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70cm reflectometer?
Can anyone point me to a website where a home built 70 cm reflectometer or
70 cm field strength meter can be built -- not using surface mount components. Thanks. |
70cm reflectometer?
On Jan 28, 1:52 pm, "Suzy" not@valid wrote:
Can anyone point me to a website where a home built 70 cm reflectometer or 70 cm field strength meter can be built -- not using surface mount components. Thanks. Are you willing to use a printed circuit board? If so, a directional coupler is pretty easy to make, microstrip style. You can even use a knife to score the copper and then peel away the unwanted copper. It's common to have one through-line and two coupled lines, one for "forward" and one for "reverse" measurements. You terminate each coupled line with Z0 (50 ohms, commonly) at one end and put a diode rectifier (or other power measuring device) at the other end. Although it's probably easiest to get a pretty good 50 ohm termination at 450MHz using surface mount parts, you can do OK with a couple 1/4 watt metal-film axial resistors. If you used 1206 size surface mount resistors, you should be able to solder them down with a decent standard iron -- they aren't all that small. If you really can't find any construction articles on making one, maybe I could put something together and post it somewhere. They're far easier to make than many people seem to think. Cheers, Tom |
70cm reflectometer?
K7ITM wrote: On Jan 28, 1:52 pm, "Suzy" not@valid wrote: Can anyone point me to a website where a home built 70 cm reflectometer or 70 cm field strength meter can be built -- not using surface mount components. Thanks. Are you willing to use a printed circuit board? If so, a directional coupler is pretty easy to make, microstrip style. You can even use a knife to score the copper and then peel away the unwanted copper. It's common to have one through-line and two coupled lines, one for "forward" and one for "reverse" measurements. You terminate each coupled line with Z0 (50 ohms, commonly) at one end and put a diode rectifier (or other power measuring device) at the other end. Although it's probably easiest to get a pretty good 50 ohm termination at 450MHz using surface mount parts, you can do OK with a couple 1/4 watt metal-film axial resistors. If you used 1206 size surface mount resistors, you should be able to solder them down with a decent standard iron -- they aren't all that small. If you really can't find any construction articles on making one, maybe I could put something together and post it somewhere. They're far easier to make than many people seem to think. Cheers, Tom Don't let the surface mount parts intimidate you. Its not that difficult especially when you are doing just a few componets. I must admit that I had a heck of a headache after building a little microphone preamp. I glue the part I am going to install to a toothpick to give me something to hold it with. Jimmie |
70cm reflectometer?
"K7ITM" wrote in message ... On Jan 28, 1:52 pm, "Suzy" not@valid wrote: Can anyone point me to a website where a home built 70 cm reflectometer or 70 cm field strength meter can be built -- not using surface mount components. Thanks. Are you willing to use a printed circuit board? If so, a directional coupler is pretty easy to make, microstrip style. You can even use a knife to score the copper and then peel away the unwanted copper. It's common to have one through-line and two coupled lines, one for "forward" and one for "reverse" measurements. You terminate each coupled line with Z0 (50 ohms, commonly) at one end and put a diode rectifier (or other power measuring device) at the other end. Although it's probably easiest to get a pretty good 50 ohm termination at 450MHz using surface mount parts, you can do OK with a couple 1/4 watt metal-film axial resistors. If you used 1206 size surface mount resistors, you should be able to solder them down with a decent standard iron -- they aren't all that small. If you really can't find any construction articles on making one, maybe I could put something together and post it somewhere. They're far easier to make than many people seem to think. Cheers, Tom Thanks very much Tom. Yes I'm certainly OK with PCBs and I was aware of the basic circuitry. But I felt that the stripline would have to be special in some way for the 435 MHz that I need. Perhaps a special length? |
70cm reflectometer?
"Suzy" not@valid wrote in message ...
Can anyone point me to a website where a home built 70 cm reflectometer or 70 cm field strength meter can be built -- not using surface mount components. Thanks. How about an SWR meter with no surface mount components? See: http://www.qsl.net/xq2fod/Electron/swr/swr.html Have fun! John |
70cm reflectometer?
"John KD5YI" wrote in message news:jFOnj.10273$fs4.5137@trnddc02... "Suzy" not@valid wrote in message ... Can anyone point me to a website where a home built 70 cm reflectometer or 70 cm field strength meter can be built -- not using surface mount components. Thanks. How about an SWR meter with no surface mount components? See: http://www.qsl.net/xq2fod/Electron/swr/swr.html Have fun! John Thanks for that. I really want to emulate a throughline (like the Bird 43 I once had) and am thinking more in the terms of a stripline on PCB (using BNC), with parallel lines to sniff the RF and diodes to convert for the meters. But what I'm usure of is the sort of dimensions I should use for 70 cms, and wther these are critical. How long should the stripline be, what spacing and so on, so that it doesn't in itself introduce errors and misleading results. The sort of thing I would imagine would be in the ARRL handbook, but I want to avoid the expense of buying the book if it's not in there at all. BTW, most of the designs I've seen are HF or for up to 2 meters at the highest. I specifically want 70 cms, 435 MHz to be exact. |
70cm reflectometer?
Suzy wrote:
I specifically want 70 cms, 435 MHz to be exact. I have a Micronta SWR/POWER METER speced for 440 MHz. I suspect it uses a small ferrite core rated for 440 MHz for the current pickup and a capacitive divider for voltage. Seems all you need is a ferrite core rated up to 440 MHz and you would be in business. Seems to me that even a ferrite core not rated for 440 MHz could be used and calibrated on a one to one basis. -- 73, Cecil http://www.w5dxp.com |
70cm reflectometer?
