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#21
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MFJ-269 Antenna/SWR/RF Analyzer
Tam/WB2TT wrote: "Tam/WB2TT" wrote in message ... Meanwhile, I measured the impedance of a Drake 100W dummy load with and without the HPF. All readings without the HPF are within Drake spec. FREQ NO HPF With HPF 4 Mhz 47j2 33j13 7MHz 47j2 55j3 14MHz 47j1 50j7 28MHz 48j2 47j3 50MHz 49j2 54j11 144MHz 53j11 74j36 Everything was connected with UHF adapters, and no coax was used. Ignore the VHF readings,. as the filter was not built that carefully. Capacitors are mica (actual values 1000, 560, 1000), and inductors appear to be 68-2 (2.2uH). If I get a chance later today, I will rewire it into the Pi configuration with the same inductors. That's about what I'd expect. The increasing X with frequency is consistent with a small amount of series stray inductance which is unavoidable in the physical construction. It can be minimized, of course, by careful construction. A typical homebrew HF filter will begin becoming poor at VHF and above due to series self inductance of the capacitors and shunt self capacitance of the inductors, plus other effects. Component selection and layout can help a lot, but it might be necessary to cascade a VHF/UHF filter with the HF filter if very wideband rejection is necessary. A network analyzer or spectrum analyzer with tracking generator or noise generator are invaluable in solving those problems. Of course, the more stuff you put in the path, the more you're likely to disturb the measurement. Roy Lewallen, W7EL |
#22
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MFJ-269 Antenna/SWR/RF Analyzer
"Roy Lewallen" wrote in message ... Tam/WB2TT wrote: Roy, I wasn't too clear, but I have 2 T networks back/back. That makes the center cap C/2. I am going to run SWCad on the Pi configuration later, and see what that does. If you've cascaded two sections, you have two 1000 pF capacitors in parallel at the center. That makes a total value of 2000 pF at that point. Roy Lewallen, W7EL It's a high pass filter to reject the AM broadcast band. So, the two 1000 PF caps are in series. Am I missing something? Tam |
#23
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MFJ-269 Antenna/SWR/RF Analyzer
Tam/WB2TT wrote:
"Roy Lewallen" wrote in message ... Tam/WB2TT wrote: Roy, I wasn't too clear, but I have 2 T networks back/back. That makes the center cap C/2. I am going to run SWCad on the Pi configuration later, and see what that does. If you've cascaded two sections, you have two 1000 pF capacitors in parallel at the center. That makes a total value of 2000 pF at that point. Roy Lewallen, W7EL It's a high pass filter to reject the AM broadcast band. So, the two 1000 PF caps are in series. Am I missing something? Sorry, I missed that you had made a T network rather than pi. In general, a tee network substituted for a pi will have the same characteristics only at one frequency, but will have different transfer and/or impedance characteristics at other frequencies. So the substitution should be done with care if characteristics are important at more than one frequency. In this case, though, if you make a tee network which has the same "half wave" characteristic as the pi at the design frequency, it'll have identical transfer characteristics (it's got the same filter response) and complementary impedance characteristics. That is, at frequencies where one network has an input impedance greater than 50 ohms, the other will have an impedance that's less, and the phase angles are the negatives of each other. And, luckily, the transformation is simple for this particular special case -- the T network reactances are also all the same and also equal to the Z0 of the "transmission line". So one is just as good as the other. The HPF equivalent doesn't of course simulate a transmission line, although the impedance transformation though the filter is unity at the design frequency. Otherwise, it works in pretty much an opposite way from the LPF. I need to correct and clarify a couple of points I made in my earlier posting. The "half wave" lowpass filter simulates a half wavelength transmission line only at and near the design frequency (where the reactances are all the same). It doesn't do a very good job either above or below that frequency. For a better general simulation of a *short* transmission line, reduce the end pi or T network components to half their values. This model improves -- in theory at least -- as more sections are added. In practice, imperfection in the components limits the quality of the approximation. But I don't think this is of particular interest in making analyzer measurements. The 7 MHz example terminated with 50 ohms will show an input impedance within 2 ohms magnitude and 2 degrees phase of 50 ohms between about 6.2 and 7.4 MHz, so it's good for the entire 40 meter band. But it will disturb measurements on lower bands. You should construct one for each band and, preferably, one for each general impedance level you expect to measure. A single one won't do for multiple bands as I implied. Roy Lewallen, W7EL |
#24
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MFJ-269 Antenna/SWR/RF Analyzer
Tam/WB2TT wrote:
It's a high pass filter to reject the AM broadcast band. So, the two 1000 PF caps are in series. Am I missing something? Sorry, I missed that you had made a T network rather than pi. In general, a tee network substituted for a pi will have the same characteristics only at one frequency, but will have different transfer and/or impedance characteristics at other frequencies. So the substitution should be done with care if characteristics are important at more than one frequency. In this case, though, if you make a tee network which has the same "half wave" characteristic as the pi at the design frequency, it'll have identical transfer characteristics (it's got the same filter response) and complementary impedance characteristics. That is, at frequencies where one network has an input impedance greater than 50 ohms, the other will have an impedance that's less, and the phase angles are the negatives of each other. And, luckily, the transformation is simple for this particular special case -- the T network reactances are also all the same and also equal to the Z0 of the "transmission line". So one is just as good as the other. The HPF equivalent doesn't of course simulate a transmission line, although the impedance transformation though the filter is unity at the design frequency. Otherwise, it works in pretty much an opposite way from the LPF. I need to correct and clarify a couple of points I made in my earlier posting. The "half wave" lowpass filter simulates a half wavelength transmission line only at and near the design frequency (where the reactances are all the same). It doesn't do a very good job either above or below that frequency(*). The 7 MHz example terminated with 50 ohms will show an input impedance within 2 ohms magnitude and 2 degrees phase of 50 ohms between about 6.2 and 7.4 MHz, so it's good for the entire 40 meter band. But it will disturb measurements on lower bands. You should construct one for each band and, preferably, one for each general impedance level you expect to measure. A single one won't do for multiple bands as I implied. (*)For a better general simulation of a *short* transmission line, use a ladder network with all the reactances equal to Z0 except the end components. For the end components, make the series L or shunt C half the value of the rest. (For example, the LPF I showed would have input and output shunt capacitors with reactance = 100 ohms, and remaining components with reactance = 50 ohms. A five-component tee type network would have input and output series inductors with reactance = 25 ohms, and the remaining components with reactance = 50 ohms.) This model improves -- in theory at least -- as more sections are added, being able to imitate longer and longer lines. In practice, imperfection in the components limits the quality of the approximation. But I don't think this is of particular interest in making analyzer measurements. The model I proposed is better for simulating a half wavelength line while providing filtering. Roy Lewallen, W7EL |
#25
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MFJ-269 Antenna/SWR/RF Analyzer
I received the device and have started to use it. My only criticism so
far is that the soft case is too tight to allow easy operation of the controls. I hope this will loosen up a bit with time. It is amazing what you can learn about an antenna in such a short time! John Ferrell W8CCW On Fri, 04 Nov 2005 19:43:07 GMT, John Ferrell wrote: I am considering ordering the MFJ-269 HF/VHF/UHF Antenna/SWR/RF Analyzer. John Ferrell W8CCW |
#26
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MFJ-269 Antenna/SWR/RF Analyzer
John Ferrell wrote:
It is amazing what you can learn about an antenna in such a short time! My MFJ-259B is a really handy little gadget for learning. So is EZNEC. -- 73, Cecil http://www.qsl.net/w5dxp |
#27
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MFJ-269 Antenna/SWR/RF Analyzer
I agree. I took the ARRL course using EZNEC and cannot praise it
enough! On Thu, 01 Dec 2005 13:41:01 GMT, Cecil Moore wrote: John Ferrell wrote: It is amazing what you can learn about an antenna in such a short time! My MFJ-259B is a really handy little gadget for learning. So is EZNEC. John Ferrell W8CCW |
#28
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MFJ-269 Antenna/SWR/RF Analyzer
I was terribly disappointed in the MFJ case and sold it on Ebay then bought
an aluminum case off Ebay that works perfectly and has room for several adapters and a cable or two as well as the instruction manual for the 269. It is a great instrument but I wish they would fix the UHF switching problem. Looks like a latching relay would solve the problem real easy, go figure. Anyway here is a URL to one of the auctions: http://cgi.ebay.com/NEW-ALUMINUM-HAR...cm dZViewItem I also bought a laptop case from the same company and both were outstanding quality. Good heavy side walls and nice interiors. Take a good look at the interior dividers on the above auction. The 269 fits exactly. Good luck and 73, Ken "John Ferrell" wrote in message ... I received the device and have started to use it. My only criticism so far is that the soft case is too tight to allow easy operation of the controls. I hope this will loosen up a bit with time. It is amazing what you can learn about an antenna in such a short time! John Ferrell W8CCW On Fri, 04 Nov 2005 19:43:07 GMT, John Ferrell wrote: I am considering ordering the MFJ-269 HF/VHF/UHF Antenna/SWR/RF Analyzer. John Ferrell W8CCW |
#29
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MFJ-269 Antenna/SWR/RF Analyzer
KEEP IT! KEEP IT! KEEP IT!
Don't ever, never unload a piece of test equipment. after 40 years in the field I warn you that you're going to need it BADLY one day after to sell it. Saandy 4Z5KS |
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