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 |
coax filter dilemma
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 |
coax filter dilemma
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coax filter dilemma
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 |
coax filter dilemma
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 |
coax filter dilemma
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coax filter dilemma
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 |
coax filter dilemma
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) |
coax filter dilemma
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 |
coax filter dilemma
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coax filter dilemma
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) |
coax filter dilemma
"Frank" wrote in message news:HMKhk.1277$nu6.889@edtnps83... 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) Hi Frank When describing the path on the Smith Chart from the "load" Z to the "rig" Z, you write "along the 10 ohm resistance circle". I would have refered to that circle as the circle of constant VSWR. Jerry KD6JDJ |
coax filter dilemma
Hi Frank
When describing the path on the Smith Chart from the "load" Z to the "rig" Z, you write "along the 10 ohm resistance circle". I would have refered to that circle as the circle of constant VSWR. Jerry KD6JDJ Of course you are correct Jerry. I realized what I had done after I posted the comments. Frank, VE6CB |
coax filter dilemma
"Frank" wrote in message news:RH1ik.1379$%b7.1159@edtnps82... Hi Frank When describing the path on the Smith Chart from the "load" Z to the "rig" Z, you write "along the 10 ohm resistance circle". I would have refered to that circle as the circle of constant VSWR. Jerry KD6JDJ Of course you are correct Jerry. I realized what I had done after I posted the comments. Frank, VE6CB Hi Frank My reply was delayed because I had anticipated that someone might ask you for clarification on Smith Chart use. I was sure you'd clear up the confusion. I even thought I might learn something new. Smith Chart use is so valuable to my thinking that I just couldnt let this thread finish without checking in with you. Jerry KD6JDJ |
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