting it as a simple shunt stub, and snipping for a
notch centred at around 98MHz.

(2) Make up stub 'B1' by temporarily connecting it as a simple shunt stub,
and snipping for a notch centred at around 93MHz.

Now, at 70MHz, the frequency response will be rolling off into the 93MHz
notch, and the RLR will be getting rapidly worse. It will be a capacitive
mismatch, so it can be corrected by adding shunt inductance in parallel
with the stub. This can be an actual inductor, or a parallel stub with its
end short circuited.

[The advantage of a stub is that you don't need additional screening. It
can be tuned by pushing a shorting pin through the coax. The short can
later be made permanent.]

So,

(3) Tune the shunt inductor/stub for best match and lowest through loss at
70MHz.

(4) Make up stub 'B2' by repeating (2) and (3), but for (say) 103MHz.

(5) Connect up the complete filter, with the 'matched' stubs separated by
the quarterwave 'A'.

You should now have a filter with minimal loss and a good match at 70MHz,
but with two deep notches at 93 and 103MHz. If you are happy with the
results, finalize any temporary short circuits etc. If you are not happy,
you can still make a few tweaks to the tuning. If all else fails, it costs
virtually nothing to replace a bit of coax which is too short.
--
Ian

Interesting Ian, but of course you cheated by adding two extra components.
Just the same, it works exactly as you describe.

In my model I used inductors with a Q of 50. Step 3, above, works out
to 47.6 nH, and for stub "b" the shunt inductor is 62.5 nH. S21 from
88 to 108 MHz is -20 db. S11 from 65 to 75 MHz is - 20 dB.
The insertion loss is a nominal 0.4 db at 70 MHz. I used open ended stubs,
not that it makes any difference to the end result. The series stub is not
very
critical, but response symmetry (S21 65 - 75, and S21 rejection 88 - 108)
appears better. Note b1 = 94.5 degrees, and b2 85.7 degrees for my
open ended stub model. Also the network is perfectly symmetrical; i.e.
S11(f) = S22(f),
and S21(f) = S12(f).

If anybody is interested I can provide JPEG copies of the response,
and schematic, including a Smith chart plot.

73,

Frank

coax filter dilemma
 

On 22 Lug, 15:12, "Frank" wrote:
I never was an expert on Smith Chart calculations, so I would adopt a
somewhat less precise (and definitely more suck-it-and-see) approach.

(1) Make 'A' a quarterwave at the centre of the FM band. This can be done
by temporarily connecting it as a simple shunt stub, and snipping for a
notch centred at around 98MHz.

(2) Make up stub 'B1' by temporarily connecting it as a simple shunt stub,
and snipping for a notch centred at around 93MHz.

Now, at 70MHz, the frequency response will be rolling off into the 93MHz
notch, and the RLR will be getting rapidly worse. It will be a capacitive
mismatch, so it can be corrected by adding shunt inductance in parallel
with the stub. This can be an actual inductor, or a parallel stub with its
end short circuited.

[The advantage of a stub is that you don't need additional screening. It
can be tuned by pushing a shorting pin through the coax. The short can
later be made permanent.]

So,

(3) Tune the shunt inductor/stub for best match and lowest through loss at
70MHz.

(4) Make up stub 'B2' by repeating (2) and (3), but for (say) 103MHz.

(5) Connect up the complete filter, with the 'matched' stubs separated by
the quarterwave 'A'.

You should now have a filter with minimal loss and a good match at 70MHz,
but with two deep notches at 93 and 103MHz. If you are happy with the
results, finalize any temporary short circuits etc. If you are not happy,
you can still make a few tweaks to the tuning. If all else fails, it costs
virtually nothing to replace a bit of coax which is too short.
--
Ian

Interesting Ian, but of course you cheated by adding two extra components.
Just the same, it works exactly as you describe.