On Jan 29, 10:23 am, "Suzy" not@valid wrote:
"K7ITM" wrote in message ... On Jan 28, 1:52 pm, "Suzy" not@valid wrote: Can anyone point me to a website where a home built 70 cm reflectometer or 70 cm field strength meter can be built -- not using surface mount components. Thanks. Are you willing to use a printed circuit board? If so, a directional coupler is pretty easy to make, microstrip style. You can even use a knife to score the copper and then peel away the unwanted copper. It's common to have one through-line and two coupled lines, one for "forward" and one for "reverse" measurements. You terminate each coupled line with Z0 (50 ohms, commonly) at one end and put a diode rectifier (or other power measuring device) at the other end. Although it's probably easiest to get a pretty good 50 ohm termination at 450MHz using surface mount parts, you can do OK with a couple 1/4 watt metal-film axial resistors. If you used 1206 size surface mount resistors, you should be able to solder them down with a decent standard iron -- they aren't all that small. If you really can't find any construction articles on making one, maybe I could put something together and post it somewhere. They're far easier to make than many people seem to think. Cheers, Tom Thanks very much Tom. Yes I'm certainly OK with PCBs and I was aware of the basic circuitry. But I felt that the stripline would have to be special in some way for the 435 MHz that I need. Perhaps a special length? The stripline (microstrip, really, if it's on one side with groundplane on the other, which is the easiest to do) can be almost any length. It will have zero coupling if it's an electrical halfwave or integral multiple of a halfwave, but otherwise, it will work. It will have the maximum coupling if it's an electrical quarter wave long, and the change in coupling versus frequency is minimum at that length, but it's usable even if it is very short. I'm using one that's about half an inch long down as low as 1MHz; the directionality is fine, and the coupling is simply very small. You can use the free- to-download RFSim99 program to get a microstrip design for a particular coupling, but what the program doesn't tell you is that the coupling they list is for a coupler an electrical quarter wave long. To a pretty close approximation, the coupling will go as the sine of the electrical length, so for example if you use the design for a 20dB coupler but make it only 1/8 wave long, the response will be about sin(45 degrees) or .707 times as much, or 3dB lower: it will be about a 23dB coupler at that length. That means the coupled port power will be 23dB lower than the input power, so if you had 10 watts going in, the coupled port would receive about 0.05 watts, which is 1.6V rms. That likely is a bit more than you need to drive a diode detector. There can be some advantage to keeping the voltage low enough, if you're using a diode detector, so that the detector is in the "square law" region: DC output _voltage_ is proportional to RF input _power_. On normal FR4 board stock 1/16" thick, leaving one side as a ground plane, a pair of traces 1/8" wide, separated by a 1/8" space, will give you about a 25dB coupler and pretty close to 50 ohms. It does depend on the particular dielectric constant of your FR4 substrate, which varies from manufacturer to manufacturer and by particular material type. In that microstrip environment, the wavelength of 450MHz signals will be on the order of 40cm, or about 4 inches for 1/4 wavelength, so you can see that your coupler doesn't have to be very long, physically. If you don't mind working with SMA connectors, you can get SMA jacks that mount directly on the edge of a 1/16" thick PC board, and that makes it very easy to make a connectorized version of a coupler. You can get SMA-to-BNC (or to other series) adapters--mpja.com have them at reasonable prices, for example--to get to other environments. If I wanted a coupler for a power/SWR meter for 450MHz, and assuming I was going to use Schottky diode detectors, I'd first think about the range of powers I wanted to handle, then select a coupling to give me perhaps 0.2V RMS at max power at the coupled port. If space permitted, I'd make the coupler an electrical quarter wave long, since its sensitivity would then be least affected by frequency (the very flat top of the sine curve). I'd use a DVM to read out the detected DC voltage (and make sure my DVM or digital panel meter or whatever wasn't affected by the 450MHz signal directly!). A 200mV full-scale 3.5 digit meter would then cover to very low SWR at full scale, and to fairly low power. The one other thing I'd want to do is insure that the meter--the directional coupler part, specifically--was optimized to 50 ohms, which would require a known good 50 ohm (assuming I wanted it to be 50 ohms) load to test. Likely I'd make some pretty accurate measurements of the board substrate thickness and relative dielectric constant before starting, so I got the trace widths and spacings right from the start. It's possible to trim the coupler up in impedance by carefully narrowing the traces, and down by adding a grounded plane above the board--adjusting the spacing to trim the impedance. Be aware that each of those will also change the coupling slightly, however. Hope that helps some! Cheers, Tom |
70cm reflectometer?
"Suzy" not@valid wrote in :
.... Thanks for that. I really want to emulate a throughline (like the Bird 43 I once had) and am thinking more in the terms of a stripline on PCB (using BNC), with parallel lines to sniff the RF and diodes to convert for the meters. But what I'm usure of is the sort of dimensions I should use for 70 cms, and wther these are critical. How long should If you make the coupling line very short wrt wavelength, you can analyse it with a lumped constant approximation. You can think of the coupled line as located in the electric field of the main line, and it will have a voltage difference to ground. Similarly, the coupled line will be cut by magnetic flux due the the current in the main line, and so a voltage will be induced end to end in the coupled line. By adjusting the resistor at one end of the coupled line, you adjust the contibution of these current and voltage derived samples, and can adjust them so balance each other (ie no meter deflection) when V/I on the main line is 50. When the coupled line is short, the characteristic impedance of the coupled line is not very critical. You should be able to achieve sufficient sensitivity for 5W pwr on 70cm with 20mm of coupled line. The Zo of the though line is more important, it is the main determinant of the insertion VSWR of the instrument, so you need to strive to achieve close to the desired Zo, presumably 50 ohms. Next, don't put the coupled line to close as it will load the main line and degrade the insertion VSWR. Then adjust the R at the end of the coupled line to null the DC output on a 50 ohm load. Repeat for the other coupled line (if you use one). Check for symmetry, ie that reversing the instrument gives exactly the same readings on the dummy load. Tom suggested Schottky diodes. Dick Smith has 1N5711 Shottky diodes (at exhorbitant prices), or you could get 1N34 germainium diodes from Jaycar. BTW, you did mention the Bird 43, the above is not frequency compensated like the Bird slugs, deflection will be frequency dependent. Owen PS: I have only met one VK YL ham named Susan, I sometimes wonder what happened to her, haven't heard her on air in decades. |
70cm reflectometer?
On Jan 29, 6:01 pm, Owen Duffy wrote:
"Suzy" not@valid wrote : ... Thanks for that. I really want to emulate a throughline (like the Bird 43 I once had) and am thinking more in the terms of a stripline on PCB (using BNC), with parallel lines to sniff the RF and diodes to convert for the meters. But what I'm usure of is the sort of dimensions I should use for 70 cms, and wther these are critical. How long should If you make the coupling line very short wrt wavelength, you can analyse it with a lumped constant approximation. You can think of the coupled line as located in the electric field of the main line, and it will have a voltage difference to ground. Similarly, the coupled line will be cut by magnetic flux due the the current in the main line, and so a voltage will be induced end to end in the coupled line. By adjusting the resistor at one end of the coupled line, you adjust the contibution of these current and voltage derived samples, and can adjust them so balance each other (ie no meter deflection) when V/I on the main line is 50. When the coupled line is short, the characteristic impedance of the coupled line is not very critical. You should be able to achieve sufficient sensitivity for 5W pwr on 70cm with 20mm of coupled line. The Zo of the though line is more important, it is the main determinant of the insertion VSWR of the instrument, so you need to strive to achieve close to the desired Zo, presumably 50 ohms. Next, don't put the coupled line to close as it will load the main line and degrade the insertion VSWR. Then adjust the R at the end of the coupled line to null the DC output on a 50 ohm load. Repeat for the other coupled line (if you use one). Check for symmetry, ie that reversing the instrument gives exactly the same readings on the dummy load. Tom suggested Schottky diodes. Dick Smith has 1N5711 Shottky diodes (at exhorbitant prices), or you could get 1N34 germainium diodes from Jaycar. BTW, you did mention the Bird 43, the above is not frequency compensated like the Bird slugs, deflection will be frequency dependent. Owen PS: I have only met one VK YL ham named Susan, I sometimes wonder what happened to her, haven't heard her on air in decades. Owen's comments reminded me that I always used to think of coupled- line hybrids as sampling the magnetic and electric fields at a point, and of course you can adjust the ratio by adjusting the load as he suggests. But then something began nagging me: his example of a 20mm (2cm) line isn't exactly short compared with a "full length" quarter wave coupled line. Because of the relatively slow propagation in stripline over FR4 PC board material, a quarter wave is only about 10 cm for 450MHz signals. But thinking of it in terms of a distributed line system, you're just terminating the coupled line in the proper impedance to not get reflections off that end; so this same thing works even with full 1/4 wave lines to adjust the directivity. However, if you have one through line and one coupled line, and you've made them very symmetric so you can swap the two and not see a difference, then if you have to terminate the coupled line in other than 50 ohms to get perfect directivity (no reflection from that port), it also says that the through line is not 50 ohms. As Owen points out, you'd really like that through line to look like 50 ohms (or other system Z0 if you wish), so it doesn't disturb the system it's installed in. There's the incentive to make the coupler symmetrical and tuned so the coupled line terminates properly in 50 ohms, to get best directivity. I'll readily admit that the details of coupled lines from a fields perspective is a bit beyond my full understanding, so there may be an error in my thinking about this, but I believe it's pretty accurate. The thing that comes first to my mind is that for stripline, the propagation velocity for even and odd modes is different, but still, I think if the lines are lightly coupled, the paragraphs I wrote above are a valid way to look at the situation. Geez, Schottky diodes should be dirt cheap these days. It's germanium that are hard to find around here, unless they are old stock. Cheers, Tom |
70cm reflectometer?
