On 22 Lug, 16:40, "Jerry" wrote:
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?

Yes, I considered that, however shorted stubs mean 1/2 wl at 98 MHz or
so, and they become more than 1m long, making the filter not that easy
to fit under the antenna, but who knows, maybe I'll be forced to
abandon open stubs if nothing works.

Thanks for all inputs

73
Francesco IZ8DWF

On Tue, 22 Jul 2008 09:04:23 -0700 (PDT),
wrote:

Open circuit stubs behave very unpredictably due to their sensitivity to their
environment. They radiate.

Yes, I considered that, however shorted stubs mean 1/2 wl at 98 MHz or
so, and they become more than 1m long, making the filter not that easy
to fit under the antenna,

Hi Francesco,

The point made about unpredictable open ends is found in their being
prone to common mode currents. You fix this through choking. Choking
can be achieved through coiling the stub. One, in a classical sense
(usually arrived at through examples oriented in microwave), does not
think of stubs coiled up; but coiling is a suitable, physical and
electrical solution. However, the open end is still prone to
environmental issues (moisture, bugs....). A shorted stub is
self-shielding, keeps out the environment, and coiling it up removes
the "too long" objection.

73's
Richard Clark, KB7QHC

On Jul 21, 11:37 am, wrote:
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

First, I apologize that I don't have time to look in detail at the
following ten postings in this thread; I did look over them quickly.
But two quick comments: First, I believe the "A" length should be
about 44 electrical degrees at 70MHz to get a 50 ohm match at 70MHz.
Second, if I were doing this and wanted to seriously attenuate
88-108MHz, I'd make a small L-C filter. At these frequencies, such a
filter should be much more compact than a coax stub filter for some
given performance. Of course, if there were specific frequencies I
wanted to get rid of, I'd put zeros on them. Coaxial cavities are
nice if you need very high Q around 100MHz, but stubs of even half-
inch coax are not particularly high Q--expect Qu about 400 at 70MHz.
A 47.7nH coil will give you your desired X=21 ohms at 70MHz, and you
can get that inductance with about three turns of #10 AWG (2.5mm),
spaced two wire diameters center to center, at about 9mm ID, and the Q
will be just about the same as the coax (in obviously a much smaller
space). But even better, a filter with max 1dB attenuation from
68-72MHz and min 55dB attenuation above 88Mhz (close to 100dB at
98MHz) can be built with four small coils and some C0G capacitors in a
shielded volume perhaps 4cm x 4cm x 15cm. (Schematic is available...)

Cheers,
Tom

On Jul 22, 10:41 am, K7ITM wrote:
....
wanted to get rid of, I'd put zeros on them. Coaxial cavities are
nice if you need very high Q around 100MHz, but stubs of even half-
inch coax are not particularly high Q--expect Qu about 400 at 70MHz.
....
I gave the Qu about 400 number based on 1/2" ID of the outer
conductor, but I guess 1/2" CellFlex is actually about 0.33" ID of the
outer. Based on that, or based on the listed attenuation specs for
the line in this frequency range, I'd have to revise my Qu estimate
down to no more than 300.

Cheers,
Tom

In message
,
writes

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?

Yes, I considered that, however shorted stubs mean 1/2 wl at 98 MHz or
so, and they become more than 1m long, making the filter not that easy
to fit under the antenna, but who knows, maybe I'll be forced to
abandon open stubs if nothing works.

Thanks for all inputs

73
Francesco IZ8DWF

Francesco, before you go, may I ask what is the purpose of the filter?
Is it to prevent the local FM stations interfering with reception? Or is
it to stop any unexpected transmitter spurious signals from being
transmitted in the FM band?
--
Ian

On 22 Lug, 21:49, Ian Jackson
wrote:
In message
,
writes





Francesco, before you go, may I ask what is the purpose of the filter?
Is it to prevent the local FM stations interfering with reception? Or is
it to stop any unexpected transmitter spurious signals from being
transmitted in the FM band?

It is to prevent desensing from strong local FM stations. Noise can go
up to S7 when beaming the local transmitters. An LC filter could be
easily made but that would need to be switched out when in TX (or made
to pass a few hundred watts), I just thougth a couple of stubs could
be easier to made. I think I was wrong.

73
Francesco IZ8DWF

Francesco, before you go, may I ask what is the purpose of the filter?
Is it to prevent the local FM stations interfering with reception? Or is
it to stop any unexpected transmitter spurious signals from being
transmitted in the FM band?

It is to prevent desensing from strong local FM stations. Noise can go
up to S7 when beaming the local transmitters. An LC filter could be
easily made but that would need to be switched out when in TX (or made
to pass a few hundred watts), I just thougth a couple of stubs could
be easier to made. I think I was wrong.

73
Francesco IZ8DWF

Francesco, You can build a notch filter with two stubs, and get a good
match, and rejection. The velocity factor of Cellflex 1/2" is 0.82, as per:
http://www2.rfsworld.com/RFS_Edition...able_30-46.pdf
The lengths of the shunt, open stubs, are reasonably critical within +/-
1cm.

I have used a Genesys optimization program with the physical parameters of
the Cellflex coax. The open shunt stub lengths are 63 cm. The series
stub is 41.3 cm. The insertion loss at 70 MHz is 0.4 db. The return
loss
at 70 MHz is 20 db. Rejection over the whole FM band is 20 db.