In my model I used inductors with a Q of 50. Step 3, above, works out
to 47.6 nH, and for stub "b" the shunt inductor is 62.5 nH. S21 from
88 to 108 MHz is -20 db. S11 from 65 to 75 MHz is - 20 dB.
The insertion loss is a nominal 0.4 db at 70 MHz. I used open ended stubs,
not that it makes any difference to the end result. The series stub is not
very
critical, but response symmetry (S21 65 - 75, and S21 rejection 88 - 108)
appears better. Note b1 = 94.5 degrees, and b2 85.7 degrees for my
open ended stub model. Also the network is perfectly symmetrical; i.e.
S11(f) = S22(f),
and S21(f) = S12(f).

If anybody is interested I can provide JPEG copies of the response,
and schematic, including a Smith chart plot.


I would be interested of course, I tried to come up with an "all coax"
filter and with minimal parts because the thing must live under the
antenna and pass all the power on TX which can be a few hundred watts
one day, that's why I was using 1/2 inch cellflex (have also 7/8 inch
but no connectors for it).
I still don't get why a perfectly reasonable filter on the smith chart
turns out to be trash when realized, I'd really like to learn
something.

73
Francesco IZ8DWF

coax filter dilemma
 

"Ian Jackson" wrote in message
...
In message 8lehk.213$GI.77@trnddc05, Jerry
writes

"Frank" wrote in message
news:_Jchk.905$nu6.310@edtnps83...

"Frank" wrote in message
news:CDchk.904$nu6.498@edtnps83...
What doesn't seem to agree between theory and practice is that
measuring any "b" stub in parallel with the dummy load shows an
impedance of about R=11 and X=11, while on the smith chart this should
be R=11 and X=21 (obviously all at 70 MHz). Why it does measure right
alone and wrong with a coaxial "T" adapter and the dummy load in
parallel?
Of course in real life I'm assuming a Vf=0.88 for the cellflex cable
and checking measures with the analyzer.

What could be wrong?
Any hint is appreciated

PS the single shunt stub of 86.8 degrees calculates to 11.7 - j 21 ohms,
when in parallel with a 50 ohm load (At 70 MHz).

Frank

Hi Frank

My calculations indicate that the length of "A" (50 ohm coax) should be
close to 48.6 degrees between two identical open stubs with a 50 ohm
termiantion on the 50 ohm line when I use your 11.7 -J21 impedance. So,
my
recommendation stands. Try a little shorter "A" if impedance match at 70
MHz is the objective.


I never was an expert on Smith Chart calculations, so I would adopt a
somewhat less precise (and definitely more suck-it-and-see) approach.

(1) Make 'A' a quarterwave at the centre of the FM band. This can be done
by temporarily connecting it as a simple shunt stub, and snipping for a
notch centred at around 98MHz.

(2) Make up stub 'B1' by temporarily connecting it as a simple shunt stub,
and snipping for a notch centred at around 93MHz.

Now, at 70MHz, the frequency response will be rolling off into the 93MHz
notch, and the RLR will be getting rapidly worse. It will be a capacitive
mismatch, so it can be corrected by adding shunt inductance in parallel
with the stub. This can be an actual inductor, or a parallel stub with its
end short circuited.

[The advantage of a stub is that you don't need additional screening. It
can be tuned by pushing a shorting pin through the coax. The short can
later be made permanent.]

So,

(3) Tune the shunt inductor/stub for best match and lowest through loss at
70MHz.

(4) Make up stub 'B2' by repeating (2) and (3), but for (say) 103MHz.

(5) Connect up the complete filter, with the 'matched' stubs separated by
the quarterwave 'A'.

You should now have a filter with minimal loss and a good match at 70MHz,
but with two deep notches at 93 and 103MHz. If you are happy with the
results, finalize any temporary short circuits etc. If you are not happy,
you can still make a few tweaks to the tuning. If all else fails, it costs
virtually nothing to replace a bit of coax which is too short.
--
Ian

Hi Ian

I have had just enough experience with filter design to know that I dont
want to get involved with someone elses choice of which to use.
I am pretty sure that you are as "expert" with Smith Chart use as you care
to be. I make no claim to being a Smith Chart expert. But, I have alot
of experience using them.