"K7ITM" wrote in message ... On Jan 29, 6:01 pm, Owen Duffy wrote: "Suzy" not@valid wrote : ... Thanks for that. I really want to emulate a throughline (like the Bird 43 I once had) and am thinking more in the terms of a stripline on PCB (using BNC), with parallel lines to sniff the RF and diodes to convert for the meters. But what I'm usure of is the sort of dimensions I should use for 70 cms, and wther these are critical. How long should If you make the coupling line very short wrt wavelength, you can analyse it with a lumped constant approximation. You can think of the coupled line as located in the electric field of the main line, and it will have a voltage difference to ground. Similarly, the coupled line will be cut by magnetic flux due the the current in the main line, and so a voltage will be induced end to end in the coupled line. By adjusting the resistor at one end of the coupled line, you adjust the contibution of these current and voltage derived samples, and can adjust them so balance each other (ie no meter deflection) when V/I on the main line is 50. When the coupled line is short, the characteristic impedance of the coupled line is not very critical. You should be able to achieve sufficient sensitivity for 5W pwr on 70cm with 20mm of coupled line. The Zo of the though line is more important, it is the main determinant of the insertion VSWR of the instrument, so you need to strive to achieve close to the desired Zo, presumably 50 ohms. Next, don't put the coupled line to close as it will load the main line and degrade the insertion VSWR. Then adjust the R at the end of the coupled line to null the DC output on a 50 ohm load. Repeat for the other coupled line (if you use one). Check for symmetry, ie that reversing the instrument gives exactly the same readings on the dummy load. Tom suggested Schottky diodes. Dick Smith has 1N5711 Shottky diodes (at exhorbitant prices), or you could get 1N34 germainium diodes from Jaycar. BTW, you did mention the Bird 43, the above is not frequency compensated like the Bird slugs, deflection will be frequency dependent. Owen PS: I have only met one VK YL ham named Susan, I sometimes wonder what happened to her, haven't heard her on air in decades. Owen's comments reminded me that I always used to think of coupled- line hybrids as sampling the magnetic and electric fields at a point, and of course you can adjust the ratio by adjusting the load as he suggests. But then something began nagging me: his example of a 20mm (2cm) line isn't exactly short compared with a "full length" quarter wave coupled line. Because of the relatively slow propagation in stripline over FR4 PC board material, a quarter wave is only about 10 cm for 450MHz signals. But thinking of it in terms of a distributed line system, you're just terminating the coupled line in the proper impedance to not get reflections off that end; so this same thing works even with full 1/4 wave lines to adjust the directivity. However, if you have one through line and one coupled line, and you've made them very symmetric so you can swap the two and not see a difference, then if you have to terminate the coupled line in other than 50 ohms to get perfect directivity (no reflection from that port), it also says that the through line is not 50 ohms. As Owen points out, you'd really like that through line to look like 50 ohms (or other system Z0 if you wish), so it doesn't disturb the system it's installed in. There's the incentive to make the coupler symmetrical and tuned so the coupled line terminates properly in 50 ohms, to get best directivity. I'll readily admit that the details of coupled lines from a fields perspective is a bit beyond my full understanding, so there may be an error in my thinking about this, but I believe it's pretty accurate. The thing that comes first to my mind is that for stripline, the propagation velocity for even and odd modes is different, but still, I think if the lines are lightly coupled, the paragraphs I wrote above are a valid way to look at the situation. Geez, Schottky diodes should be dirt cheap these days. It's germanium that are hard to find around here, unless they are old stock. Cheers, Tom I'd like to thank all you kind fellows for your assistance, but unfortunately a lot of it is rather above my head. I need to be pointed at an article that spells out exactly what dimensions, how to build etc. (eg is there one in the ARRL handbook?) And Owen, no in this case Suzy if not short for Susan, so I'm not the person you knew. |
70cm reflectometer?
On Jan 30, 9:57 am, "Suzy" not@valid wrote:
.... I'd like to thank all you kind fellows for your assistance, but unfortunately a lot of it is rather above my head. I need to be pointed at an article that spells out exactly what dimensions, how to build etc. (eg is there one in the ARRL handbook?) And Owen, no in this case Suzy if not short for Susan, so I'm not the person you knew. OK, back to the first posting I made here. How about if I build one, test it, and post the design and results somewhere? As I noted in one of my postings, if I were to make one, I'd first consider how much power I wanted to read, full scale, so if my offer is appealing, let me know how much power you want to measure, max. Don't tell me a kilowatt if you really are going to use it at 10 watts, because if I design for a kilowatt, 10 watts will be low enough that you won't be able to read it very well. In fact, I'd propose 10 watts as a reasonable full scale for a lot of ham uses. Cheers, Tom |
70cm reflectometer?
