Re: Topband: Bias Tee Measurements (Test Data)
Tom, you said Any test is meaningless, and I don't understand where you are coming from on that statement. If you look at my test, I tested with and without the bias Tee, and I tested with a well regulated bench top linear power supply, and the results are similar so I don't see how you can say you can't do that. I then went and tested with a wall wart power supply that produced 1.5 volts peak to peak ripple when under my 100 ohm load on the Bias Tee DC port, and it did indeed produce noisy data as you said would happen. I believe my test shows that you can indeed make valid measurements as long as you are using a well regulated supply. Please explain in more detail why you said it can't be done? Thanks, Don (wd8dsb) On Sat, Aug 23, 2014 at 11:29 PM, Tom W8JI w...@w8ji.com wrote: We cannot test impedance that way with a DC coupled impedance meter and get good data with any noise in the supply. Any test is meaningless. The .1 capacitor will just couple any distortion or ripple in the AC to the analyzer, where it would show as jitter or false readings. It would be a valid test if a moderately low resistance 200-500 uH RF choke shunted the analyzer (receiver) port. - Original Message - From: Don Kirk wd8...@gmail.com To: topband topband@contesting.com Sent: Saturday, August 23, 2014 10:48 PM Subject: Topband: Bias Tee Measurements (Test Data) Late last year Pete (N4ZR) reported problems with his home brew Bias Tee that included radical shifts in measured impedance when DC voltage was applied to his Bias Tee. Today I decided to make impedance measurements on a simple Bias Tee circuit I recently proposed for Dwight (NS9I) who was looking for a method of switching pennants via the feedline. I was not able to duplicate the problems that Pete reported, but I did notice unstable (noisy) impedance measurements when using a DC power supply on the Bias Tee that had a lot of ripple when under load, and below is my test data. My proposed Bias Tee schematic for NS9I is on my Pennant website at http://sites.google.com/site/pennantflagantennas/ -- *Test Data Using Resistor as the RF load * 66 foot of RG58U coax (measured Zo = 56 ohms) Test Frequency = 4.545 Mhz (frequency where the coax was an electrical 1/2 wavelength) RF Load = 50 ohm resistor No Bias Tee (Bias Tee bypassed) : R = 49, X = 0 Bias Tee (with 100 ohm 10 watt resistor connected to the Bias Tee DC output port = 120mA load when 12 volts is applied), and well regulated DC supply : 0 Vdc R = 51, X = 0 +12 Vdc R = 51, X =0 -12 Vdc R = 51, X = 0 Note : when using a DC supply that had 1.5 volts peak to peak ripple the measured R was jumping around between 46 and 53 ohms - *Test Data Using Transformer with the RF resistive load *66 foot of RG58U coax (measured Zo = 56 ohms) Test Frequency = 4.545 Mhz (frequency where the coax was an electrical 1/2 wavelength) RF Load : Transformer BN-73-202 Binocular core (Primary = 3 turns, Secondary = 12 turns) with 1K resistor connected to secondary Bias Tee (with 100 ohm 10 watt resistor connected to the Bias Tee DC output port = 120mA load when 12 volts is applied), and well regulated DC supply : 0 Vdc R = 55, X = 0 +12 Vdc R = 55, X =0 -12 Vdc R = 55, X = 0 Note : when using a DC supply that had 1.5 volts peak to peak ripple the measured R was jumping around between 49 and 58 ohms Note : For the above tests I was using an antenna analyzer that I designed and built last year, and the detector is based on the VK5JST antenna analyzer. The diodes used in the detector are germanium which have a high enough voltage rating to allow impedance measurements on the Bias Tee without the concern that W8JI had about detector diodes being damaged due to high voltage exposure when measuring Bias Tees. I repeated the 1st test shown above (Resistor as the RF load) between 1.4 and 12.5 Mhz and the results were similar (no change in measured impedance between 0, -12, and + 12 Vdc applied Bias Tee voltage). - *Conclusion* The Bias Tee (concept based on an AD5X Bias Tee design) measured impedance does not change between an applied voltage of 0 and +/-12 volts DC when using a well regulated supply (based on the 120 mA load used in my test) when tested between 1.4 and 12.5 Mhz. Just FYI, Don (wd8dsb) _ Topband Reflector Archives - http://www.contesting.com/_topband - No virus found in this message. Checked by AVG - www.avg.com Version: 2014.0.4745 / Virus Database: 4007/8088 - Release Date:
Re: Topband: Bias Tee Measurements (Test Data)