I suspect that you know that impedances move along the lines (circles) of
constant R when pure reactance is added in series. And impedances move
along lines of constant conductance when reactance is added in shunt.
The chart with both lines of constant Resistance and constant Conductance
can be made by overlaying a second Smith Chart on the other with the line of
zero reactance aligned and the hi R flipped to overlay the low R of the
other chart.

Any impedance, with a REAL resistance will plot somewhere on the Smith
Chart. The outer boundrie of the Chart is marked in wavelengths. A line
drawn from the chart center thru the plotted impedance intersects the outer
boundry to identify a starting point. When that "load impedance " is seen
thru a length of lossles transmission line, it will plot somewhere on a
circle whoes center is the chart center and which passes thru the plotted
impedance.

In Frank's case, the where the load impedance is 11 -J21, the chart shows
that 0.135 lambda of line length is required to move the load impedance to
where it intersects the circle of constant Admittance, where the second stub
moves the impedance to a good match.

Jerry KD6JDJ

coax filter dilemma
 

wrote in message
...
On 22 Lug, 15:12, "Frank" wrote:
I never was an expert on Smith Chart calculations, so I would adopt a
somewhat less precise (and definitely more suck-it-and-see) approach.

(1) Make 'A' a quarterwave at the centre of the FM band. This can be
done
by temporarily connecting it as a simple shunt stub, and snipping for a
notch centred at around 98MHz.

(2) Make up stub 'B1' by temporarily connecting it as a simple shunt
stub,
and snipping for a notch centred at around 93MHz.

Now, at 70MHz, the frequency response will be rolling off into the
93MHz
notch, and the RLR will be getting rapidly worse. It will be a
capacitive
mismatch, so it can be corrected by adding shunt inductance in parallel
with the stub. This can be an actual inductor, or a parallel stub with
its
end short circuited.

[The advantage of a stub is that you don't need additional screening.
It
can be tuned by pushing a shorting pin through the coax. The short can
later be made permanent.]

So,

(3) Tune the shunt inductor/stub for best match and lowest through loss
at
70MHz.

(4) Make up stub 'B2' by repeating (2) and (3), but for (say) 103MHz.

(5) Connect up the complete filter, with the 'matched' stubs separated
by
the quarterwave 'A'.

You should now have a filter with minimal loss and a good match at
70MHz,
but with two deep notches at 93 and 103MHz. If you are happy with the
results, finalize any temporary short circuits etc. If you are not
happy,
you can still make a few tweaks to the tuning. If all else fails, it
costs
virtually nothing to replace a bit of coax which is too short.
--
Ian

Interesting Ian, but of course you cheated by adding two extra
components.
Just the same, it works exactly as you describe.

In my model I used inductors with a Q of 50. Step 3, above, works out
to 47.6 nH, and for stub "b" the shunt inductor is 62.5 nH. S21 from
88 to 108 MHz is -20 db. S11 from 65 to 75 MHz is - 20 dB.
The insertion loss is a nominal 0.4 db at 70 MHz. I used open ended
stubs,
not that it makes any difference to the end result. The series stub is
not
very
critical, but response symmetry (S21 65 - 75, and S21 rejection 88 - 108)
appears better. Note b1 = 94.5 degrees, and b2 85.7 degrees for my
open ended stub model. Also the network is perfectly symmetrical; i.e.
S11(f) = S22(f),
and S21(f) = S12(f).

If anybody is interested I can provide JPEG copies of the response,
and schematic, including a Smith chart plot.


I would be interested of course, I tried to come up with an "all coax"
filter and with minimal parts because the thing must live under the
antenna and pass all the power on TX which can be a few hundred watts
one day, that's why I was using 1/2 inch cellflex (have also 7/8 inch
but no connectors for it).
I still don't get why a perfectly reasonable filter on the smith chart
turns out to be trash when realized, I'd really like to learn
something.

73
Francesco IZ8DWF

Hi Frank

I am confident that you chose open ended stubs for good reason. Open
circuit stubs behave very unpredictably due to their sensitivity to their
environment. They radiate. Can you consider using longer stubs, and
making them shorted?

Jerry KD6JDJ