On Jan 29, 4:26*pm, K7ITM wrote:
... To a pretty close approximation, the coupling will go as the sine of the electrical length, so for example if you use the design for a 20dB coupler but make it only 1/8 wave long, the response will be about sin(45 degrees) or .707 times as much, or 3dB lower: *it will be about a 23dB coupler at that length. * Actually, the sine approximation isn't all that good. Since it's not been easy for me to find on the web and others may have similar trouble finding it, I'll reproduce a better equation he coupling = (Zeven-Zodd)*sin(theta) / [2*Z0*cos(theta) + j*(Zeven +Zodd)*sin(theta)] whe Zeven and Zodd are the even-mode and odd-mode impedances of the coupled line pair: Zeven is the impedance seen when testing one line of the pair, when both lines are driven by the same voltage. Zodd is the impedance seen when testing one line of the pair, when the two lines are driven by equal-amplitude but 180 degree out of phase voltages. Z0 = sqrt(Zeven*Zodd) theta = electrical length of the coupled line section To find the coupling in dB, just take 20*log10(coupling). You can get the even and odd mode impedances from a program like altc, available at http://altc.sourceforge.net/. You may also be able to find formulas that give them to you for common structures like stripline and microstrip just from the dimensions and relative dielectric constant of the substrate. In the hopes this may be valuable info for one or more of the lurkers... Cheers, Tom |
70cm reflectometer?
"K7ITM" wrote in message ... On Jan 30, 9:57 am, "Suzy" not@valid wrote: ... I'd like to thank all you kind fellows for your assistance, but unfortunately a lot of it is rather above my head. I need to be pointed at an article that spells out exactly what dimensions, how to build etc. (eg is there one in the ARRL handbook?) And Owen, no in this case Suzy if not short for Susan, so I'm not the person you knew. OK, back to the first posting I made here. How about if I build one, test it, and post the design and results somewhere? As I noted in one of my postings, if I were to make one, I'd first consider how much power I wanted to read, full scale, so if my offer is appealing, let me know how much power you want to measure, max. Don't tell me a kilowatt if you really are going to use it at 10 watts, because if I design for a kilowatt, 10 watts will be low enough that you won't be able to read it very well. In fact, I'd propose 10 watts as a reasonable full scale for a lot of ham uses. Cheers, Tom Hi Tom That's a kind offer, but you needn't go to all the trouble of building it. Just a pointer at the practical design will do (but no complex theory -- over my head!). I want to investigate various 70 cm antennas (central frequency in Australia 435 MHz). TX is switchable 5 10 20 watts. I want to standardise on BNC, and have readouts on analogue meters (probably 1 mA movements) |
70cm reflectometer?
"Suzy" not@valid wrote in message ... "K7ITM" wrote in message ... On Jan 30, 9:57 am, "Suzy" not@valid wrote: ... I'd like to thank all you kind fellows for your assistance, but unfortunately a lot of it is rather above my head. I need to be pointed at an article that spells out exactly what dimensions, how to build etc. (eg is there one in the ARRL handbook?) And Owen, no in this case Suzy if not short for Susan, so I'm not the person you knew. OK, back to the first posting I made here. How about if I build one, test it, and post the design and results somewhere? As I noted in one of my postings, if I were to make one, I'd first consider how much power I wanted to read, full scale, so if my offer is appealing, let me know how much power you want to measure, max. Don't tell me a kilowatt if you really are going to use it at 10 watts, because if I design for a kilowatt, 10 watts will be low enough that you won't be able to read it very well. In fact, I'd propose 10 watts as a reasonable full scale for a lot of ham uses. Cheers, Tom Hi Tom That's a kind offer, but you needn't go to all the trouble of building it. Just a pointer at the practical design will do (but no complex theory -- over my head!). I want to investigate various 70 cm antennas (central frequency in Australia 435 MHz). TX is switchable 5 10 20 watts. I want to standardise on BNC, and have readouts on analogue meters (probably 1 mA movements) Oh, and no surface mount components. Eyes not good enough! |
70cm reflectometer?
On Jan 30, 2:08 pm, "Suzy" not@valid wrote:
"K7ITM" wrote in message ... On Jan 30, 9:57 am, "Suzy" not@valid wrote: ... I'd like to thank all you kind fellows for your assistance, but unfortunately a lot of it is rather above my head. I need to be pointed at an article that spells out exactly what dimensions, how to build etc. (eg is there one in the ARRL handbook?) And Owen, no in this case Suzy if not short for Susan, so I'm not the person you knew. OK, back to the first posting I made here. How about if I build one, test it, and post the design and results somewhere? As I noted in one of my postings, if I were to make one, I'd first consider how much power I wanted to read, full scale, so if my offer is appealing, let me know how much power you want to measure, max. Don't tell me a kilowatt if you really are going to use it at 10 watts, because if I design for a kilowatt, 10 watts will be low enough that you won't be able to read it very well. In fact, I'd propose 10 watts as a reasonable full scale for a lot of ham uses. Cheers, Tom Hi Tom That's a kind offer, but you needn't go to all the trouble of building it. Just a pointer at the practical design will do (but no complex theory -- over my head!). I want to investigate various 70 cm antennas (central frequency in Australia 435 MHz). TX is switchable 5 10 20 watts. I want to standardise on BNC, and have readouts on analogue meters (probably 1 mA movements) OK, I gave this some thought last night. I see a couple problems... Though you could use 1mA meter movements, that puts you at a detected power level high enough that the meter scale won't be linear in power, assuming Schottky or germanium diode detectors. To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. There's a reason that Bird power meters use a sensitive microammeter movement. (I think I've heard 30uA full scale, but I'm not sure about that.) Anyway, that's why I suggested using a DVM for readout. The second problem is, if you want to implement a microstrip design, how do you get the trace width right? If you're afraid of surface mount parts, how will you control the trace width to +/- a fraction of a millimeter? On 1.6mm thick PC board, assuming FR4 with a relative dielectric constant of 4.75, you'd like to have a trace width about 2.78mm to get a 50 ohm line. If your trace is 3.5mm wide, you get a bit under 44 ohms, and if your trace comes out 2.0mm wide, you get a line that's almost 60 ohms. If you can do the PC board photographically and have confidence that you can control the trace width to within 0.1mm, that would work. If you're doing it by scribing the copper and pulling up unwanted copper, I think you'll have to be working under a pretty good microscope to get to much closer than a mm of the desired width-- or maybe cut it on a milling machine. I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. But there's still the problem of making the two (or three) all the same width. Not knowing how you might be able to do this, I'm rather discouraged about how this would come out. Maybe there's a better way to make the coupled lines that's easier for a typical ham with minimal shop facilities to handle. Or maybe if there was enough interest, someone could make some boards with guaranteed performance. On the positive side, I did find BNC jacks that edge-mount on PC boards, so that part of it becomes easy at least. Cheers, Tom |
70cm reflectometer?