Hi Tom, I also just went and changed the RF load resistor to 200 ohms and checked readings at various frequencies where the impedance was part reactive. I also added a 3rd part to the test in which I physically removed (disconnected) the DC power supply from the Bias Tee. All tests done using my well regulated powder supply set at 12 Vdc for the Bias Tee DC voltage and 100 ohm load on the Bias Tee DC output port. Impedance readings are identical with the Bias Tee connected to the DC power supply versus not connected to the DC power supply and slightly different when the Bias Tee is removed from the circuit (all of which we should expect). Freq : 5.672 Mhz Without Bias Tee : R = 50, X = 45 With Bias Tee : R = 46, X = 39 With Bias Tee (but power supply connections removed from the circuit) : R = 46, X = 39 Freq : 1.501 Mhz Without Bias Tee : R = 26, X = 27 With Bias Tee : R = 31, X = 32 With Bias Tee (but power supply connections removed from the circuit) : R = 31, X = 32 The Bias Tee has a slight impact on the impedance but it has nothing to do with the DC power supply that powers the Bias Tee (as long as it's a well regulated supply), and the slight change in impedance is what we would expect with a well designed Bias Tee (the impedance change is real and due to the component selection within the bias tee). Therefore I say (based on my tests) that valid impedance measurements can be made on a Bias Tee using an antenna analyzer as long as the DC power supply feeding the Bias Tee is well regulated. I understand your concerns about the voltage ratings on the detector diodes used in the MFJ antenna analyzer not being adequate for measuring Bias Tee impedance, but that's an entirely different issue. Just FYI, and hope you agree. Don (wd8dsb) On Sun, Aug 24, 2014 at 2:06 AM, Don Kirk wd8...@gmail.com wrote: Tom, you said Any test is meaningless, and I don't understand where you are coming from on that statement. If you look at my test, I tested with and without the bias Tee, and I tested with a well regulated bench top linear power supply, and the results are similar so I don't see how you can say you can't do that. I then went and tested with a wall wart power supply that produced 1.5 volts peak to peak ripple when under my 100 ohm load on the Bias Tee DC port, and it did indeed produce noisy data as you said would happen. I believe my test shows that you can indeed make valid measurements as long as you are using a well regulated supply. Please explain in more detail why you said it can't be done? Thanks, Don (wd8dsb) _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bias Tee Measurements (Test Data)
Tom, you said Any test is meaningless, and I don't understand where you are coming from on that statement. If you look at my test, I tested with and without the bias Tee, and I tested with a well regulated bench top linear power supply, and the results are similar so I don't see how you can say you can't do that. I then went and tested with a wall wart power supply that produced 1.5 volts peak to peak ripple when under my 100 ohm load on the Bias Tee DC port, and it did indeed produce noisy data as you said would happen. I believe my test shows that you can indeed make valid measurements as long as you are using a well regulated supply. I understood your test to be an indictment of using an unregulated supply, and that a well regulated supply cures issues. Of course what you intended to convey and what I thought you intended to convey might be two different things. If you only meant the filtered dc (unrelated to regulation or lack of regulation) allowed a reading in that case, I agree. It allows a reading in that case, BUT it is still dangerous to use a low voltage diode test device on that line to test things. Because it can damage test equipment, I do not think it is a valid public test protocol for the general population. The issue is not regulation. The issue is noise or ripple making it into the analyzer port. We can run well regulated dc with ripple, or even unfiltered unregulated dc into the line and just clean it up at the relay end so the relay does not chatter, and get a valid dc test as long as test equipment is not sensitive to low frequency noise. We could never test ac, irrespective of filtering, regulation, or waveform, without skewing measurements. This means testing a four-way system with a bias T and ac, or rippled + or - dc, or with cable ground loop ac voltage offsets on the shield, can result in false readings. It won't affect the receiver at all, but we might think the system has a problem or damage our test gear. The best way to improve the test method and increase reliability is to make the measurement device insensitive to offset on the output. It is better to remove ground loop low frequency bias or coupling through the capacitor by making it a highpass filter for low frequencies, although I probably still would not switch the relays with my test equipment connected unless I confirmed no transients first. :-) The problem actually comes from the reactance of the series coupling capacitor and the sensitivity of many cheap measuring devices to out of band voltages. A solution that reduces low frequency offset from external ground loops and allows ac or unfiltered dc operation, is adding a shunting choke on the RX port. So to clarify, I am saying: 1.) the problem is not regulation, it is noise or ripple 2.) a filtered or regulated supply does not solve the ac mode test issue 3.) switching can result in a high voltage transient that can damage test gear 4.) low frequency ground loops might still inject ripple on long cable