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using an HP 8405A to measure SWR ?
On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror
wrote: The coupler measures about -14 dbm. What does this mean? Are you trying to tell us that the power on the coupler port is 14dB less than the through power? What has dBm got to do with it? You didn't report the power in the coupler port with a s/c and / or o/c at the measurement plane. Did you perform this cal? (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) That is understandable. How much coax between the A probe and the load, and the B probe and the load... how many degress does this total electrical length change for a 1% change in frequency? Does that explain some of the phase sensitivity? The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2 foot square aluminum plate. This antenna should have an input impedance less then 20 Ohms. Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... How can it measure very close to 50 Ohms? Is there something wrong with this analysis? See if you can trust your measurements on known loads before wondering why the unknown load isn't what you expect when using unknown measurement technology... too many unknowns. Owen -- |
#2
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using an HP 8405A to measure SWR ?
On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy wrote:
(The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) ...Does that explain some of the phase sensitivity? Hi Owen, Being very touchy, especially to the specifics of sitting very still, sounds like classic common mode problems. 73's Richard Clark, KB7QHC |
#3
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using an HP 8405A to measure SWR ?
On Mon, 02 Jan 2006 23:06:43 -0800, Richard Clark
wrote: On Tue, 03 Jan 2006 06:42:53 GMT, Owen Duffy wrote: (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) ...Does that explain some of the phase sensitivity? Hi Owen, Being very touchy, especially to the specifics of sitting very still, sounds like classic common mode problems. Yes it does Richard. I saw Roy's response regarding isolation of the feedline, and it is a valid comment. My comment was towards the reported frequency sensitivity... until the effect of the propagation delay is removed from the results, the underlying impedance is obsured. Owen -- |
#4
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using an HP 8405A to measure SWR - update
Thanks,
I went back through the calibration procedure. 1. The meter appears to be performing as it is supposed to. The 'static readings work repeatably and predictably. With a balanced configuration shorts and opens read correctly. 2. The pad I was using appears to have a frequency shift. With the 'legs balanced for length' the measurement would change with frequency. This really confused the measurement process. I put it aside. 3. I have some 23 Ohm couplers. These of course confuse readings when used as normal couplers. However if they are used in a truly balanced configuration - that is the legs are configured as close to identical as possible then they appear to work. I am going to shelve this until I find or make a 50 Ohm bi-directional coupler. Thanks again - Dan Owen Duffy wrote: On Mon, 02 Jan 2006 16:10:02 -0800, dansawyeror wrote: The coupler measures about -14 dbm. What does this mean? Are you trying to tell us that the power on the coupler port is 14dB less than the through power? What has dBm got to do with it? You didn't report the power in the coupler port with a s/c and / or o/c at the measurement plane. Did you perform this cal? (The phase angle and reflected ware very touchy. It was almost impossible to adjust by changing frequency. It was easier to 'adjust' it by sitting very still and moving my arm.) That is understandable. How much coax between the A probe and the load, and the B probe and the load... how many degress does this total electrical length change for a 1% change in frequency? Does that explain some of the phase sensitivity? The antenna is a copy from the ARRL handbook. It is a 4 inch segment, a 1 inch long by 3/4 inch diameter 5 turn coil, and a 4 inch tip. It is mounted over a 2 foot square aluminum plate. This antenna should have an input impedance less then 20 Ohms. Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... How can it measure very close to 50 Ohms? Is there something wrong with this analysis? See if you can trust your measurements on known loads before wondering why the unknown load isn't what you expect when using unknown measurement technology... too many unknowns. Owen -- |
#5
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using an HP 8405A to measure SWR
I now have use of a b-directional coupler for HF and would like to perform the
following 'tests' however I do not understand them. Can you explain the last a different way. I don't understand "two 50 Ohm loads in parallel with the stub". Tests of stubs are now clearly reading correct for various lengths across frequencies. That is once the 1/4 wave is determined the next odd and even is very predicable. Thanks, Dan Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... |
#6
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using an HP 8405A to measure SWR
On Thu, 05 Jan 2006 22:24:54 -0800, dansawyeror
wrote: I now have use of a b-directional coupler for HF and would like to perform the following 'tests' however I do not understand them. Can you explain the last a different way. I don't understand "two 50 Ohm loads in parallel with the stub". Parallel two 50 ohm loads with a tee piece, then use another tee to put the stub in parallel. The tees will be imperfect, but at HF, the impact should not be major. Tests of stubs are now clearly reading correct for various lengths across frequencies. That is once the 1/4 wave is determined the next odd and even is very predicable. Do you get the correct answers for 1/8, 3/8 wave stubs in parallel with a 50 ohm load? (1/8 wave s/c stub has an impedance close to 0+jRo, and you need to put it in parallel with the dummy load, so series equivalent is 25+j25. 25 ohms in parallel with 1.8 wave s/c stub should be around 20+j10. Put a 25 ohm load at the end of a metre of coax and check its transformation at different frequencies (equivalent to quarter wave, eight wave etc) against your Smith Chart prog. They are some examples, work some out for what you have at hand. Prove that your measurement system works on predictable loads. Owen Thanks, Dan Don't you need to measure some "known" loads. Why not try a 50 ohm load tee'd to a s/c stub (quarter wave at a known frequency) and see if you get the predictable results at different frequencies around resonance. Then try two 50 ohm loads in parallel with the stub. (339mm of RG58C/U should have a Z of around 6000+j0 ohms at around 146MHz, at half that frequency it should be 0.85+j50, etc... -- |
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