Hello Tom
My responses** "K7ITM" wrote in message ... On Jan 30, 2:08 pm, "Suzy" not@valid wrote: "K7ITM" wrote in message ... On Jan 30, 9:57 am, "Suzy" not@valid wrote: ... I'd like to thank all you kind fellows for your assistance, but unfortunately a lot of it is rather above my head. I need to be pointed at an article that spells out exactly what dimensions, how to build etc. (eg is there one in the ARRL handbook?) And Owen, no in this case Suzy if not short for Susan, so I'm not the person you knew. OK, back to the first posting I made here. How about if I build one, test it, and post the design and results somewhere? As I noted in one of my postings, if I were to make one, I'd first consider how much power I wanted to read, full scale, so if my offer is appealing, let me know how much power you want to measure, max. Don't tell me a kilowatt if you really are going to use it at 10 watts, because if I design for a kilowatt, 10 watts will be low enough that you won't be able to read it very well. In fact, I'd propose 10 watts as a reasonable full scale for a lot of ham uses. Cheers, Tom Hi Tom That's a kind offer, but you needn't go to all the trouble of building it. Just a pointer at the practical design will do (but no complex theory -- over my head!). I want to investigate various 70 cm antennas (central frequency in Australia 435 MHz). TX is switchable 5 10 20 watts. I want to standardise on BNC, and have readouts on analogue meters (probably 1 mA movements) OK, I gave this some thought last night. I see a couple problems... Though you could use 1mA meter movements, that puts you at a detected power level high enough that the meter scale won't be linear in power, assuming Schottky or germanium diode detectors. **Pardon my ignorance, but isn't it just a case of using an op amp or whatever to suit whatever meter movement I have? ANyway, I have now sourced a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1 mA one has a 210 ohm resistance. BTW, I'n not bothered about the linear issue. I will be having two meters to show forward and relected powers simultaneously. I don't want to clculate actual SWR. To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. There's a reason that Bird power meters use a sensitive microammeter movement. (I think I've heard 30uA full scale, but I'm not sure about that.) Anyway, that's why I suggested using a DVM for readout. The second problem is, if you want to implement a microstrip design, how do you get the trace width right? If you're afraid of surface mount parts, how will you control the trace width to +/- a fraction of a millimeter? On 1.6mm thick PC board, assuming FR4 with a relative dielectric constant of 4.75, you'd like to have a trace width about 2.78mm to get a 50 ohm line. If your trace is 3.5mm wide, you get a bit under 44 ohms, and if your trace comes out 2.0mm wide, you get a line that's almost 60 ohms. If you can do the PC board photographically and have confidence that you can control the trace width to within 0.1mm, that would work. If you're doing it by scribing the copper and pulling up unwanted copper, I think you'll have to be working under a pretty good microscope to get to much closer than a mm of the desired width-- or maybe cut it on a milling machine. I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? But there's still the problem of making the two (or three) all the same width. Not knowing how you might be able to do this, I'm rather discouraged about how this would come out. Maybe there's a better way to make the coupled lines that's easier for a typical ham with minimal shop facilities to handle. Or maybe if there was enough interest, someone could make some boards with guaranteed performance. On the positive side, I did find BNC jacks that edge-mount on PC boards, so that part of it becomes easy at least. Cheers, Tom Cheers |
70cm reflectometer?
On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote:
I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? This is called residual SWR in a reflectometer. You load it with a known good load, and what SWR you find (or what is exhibited by the two meters) inhabits the reflectometer itself. Then you flip it over and apply your source into the goesoutta with the known load on the comesinna. You then proceed to reduce the residual SWR in both directions. Finding a good load is another matter, and I reported one (a precision RF resistor) with specific characteristics here last week. Consult the thread "RF Power Resistors from Caddock." All of $10-$20 to accomplish. 73's Richard Clark, KB7QHC |
70cm reflectometer?
On Jan 31, 11:12 am, "Suzy" not@valid wrote:
Hello Tom My responses** "K7ITM" wrote in message .... OK, I gave this some thought last night. I see a couple problems... Though you could use 1mA meter movements, that puts you at a detected power level high enough that the meter scale won't be linear in power, assuming Schottky or germanium diode detectors. **Pardon my ignorance, but isn't it just a case of using an op amp or whatever to suit whatever meter movement I have? ANyway, I have now sourced a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1 mA one has a 210 ohm resistance. Ah, OK. I had assumed you wanted to use just the meters, with no amplifier. With amplifiers, the 1mA meters will be fine. But we need op amps that have very low offset voltage and drift--I would prefer to set the meter full scale to correspond to around a millivolt or two of detected DC. I suppose if you have a way to zero the offset and it doesn't drift, that'll be OK. Then we need to make sure the amplifier is reasonably immune to 450MHz signals floating around... This is all "do-able" but there are some details you'll have to pay attention to. I'd suggest using a couple of the RF power detector ICs available from Analog Devices or Linear Technology, but we're back to surface mount stuff again at that point. BTW, I'n not bothered about the linear issue. I will be having two meters to show forward and relected powers simultaneously. I don't want to clculate actual SWR. Well, yes, but wouldn't you want to know whether "0.1" on the meter represented 1/10 the power of "1.0" on the meter, rather than 1/100 of the power?? If you don't pay a little attention to the level of RF the detector is actually detecting, you're liable to have that problem. .... I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? What do you mean "no" test gear? You'll have the directional coupler with meters itself, and a power source. The only other thing you need is a 50 ohm load to put on the coupler output. Is it not worth having at least a load you can trust? With a known good load, you feed some power through the coupler and adjust for zero indicated return; turn the coupler around and make sure the other port also reads zero. With an open or short load you should get equal readings on the forward and reverse meters. (I suppose you need two couplers and a good load to insure that the through line of the coupler is also the same impedance as the load...) I have a good network analyzer on my bench at work, but without a calibration load to test and calibrate it with, I don't know how good its readings really are. Cheers, Tom |
70cm reflectometer?
K7ITM wrote:
To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com |
70cm reflectometer?
On Jan 31, 12:31 pm, Cecil Moore wrote:
K7ITM wrote: To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com See earlier posting in this thread. See various Avago ap notes, such as AN 969. A diode detector run at low input provides an output DC voltage that's a constant times the square of the input RF voltage. If the input voltage is, or is assumed to be, at some constant resistive load impedance, the DC output is linear with RF power input. The proportionality is temperature dependent, but if two detectors are constructed the same and run at the same temperature, and run in the signal level region where that relationship holds, then the ratio of the output DC voltages is a very good approximation of the ratio of the input RF power levels, and thus is useful for finding the SWR if the detectors are attached to the forward and reverse ports of a good directional coupler. Top end of the useful "linear power" range using an HSMS-2850 single diode detector is about 10mV DC output. If you can measure the DC accurately down to 1uV (a bit tough, given thermal emfs, but possible), that gives you about a 10000:1 power range, or 100:1 RF input voltage range -- or about 1.02:1 SWR. Chances are very good that a home-built coupler won't be accurately enough matched to 50+j0 ohms to worry about anything that low anyway, even if you had a reason to care about it. Cheers, Tom |
70cm reflectometer?
K7ITM wrote:
See earlier posting in this thread. Thanks Tom, when I said "linear power scale", I meant e.g. a meter reading where 2000 watts is full scale and 1000 watts is half scale. I have seen such meters but not without a digital or analog computer on the front end. -- 73, Cecil http://www.w5dxp.com |
70cm reflectometer?
