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Re: LCR inductance measurement and determination of the coil-capacitance
Good to see some interest in the subject. Four members have requested access to the Google spreadsheet which I granted. Presumably, the direct link requires to have a Google account. |
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LCR inductance measurement and determination of the coil-capacitance
Hi Coil capacitance can be derived with ease from the coil's resonant frequency, however accurate measurements of coil resonance are not so easy to perform. What is easy, is measuring the coil inductance with the FA-VA5 on a range of frequencies and deriving the coil capacitance from the measurement data. An ideal inductance has no capacitance at all and therefore the inductance L is frequency-independent. Real inductances (coils)? always have a more or less noticeable coil capacitance which makes the "effective inductance" (what the FA-VA5 measures) frequency dependent. The valid equivalent circuit of a practical "real-world" coil is simply that of a parallel-tuned resonant circuit. Validity exists at least up to the resonant frequency which is naturally quite far above the frequency of operation of the coil (at which the coil is used). If we go higher in frequency the equivalent circuit loses validity due to "higher order effects" which are beyond the scope of this message. I have prepared a Google-sheet for finding the coil capacitance from FA-VA5 inductance (LCR) measurements. Below is a link to the sheet.? The idea is to calculate the admittance (1/reactance) of both the measured L value and the L value derived from the equivalent circuit. We start with a guess-value (small C) for the coil capacitance and observe the resulting two curves of L versus frequency. When both curves coincide, the value of the parallel capacitance is correct and the equivalent circuit is valid.?
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Re: Calibration Question
Hello Hans
did you do a "master calibration" as described on page-12 of the manual ? The data of the "current calibration" on page 11 is lost when the unit is switched off (page 11, last par). This might explain your problem with a 25 Ohm load.? I am not aware of using electrical length (delay) data in the VA5 calibration. Maybe someone else can answer that question. If the electrical length of the calibration standard is small, it will only affect measurement accuracy at relatively high frequencies. Certainly not a problem on HF. 73 Dieter, VK3FFB |
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Locked
USB cable for FA-VA5 operation via PC
It is perhaps time to highlight once more that many so-called USB cables that seem to suit the FA-VA5 are actually? |
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FA-VA5 measuring bandpass filter passband VSWR
I have often wondered whether my FA-VA5 can accurately measure a UHF (70cm band) bandpass filter I made recently.
Well, I tried it out today and the answer is: yes, it can.? While a filter is clearly a 2-port device with input and output, there is a way to measure at least the passband VSWR. The latter is a good indication of proper tuning of the filter. The idea of a filter is to deliver a filtered output with only the desired frequency spectrum of interest. Take for example a simple USB-type SDR and connect it to a good outdoor antenna. The poor SDR may get quite overwhelmed by the strong RF signals at its input. Most antennas have little or no real selectivity. (One exception is perhaps the magnetic loop antenna). So, the filter cures that, and often the difference is quite remarkable. Back to the measurement. The filter under test was terminated at its output with a good 50 Ohm termination. In this arrangement, the input impedance should be close to 50 Ohms over the entire filter passband, meaning low VSWR of course.? Outside the passband, the VSWR deteriorates sharply because all unwanted signals outside the pass band shall be reflected back and not delivered to the output.? So, by terminating the filter output with 50 Ohms, we "convert" the 2-port filter to a 1-port device and that is what the FA-VA5 can handle. The results shown in the images are quite impressive and not far from measurements with a big VNA boat-anchor machine. In cases where a filter is deployed somewhere and taking it out is a problem, the FA-VA5 can sure be used to check the tuning status. Perhaps a curing re-tune can also be performed. Any comments or questions - pls reply or send me an e-mail. 73 Dieter, VK3FFB |
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Printing problem
¿ªÔÆÌåÓýA cordial greeting to all.I am a new user of FA-VA5 and VNWA software. I have a problem: when I print (File/Print) a green band always appears as in the image below. - What does this band mean? - Is there a way to not make it appear? Thanks for any suggestions. 73 de ik3ogn GFranco |
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Re: Measuring Coax Loss
Hi Peter,
sorry for the delay. For measuring cable loss, you can use RL (return loss). You disconnect the cable at the antenna side and?leave the cable unterminated. The RL you measure is then twice the cable loss or Cable Loss = 0.5 x RL This is so because the forward wave and the reflected wave are attenuated. In a way it is like having twice the actual cable length :) I hope this helps. 73 Dieter, VK3FFB |
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Measuring traps with the FA-VA5
Hi Matt
Sounds good. Here is some advice regarding the measurement of traps. I have used this method successfully when I made a trapped dipole for 18 and 14 MHz. When measuring a trap, we have to consider that a trap is a parallel-tuned circuit. It is therefore not advisable to measure traps directly with the FA-VA5. Any small amount of capacitance will falsify the measurement.? I found the best way to measure traps is by using the FA-VA5 like we would use a Dip-meter. We can do this by setting the FA-VA5 to VSWR sweep mode with a narrow sweep width of say 1..2 MHz. You may need to refer to the manual (pages 15 to 17). Then we connect a suitable one or two-winding coupling coil made from solid wire to the FA-VA5 BNC socket. Coil diameter is not critical and should suit the trap coil. The idea is to couple loosely to the trap and observe a small VSWR dip in the VSWR graph at coil resonance. In order to obtain best results, the weakest?coupling that still gives a visible VSWR dip will ensure highest accuracy. The marker function may come in very handy. Keep in mind that during marker adjustment, the FA-VA5 does not make new measurements ! This worked rather well here and I found very good agreement with a measurement made with my boat-anchor Agilent 8753 analyser.? During trap measurement, the trap should be placed far away from any metal. You can support it with non-metallic supports. If the above is not entirely clear, please let me know. 73 Dieter, VK3FFB |
Dieter VK3FFB
Wes N7WS