runs, or with poor shield connections 5.) regulation will still not allow an ac switch test There is a second caveat I have about switching high impedance lines. We have to be very careful about relay contact and wiring capacitance. Just 10 pF of contact capacitance is 8k ohms on 160 meters. We only have a coupled load to leakage path ratio of 10 times if we switch an 800 ohm line. That same leakage path to load becomes a 100 ratio if we use a transformer to 80 ohms at the switch point. Now I absolutely understand we will see an empirical good F/B ratio with some pretty dismal relay isolation, and of course it will not show as an SWR issue at all. I'm not disputing switching high Z lines will still make many people happy. When I build a system, I probably look at it differently. If I could have 15-20 dB of relay isolation when switching or 30-60 dB of relay isolation, and the difference only costs a couple dollars, I'd probably give up a cheeseburger and buy the piece of mind from switching low impedance points. If the transformers were $25 dollars each, I might not do that. My cheapness might take over. :-) Simplicity is a wonderful thing as long as the saving a few pennies does not cost us dollars of joy over time. When all the little mistakes we don't notice are added, we might be worse off than we assume. I'm simply offering a more reliable or accurate way to do things. 73 Tom _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Bias Tee Measurements (Test Data)
Hi Tom, Thanks for the clarification, and I don't disagree with your most recent posting except I consider a well regulated supply to be one that has low ripple, but that might be where you and I went astray on this posting (sorry about that). My posting was not to tell folks that they should repeat my same test since my antenna analyzer has diodes that likely have a higher voltage rating than some of the antenna analyzers commercially sold. I was just trying to follow up on the postings earlier this year regarding Petes Bias Tee problems in an attempt to shed some light on the topic. P.S. another ham thought it would be a good idea if I repeated my test using a battery, so here are the results (same feedline with 200 ohm RF load and 100 ohm DC load), and the battery yielded the same results as my well regulated DC supply (well regulated meaning low ripple and stable average voltage). Freq : 1.505 Mhz Bias Tee (+12 Vdc regulated supply) : R = 30, X = 32 Bias Tee (-12 Vdc regulated supply) : R = 30, X = 32 Bias Tee (0 Vdc regulated supply) : R = 30, X = 32 Bias Tee (no power supply, no battery) : R = 30, X = 32 Bias Tee (+13.06 Vdc SLA battery) : R = 30, X = 32 Bias Tee (-13.06 Vdc SLA battery) : R = 30, X = 32 No Bias Tee : R = 26, X = 27 73, and thanks for the lively discussion. Don On Sun, Aug 24, 2014 at 10:52 AM, Tom W8JI w...@w8ji.com wrote: Tom, you said Any test is meaningless, and I don't understand where you are coming from on that statement. If you look at my test, I tested with and without the bias Tee, and I tested with a well regulated bench top linear power supply, and the results are similar so I don't see how you can say you can't do that. I then went and tested with a wall wart power supply that produced 1.5 volts peak to peak ripple when under my 100 ohm load on the Bias Tee DC port, and it did indeed produce noisy data as you said would happen. I believe my test shows that you can indeed make valid measurements as long as you are using a well regulated supply. I understood your test to be an indictment of using an unregulated supply, and that a well regulated supply cures issues. Of course what you intended to convey and what I thought you intended to convey might be two different things. If you only meant the filtered dc (unrelated to regulation or lack of regulation) allowed a reading in that case, I agree. It allows a reading in that case, BUT it is still dangerous to use a low voltage diode test device on that line to test things. Because it can damage test equipment, I do not think it is a valid public test protocol for the general population. The issue is not regulation. The issue is noise or ripple making it into the analyzer port. We can run well regulated dc with ripple, or even unfiltered unregulated dc into the line and just clean it up at the relay end so the relay does not chatter, and get a valid dc test as long as test equipment is not sensitive to low frequency noise. We could never test ac, irrespective of filtering, regulation, or waveform, without skewing measurements. This means testing a four-way system with a bias T and ac, or rippled + or - dc, or with cable ground loop ac voltage offsets on the shield, can result in false readings. It won't affect the receiver at all, but we might think the system has a problem or damage our test gear. The best way to improve the test method and increase reliability is to make the measurement device insensitive to offset on the output. It is better to remove ground loop low frequency bias or coupling through the capacitor by making it a highpass filter