K7ITM wrote in news:9e844e58-a673-4ec0-9a0b-ec15f8cc8f30
@c4g2000hsg.googlegroups.com: On Jan 31, 12:31 pm, Cecil Moore wrote: K7ITM wrote: To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com See earlier posting in this thread. See various Avago ap notes, such as AN 969. A diode detector run at low input provides an output DC voltage that's a constant times the square of the input RF voltage. If the input voltage is, or is assumed to be, at some constant resistive load impedance, the DC output is linear with RF power input. The proportionality is temperature dependent, but if two detectors are constructed the same and run at the same temperature, and run in the signal level region where that relationship holds, then the ratio of the output DC voltages is a very good approximation of the ratio of the input RF power levels, and thus is useful for finding the SWR if the detectors are attached to the forward and reverse ports of a good directional coupler. Top end of the useful "linear power" range using an HSMS-2850 single diode detector is about 10mV DC output. If you can measure the DC accurately down to 1uV (a bit tough, given thermal emfs, but possible), that gives you about a 10000:1 power range, or 100:1 RF input voltage range -- or about 1.02:1 SWR. Chances are very good that a home-built coupler won't be accurately enough matched to 50+j0 ohms to worry about anything that low anyway, even if you had a reason to care about it. Cheers, Tom Tom, This is further from Suzy's needs, but... Operation of a diode detector in the square law region isn't out of the question, but it takes some serious gain to drive a meter. There are some good chopper stabilised op amps out there that have uV offset levels and single supply rail and input to below the negative rail eg LTC1050. Another alternative is the AD8307AN log amps for a linear dBW scale. You could even use one on FWD and REF detectors and difference the outputs in an op amp for a direct indicating VSWR or RL scale. I have thought of getting one of these chips and seeing whether its response is fast enough to drive a PEP amplifier for SSB telephony. Back to Suzy's problem... The instrument downstream of the sampler is not so much the issue as building and calibrating a sampler when you have no test gear. Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good buy. It has HF to 70cm (two independent couplers, ie four coax connectors), and works pretty well. For a dummy load, the market was flooded with terminations from 25W to about 60W that had been scrapped from AMPS base station equipment, and they were sold at hamfests for $20 or so, you may find them if you look around. Owen |
70cm reflectometer?
Owen Duffy wrote:
Operation of a diode detector in the square law region isn't out of the question, but it takes some serious gain to drive a meter. There are some good chopper stabilised op amps out there that have uV offset levels and single supply rail and input to below the negative rail eg LTC1050. If the diode is DC-biased and the bias subtracted out during calibration, doesn't that improve the low power accuracy considerably? -- 73, Cecil http://www.w5dxp.com |
70cm reflectometer?
"Richard Clark" wrote in message ... On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote: I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? This is called residual SWR in a reflectometer. You load it with a known good load, and what SWR you find (or what is exhibited by the two meters) inhabits the reflectometer itself. Then you flip it over and apply your source into the goesoutta with the known load on the comesinna. You then proceed to reduce the residual SWR in both directions. Finding a good load is another matter, and I reported one (a precision RF resistor) with specific characteristics here last week. Consult the thread "RF Power Resistors from Caddock." All of $10-$20 to accomplish. 73's Richard Clark, KB7QHC I'm in Australia! |
70cm reflectometer?
"K7ITM" wrote in message ... On Jan 31, 11:12 am, "Suzy" not@valid wrote: Hello Tom My responses** "K7ITM" wrote in message ... OK, I gave this some thought last night. I see a couple problems... Though you could use 1mA meter movements, that puts you at a detected power level high enough that the meter scale won't be linear in power, assuming Schottky or germanium diode detectors. **Pardon my ignorance, but isn't it just a case of using an op amp or whatever to suit whatever meter movement I have? ANyway, I have now sourced a 100 microamp meter (MU65) with a 3.9K resistance (sounds strange as the 1 mA one has a 210 ohm resistance. Ah, OK. I had assumed you wanted to use just the meters, with no amplifier. With amplifiers, the 1mA meters will be fine. But we need op amps that have very low offset voltage and drift--I would prefer to set the meter full scale to correspond to around a millivolt or two of detected DC. I suppose if you have a way to zero the offset and it doesn't drift, that'll be OK. Then we need to make sure the amplifier is reasonably immune to 450MHz signals floating around... This is all "do-able" but there are some details you'll have to pay attention to. I'd suggest using a couple of the RF power detector ICs available from Analog Devices or Linear Technology, but we're back to surface mount stuff again at that point. BTW, I'n not bothered about the linear issue. I will be having two meters to show forward and relected powers simultaneously. I don't want to clculate actual SWR. Well, yes, but wouldn't you want to know whether "0.1" on the meter represented 1/10 the power of "1.0" on the meter, rather than 1/100 of the power?? If you don't pay a little attention to the level of RF the detector is actually detecting, you're liable to have that problem. ... I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? What do you mean "no" test gear? You'll have the directional coupler with meters itself, and a power source. The only other thing you need is a 50 ohm load to put on the coupler output. Is it not worth having at least a load you can trust? With a known good load, you feed some power through the coupler and adjust for zero indicated return; turn the coupler around and make sure the other port also reads zero. With an open or short load you should get equal readings on the forward and reverse meters. (I suppose you need two couplers and a good load to insure that the through line of the coupler is also the same impedance as the load...) I have a good network analyzer on my bench at work, but without a calibration load to test and calibrate it with, I don't know how good its readings really are. Cheers, Tom Having been told so by two of you, looks like I'll have to trade a rolling pin for a "known good load". Trouble is, there's probably no source here in Australia. |
70cm reflectometer?
K7ITM wrote:
On Jan 31, 12:31 pm, Cecil Moore wrote: K7ITM wrote: To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com And what makes one think that a standard meter is linear (unless your standards are +/- 10% ) See earlier posting in this thread. See various Avago ap notes, such as AN 969. A diode detector run at low input provides an output DC voltage that's a constant times the square of the input RF voltage. If the input voltage is, or is assumed to be, at some constant resistive load impedance, the DC output is linear with RF power input. Not necessarily true for a coupler.. The proportionality is temperature dependent, but if two detectors are constructed the same and run at the same temperature, and run in the signal level region where that relationship holds, then the ratio of the output DC voltages is a very good approximation of the ratio of the input RF power levels, and thus is useful for finding the SWR if the detectors are attached to the forward and reverse ports of a good directional coupler. where "good" is the operative word Top end of the useful "linear power" range using an HSMS-2850 single diode detector is about 10mV DC output. If you can measure the DC accurately down to 1uV (a bit tough, given thermal emfs, but possible), that gives you about a 10000:1 power range, or 100:1 RF input voltage range -- or about 1.02:1 SWR. Chances are very good that a home-built coupler won't be accurately enough matched to 50+j0 ohms to worry about anything that low anyway, even if you had a reason to care about it. Which is why "real instruments" have some form of calibration. With today's technology, there's really no excuse for not putting some form of calibration into the logic between measurement and display, unless all you're looking for is the RF equivalent of a battery and test lamp. Heck, if you MUST use all analog designs and you're at less than 3GHz, don't fool with diodes, use the less expensive, more sensitive, and more accurate power measuring chips from Analog Devices. Example: AD8310, DC-440MHz, 90+dB dynamic range (-91 to +4dBm) linear to 0.4dB, stable over temp(-40 to +85) +/-1 dB or the 8319, 1MHz to 10GHz, 40dB range, similar accuracy they also come in dual versions and versions with phase comparators.. These days, there's relatively few applications where a straight diode detector would be better: 1) Absolute lowest cost in mass manufacturing with relaxed performance requirements(so you can use a really cheap silicon diode) (0.10 for a diode vs $3 for a chip is a $30 difference in the retail list price) 2) very fast rise time requirements The chips have response times in the 10 nS and slower range. The right diode in the right mount can get sub nanoseconds. 3) frequencies 10 GHz The chips don't go there yet. Cheers, Tom |
70cm reflectometer?