Using the Smith Chart to design a filter is tricky, since you are dealing
with
multiple frequencies, and three variables.

Let me know if you would like to see my results, will be glad to e-mail them
as JPEGs.

73,

Frank (VE6CB)

On 23 Lug, 02:50, "Frank" wrote:

Francesco, You can build a notch filter with two stubs, and get a good
match, and rejection. The velocity factor of Cellflex 1/2" is 0.82, as per:http://www2.rfsworld.com/RFS_Edition...able_30-46.pdf
The lengths of the shunt, open stubs, are reasonably critical within +/-
1cm.

I have used a Genesys optimization program with the physical parameters of
the Cellflex coax. The open shunt stub lengths are 63 cm. The series
stub is 41.3 cm. The insertion loss at 70 MHz is 0.4 db. The return
loss
at 70 MHz is 20 db. Rejection over the whole FM band is 20 db.


ok, with cut and try arrived at similar results, shunts are 64 cm and
series is 47 cm, the antenna analyzer is happy, I'm not so happy
because I don't understand where's my original error.

Using the Smith Chart to design a filter is tricky, since you are dealing
with
multiple frequencies, and three variables.

well, yes, but actually the design started with the shorts at 98 MHz
which are easy, then the only unknown variable becomes the series stub
that gives a 50 ohm match at 70 MHz, I still don't get why the
matching section appear to be 3/8 wl on a smith chart and actually is
1/8 like if I inverted the sense of load/generator (yes, I worked with
admittances for shunt stubs).


Let me know if you would like to see my results, will be glad to e-mail them
as JPEGs.

sure, my email address is valid, thank you very much.

73
Francesco IZ8DWF

In message
,
writes
On 22 Lug, 21:49, Ian Jackson
wrote:
In message
,
writes





Francesco, before you go, may I ask what is the purpose of the filter?
Is it to prevent the local FM stations interfering with reception? Or is
it to stop any unexpected transmitter spurious signals from being
transmitted in the FM band?

It is to prevent desensing from strong local FM stations. Noise can go
up to S7 when beaming the local transmitters. An LC filter could be
easily made but that would need to be switched out when in TX (or made
to pass a few hundred watts), I just thougth a couple of stubs could
be easier to made. I think I was wrong.


No, you ARE correct. For non-experts like me, simple stub filters are
often very easy to make. This is especially true if you need to pass
only one frequency, and reject another. The one thing that you must do
at the 'pass' frequency is to tune out the unwanted reactance of the
rejection stub by adding a shunt inductor (where the 'stop' frequency is
on the HF side of the pass) or a shunt capacitor (where the 'stop'
frequency is on the LF side of the pass).

It helps if you should have (or have assess to) some test equipment
which shows you a swept frequency response and impedance match, but you
can also tune them using individual frequency measurements.

One great advantage of coax stub filters is that, unlike LC filters, you
do not need to screen each section. You can coil up the coax neatly, and
secure it with cable ties. However, it is sometimes a good idea to
ensure that the coax outers are completely 'RF dead' by bonding one to
another at several places (ie more than simply where one piece of coax
is connected to another). Of course, you can put the whole filter in a
metal box, and make sure that coax braid is connected to the box in
several places (and especially at the open end of the shunt stubs).

Of course, you can also make 'hybrid' filters which make use of the best
features of LC circuits and stub circuits. I made such a filter for use
with an FM CB radio which I had modified for use at the HF end of 10m. I
found that I was still radiating (and receiving) some signal on the
original CB band. The only practical way to eliminate this problem was
to use an external filter which rejected 27 to 28MHz, but passed 29.3 to
29.7MHz. I did this by using two LC 'suck-blow' (stop-pass) filters in
individual metal boxes, separated by a quarterwave of coax (coiled up,
of course).

On the internet, there is quite a lot of information about coaxial stub
filters (try a Google search). Anyway, may I wish you success, whatever
filter you choose to use!
--
Ian

well, yes, but actually the design started with the shorts at 98 MHz
which are easy, then the only unknown variable becomes the series stub
that gives a 50 ohm match at 70 MHz, I still don't get why the
matching section appear to be 3/8 wl on a smith chart and actually is
1/8 like if I inverted the sense of load/generator (yes, I worked with
admittances for shunt stubs).


Let me know if you would like to see my results, will be glad to e-mail
them
as JPEGs.

sure, my email address is valid, thank you very much.

73
Francesco IZ8DWF

Francesco, I will be e-mailing my results for your interest. We seem to
have arrived at the same conclusions.

I think I understand where you are confused. Initially, starting at the
center of the Smith Chart, the shunt open stub moves down, along the
constant conductance circle (0.2S). At 0.175 WL at 70 MHz (0.25
WL at 98 MHz) - 62 cm, the input impedance is 10 - j 20 ohms.
Next; the series section of transmission line moves clockwise along
the 10 ohm resistance circle. At 0.125 WL (70 MHz) - 44 cm, the impedance
reaches 10 + j 20. The final shunt section then moves the impedance
to the center of the Smith Chart along the 0.2 S circle.

73,

Frank (VE6CB)