for low frequencies, although I probably still would not switch the relays with my test equipment connected unless I confirmed no transients first. :-) The problem actually comes from the reactance of the series coupling capacitor and the sensitivity of many cheap measuring devices to out of band voltages. A solution that reduces low frequency offset from external ground loops and allows ac or unfiltered dc operation, is adding a shunting choke on the RX port. So to clarify, I am saying: 1.) the problem is not regulation, it is noise or ripple 2.) a filtered or regulated supply does not solve the ac mode test issue 3.) switching can result in a high voltage transient that can damage test gear 4.) low frequency ground loops might still inject ripple on long cable runs, or with poor shield connections 5.) regulation will still not allow an ac switch test There is a second caveat I have about switching high impedance lines. We have to be very careful about relay contact and wiring capacitance. Just 10 pF of contact capacitance is 8k ohms on 160 meters. We only have a coupled load to leakage path ratio of 10 times if we switch an 800 ohm line. That same leakage path to load becomes a 100 ratio if we use a transformer to 80 ohms at the switch point. Now I absolutely understand we will see an empirical good
Re: Topband: Bias Tee Measurements (Test Data)
We cannot test impedance that way with a DC coupled impedance meter and get good data with any noise in the supply. Any test is meaningless. The .1 capacitor will just couple any distortion or ripple in the AC to the analyzer, where it would show as jitter or false readings. It would be a valid test if a moderately low resistance 200-500 uH RF choke shunted the analyzer (receiver) port. - Original Message - From: Don Kirk wd8...@gmail.com To: topband topband@contesting.com Sent: Saturday, August 23, 2014 10:48 PM Subject: Topband: Bias Tee Measurements (Test Data) Late last year Pete (N4ZR) reported problems with his home brew Bias Tee that included radical shifts in measured impedance when DC voltage was applied to his Bias Tee. Today I decided to make impedance measurements on a simple Bias Tee circuit I recently proposed for Dwight (NS9I) who was looking for a method of switching pennants via the feedline. I was not able to duplicate the problems that Pete reported, but I did notice unstable (noisy) impedance measurements when using a DC power supply on the Bias Tee that had a lot of ripple when under load, and below is my test data. My proposed Bias Tee schematic for NS9I is on my Pennant website at http://sites.google.com/site/pennantflagantennas/ -- *Test Data Using Resistor as the RF load * 66 foot of RG58U coax (measured Zo = 56 ohms) Test Frequency = 4.545 Mhz (frequency where the coax was an electrical 1/2 wavelength) RF Load = 50 ohm resistor No Bias Tee (Bias Tee bypassed) : R = 49, X = 0 Bias Tee (with 100 ohm 10 watt resistor connected to the Bias Tee DC output port = 120mA load when 12 volts is applied), and well regulated DC supply : 0 Vdc R = 51, X = 0 +12 Vdc R = 51, X =0 -12 Vdc R = 51, X = 0 Note : when using a DC supply that had 1.5 volts peak to peak ripple the measured R was jumping around between 46 and 53 ohms - *Test Data Using Transformer with the RF resistive load *66 foot of RG58U coax (measured Zo = 56 ohms) Test Frequency = 4.545 Mhz (frequency where the coax was an electrical 1/2 wavelength) RF Load : Transformer BN-73-202 Binocular core (Primary = 3 turns, Secondary = 12 turns) with 1K resistor connected to secondary Bias Tee (with 100 ohm 10 watt resistor connected to the Bias Tee DC output port = 120mA load when 12 volts is applied), and well regulated DC supply : 0 Vdc R = 55, X = 0 +12 Vdc R = 55, X =0 -12 Vdc R = 55, X = 0 Note : when using a DC supply that had 1.5 volts peak to peak ripple the measured R was jumping around between 49 and 58 ohms Note : For the above tests I was using an antenna analyzer that I designed and built last year, and the detector is based on the VK5JST antenna analyzer. The diodes used in the detector are germanium which have a high enough voltage rating to allow impedance measurements on the Bias Tee without the concern that W8JI had about detector diodes being damaged due to high voltage exposure when measuring Bias Tees. I repeated the 1st test shown above (Resistor as the RF load) between 1.4 and 12.5 Mhz and the results were similar (no change in measured impedance between 0, -12, and + 12 Vdc applied Bias Tee voltage). - *Conclusion* The Bias Tee (concept based on an AD5X Bias Tee design) measured impedance does not change between an applied voltage of 0 and +/-12 volts DC when using a well regulated supply (based on the 120 mA load used in my test) when tested between 1.4 and 12.5 Mhz. Just FYI, Don (wd8dsb) _ Topband Reflector Archives - http://www.contesting.com/_topband - No virus found in this message. Checked by AVG - www.avg.com Version: 2014.0.4745 / Virus Database: 4007/8088 - Release Date: 08/23/14 _ Topband Reflector Archives - http://www.contesting.com/_topband