"K7ITM" wrote in message ... On Jan 31, 12:31 pm, Cecil Moore wrote: K7ITM wrote: To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com See earlier posting in this thread. See various Avago ap notes, such as AN 969. A diode detector run at low input provides an output DC voltage that's a constant times the square of the input RF voltage. If the input voltage is, or is assumed to be, at some constant resistive load impedance, the DC output is linear with RF power input. The proportionality is temperature dependent, but if two detectors are constructed the same and run at the same temperature, and run in the signal level region where that relationship holds, then the ratio of the output DC voltages is a very good approximation of the ratio of the input RF power levels, and thus is useful for finding the SWR if the detectors are attached to the forward and reverse ports of a good directional coupler. Top end of the useful "linear power" range using an HSMS-2850 single diode detector is about 10mV DC output. If you can measure the DC accurately down to 1uV (a bit tough, given thermal emfs, but possible), that gives you about a 10000:1 power range, or 100:1 RF input voltage range -- or about 1.02:1 SWR. Chances are very good that a home-built coupler won't be accurately enough matched to 50+j0 ohms to worry about anything that low anyway, even if you had a reason to care about it. Cheers, Tom Much too theoretical for me! Like Cecil's posts in that other longgoing thread, it leaves me glazed! |
70cm reflectometer?
"Owen Duffy" wrote in message ... K7ITM wrote in news:9e844e58-a673-4ec0-9a0b-ec15f8cc8f30 @c4g2000hsg.googlegroups.com: On Jan 31, 12:31 pm, Cecil Moore wrote: K7ITM wrote: To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com See earlier posting in this thread. See various Avago ap notes, such as AN 969. A diode detector run at low input provides an output DC voltage that's a constant times the square of the input RF voltage. If the input voltage is, or is assumed to be, at some constant resistive load impedance, the DC output is linear with RF power input. The proportionality is temperature dependent, but if two detectors are constructed the same and run at the same temperature, and run in the signal level region where that relationship holds, then the ratio of the output DC voltages is a very good approximation of the ratio of the input RF power levels, and thus is useful for finding the SWR if the detectors are attached to the forward and reverse ports of a good directional coupler. Top end of the useful "linear power" range using an HSMS-2850 single diode detector is about 10mV DC output. If you can measure the DC accurately down to 1uV (a bit tough, given thermal emfs, but possible), that gives you about a 10000:1 power range, or 100:1 RF input voltage range -- or about 1.02:1 SWR. Chances are very good that a home-built coupler won't be accurately enough matched to 50+j0 ohms to worry about anything that low anyway, even if you had a reason to care about it. Cheers, Tom Tom, This is further from Suzy's needs, but... Operation of a diode detector in the square law region isn't out of the question, but it takes some serious gain to drive a meter. There are some good chopper stabilised op amps out there that have uV offset levels and single supply rail and input to below the negative rail eg LTC1050. Another alternative is the AD8307AN log amps for a linear dBW scale. You could even use one on FWD and REF detectors and difference the outputs in an op amp for a direct indicating VSWR or RL scale. I have thought of getting one of these chips and seeing whether its response is fast enough to drive a PEP amplifier for SSB telephony. Back to Suzy's problem... The instrument downstream of the sampler is not so much the issue as building and calibrating a sampler when you have no test gear. Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good buy. It has HF to 70cm (two independent couplers, ie four coax connectors), and works pretty well. For a dummy load, the market was flooded with terminations from 25W to about 60W that had been scrapped from AMPS base station equipment, and they were sold at hamfests for $20 or so, you may find them if you look around. Owen Thanks Owen. BTW, what type of coax connector? Not PAL surely! |
70cm reflectometer?
Suzy wrote:
"Richard Clark" wrote in message ... On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote: I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? This is called residual SWR in a reflectometer. You load it with a known good load, and what SWR you find (or what is exhibited by the two meters) inhabits the reflectometer itself. Then you flip it over and apply your source into the goesoutta with the known load on the comesinna. You then proceed to reduce the residual SWR in both directions. Finding a good load is another matter, and I reported one (a precision RF resistor) with specific characteristics here last week. Consult the thread "RF Power Resistors from Caddock." All of $10-$20 to accomplish. 73's Richard Clark, KB7QHC I'm in Australia! http://www.caddock.com/ has a link to their Australian distributor (granted, they may have some punitive minimum order). A begging letter to Caddock for a sample might work? Or, do the big mailorder companies ship to Australia (the parts are in the few bucks range.. postage and shipping could be more) |
70cm reflectometer?
At the risk of thoroughly boring you all, I'll summarize the position to
date. I have a good workshop with power tools and I like metal bashing and am quite happy with PCBs. I even have an electronic calliper so can measure thickness accurately. However, my eyes will not allow very fine work like SMD. I am looking to build an SWR meter using two 1 mA meters, one for forward and one for reverse. I am looking for a practical (non-theoretical) article on how to build one. I am wondering if there is one the ARRL handboook before I go to the exp-ense of buying one here in Australia. Most of the beautifully argued theory on here is way way beyond me. Any pointers to a suitable article? |
70cm reflectometer?
"Jim Lux" wrote in message ... Suzy wrote: "Richard Clark" wrote in message ... On Fri, 1 Feb 2008 06:12:14 +1100, "Suzy" not@valid wrote: I suppose there's still the possibility of cutting the trace a bit narrow on purpose and adjusting the impedance by adding a grounded plate above the board. It could be spaced an adjustable distance away by mounting it with threaded rods (long screws), and adjusted to make the traces 50 ohms. **But how do you check that in a workshop with no test gear? This is called residual SWR in a reflectometer. You load it with a known good load, and what SWR you find (or what is exhibited by the two meters) inhabits the reflectometer itself. Then you flip it over and apply your source into the goesoutta with the known load on the comesinna. You then proceed to reduce the residual SWR in both directions. Finding a good load is another matter, and I reported one (a precision RF resistor) with specific characteristics here last week. Consult the thread "RF Power Resistors from Caddock." All of $10-$20 to accomplish. 73's Richard Clark, KB7QHC I'm in Australia! http://www.caddock.com/ has a link to their Australian distributor (granted, they may have some punitive minimum order). A begging letter to Caddock for a sample might work? Or, do the big mailorder companies ship to Australia (the parts are in the few bucks range.. postage and shipping could be more) Thanks very much for that, but I am hoping to get a Bird Termaline 50W dummy load, which I hope will enable me to progress. Thank you all for holding this floury (not flowery!) hand. |
70cm reflectometer?
"Suzy" not@valid wrote in message ... At the risk of thoroughly boring you all, I'll summarize the position to date. I have a good workshop with power tools and I like metal bashing and am quite happy with PCBs. I even have an electronic calliper so can measure thickness accurately. However, my eyes will not allow very fine work like SMD. I am looking to build an SWR meter using two 1 mA meters, one for forward and one for reverse. I am looking for a practical (non-theoretical) article on how to build one. I am wondering if there is one the ARRL handboook before I go to the exp-ense of buying one here in Australia. Most of the beautifully argued theory on here is way way beyond me. Any pointers to a suitable article? And further to that, I am hoping to shortly get a Bird Termaline 50W dummy load, so I'll have something to work with! |
70cm reflectometer?
"Suzy" not@valid wrote in :
Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good buy. It has HF to 70cm (two independent couplers, ie four coax connectors), and works pretty well. N type on the VHF/UHF coupler, UHF type on the other coupler. For a dummy load, the market was flooded with terminations from 25W to about 60W that had been scrapped from AMPS base station equipment, and they were sold at hamfests for $20 or so, you may find them if you look around. N type. Owen Thanks Owen. BTW, what type of coax connector? Not PAL surely! No, but they are not all that bad. Almost no one manufactured VHF land mobiles here with UHF connectors, but they did use PAL (Belling & Lee) once (Pye Reporters for instance). Owen |
70cm reflectometer?
Jim Lux wrote in news:fntlce$h96$1
@news.jpl.nasa.gov: http://www.caddock.com/ has a link to their Australian distributor (granted, they may have some punitive minimum order). A begging letter to Caddock for a sample might work? Or, do the big mailorder companies ship to Australia (the parts are in the few bucks range.. postage and shipping could be more) Suzy, I had a look at Farnell (au) for these type of resistors recently (Caddock are not the only maker). My recollection is that a 100W resistor is likely to be good for 50W - 60W in a practical application, and they were about $28 ea + $12 shipping, and IIRC minimum order quantity was one. That is not to recommend them for a simple accurate load for 70cm. Owen |
70cm reflectometer?
"Owen Duffy" wrote in message ... Jim Lux wrote in news:fntlce$h96$1 @news.jpl.nasa.gov: http://www.caddock.com/ has a link to their Australian distributor (granted, they may have some punitive minimum order). A begging letter to Caddock for a sample might work? Or, do the big mailorder companies ship to Australia (the parts are in the few bucks range.. postage and shipping could be more) Suzy, I had a look at Farnell (au) for these type of resistors recently (Caddock are not the only maker). My recollection is that a 100W resistor is likely to be good for 50W - 60W in a practical application, and they were about $28 ea + $12 shipping, and IIRC minimum order quantity was one. That is not to recommend them for a simple accurate load for 70cm. Owen But hopefully the Bird Termaline 50W model 8085 will do the job? |
70cm reflectometer?
Suzy wrote:
At the risk of thoroughly boring you all, I'll summarize the position to date. I have a good workshop with power tools and I like metal bashing and am quite happy with PCBs. I even have an electronic calliper so can measure thickness accurately. However, my eyes will not allow very fine work like SMD. I am looking to build an SWR meter using two 1 mA meters, one for forward and one for reverse. I am looking for a practical (non-theoretical) article on how to build one. I am wondering if there is one the ARRL handboook before I go to the exp-ense of buying one here in Australia. Most of the beautifully argued theory on here is way way beyond me. Any pointers to a suitable article? One might find that you can BUY a surplus directional coupler for 440MHz fairly cheaply. Check Ebay, etc. Then it's just a matter of wiring up electronics to the coupled ports. |
70cm reflectometer?
Suzy wrote:
"Owen Duffy" wrote in message ... Jim Lux wrote in news:fntlce$h96$1 : http://www.caddock.com/ has a link to their Australian distributor (granted, they may have some punitive minimum order). A begging letter to Caddock for a sample might work? Or, do the big mailorder companies ship to Australia (the parts are in the few bucks range.. postage and shipping could be more) Suzy, I had a look at Farnell (au) for these type of resistors recently (Caddock are not the only maker). My recollection is that a 100W resistor is likely to be good for 50W - 60W in a practical application, and they were about $28 ea + $12 shipping, and IIRC minimum order quantity was one. That is not to recommend them for a simple accurate load for 70cm. Owen But hopefully the Bird Termaline 50W model 8085 will do the job? in AU, have you checked surplus places like http://www.users.bigpond.com/alandevlin/forsale.html Indeed, it shows a HP 774D as being sold, but perhaps he'd have other stuff. |
70cm reflectometer?
"Suzy" not@valid wrote in :
But hopefully the Bird Termaline 50W model 8085 will do the job? I think it is rated to 3GHz, look after it and it will be very useful. People tend to overlook the value of a known dummy load, it is a very useful piece of test equipment, and one of the first you should own. If it is used, take the opportunity to test it some time. With that in you kit, you have a chance of calibrating a VSWR meter sampler. You might find the following article of interest, skip the maths if it gives you a headache, but the discussion towards the end is relevant to setting up / testing a VSWR meter: http://www.vk1od.net/VSWR/VSWRMeter.htm .. Perhaps you have some trusted ham neighbours who can help you out. Owen |
70cm reflectometer?
On Jan 31, 1:46 pm, Cecil Moore wrote:
K7ITM wrote: See earlier posting in this thread. Thanks Tom, when I said "linear power scale", I meant e.g. a meter reading where 2000 watts is full scale and 1000 watts is half scale. I have seen such meters but not without a digital or analog computer on the front end. -- 73, Cecil http://www.w5dxp.com Right, and I'm saying that with a coupler or sampler that picks off, say, 1e-7 of the power (0.2 milliwatts of RF out for 2000 watts in/ through), a diode detector that shows that power to be full scale on a linear meter will show 1000 watts at half scale. That assumes that 0.2 milliwatts is low enough to get you into the square law region of the detector; for an HSMS-2850 diode, that's a bit high. Because the power at which the square law holds accurately is so low (and the detector output is so low) you need an electronic way to read that output; a simple meter movement isn't likely to do the trick. But there's no need for any fancy processing. |
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