On 8/13/20 11:10 AM, K2STP Chris wrote:
So, when he hooks up the VNA to the coax in the shack to test, will he not need to use the TDR function to isolate the length of coax cable to the antenna? In other words, wont the long length of coax feed line affect the vna readings?
The calibration process "calibrates out" all the variations up to the "reference plane", which is where you put the short, open, load, thru cal standards.
Essentially, what happens mathematically is that you put a short on the end (which has a reflection coefficient(Gamma) of -1) - you measure all the values (Tx, Rx0) and you adjust the calibration parameters so that whenever those particular values are read, the display reads Gamma=-1.
it's way more complex in reality, because you have multiple "knowns" (short, open, load, thru), and multiple variables, so it winds up being a non-linear "solve multiple equations with multiple unknowns". Fortunately, it's been solved, in the generalized case, and that's what the calibration function does.
It's just a more complex mathematics than calculating VSWR from forward and reflected powers on a Bird wattmeter - it can incorporate a variety of systematic effects (time delay and loss in a transmission line from test set to the reference plane) and even more important in some cases, it can allow you to know the uncertainty of the resulting measurement. If I'm testing an amplifier, and I need to know that the S11 magnitude is <-20dB everywhere, and the measurement uncertainty is 1 dB, then to "pass" I have to measure -21 dB. If the uncertainty is 10dB, then to pass, I have to measure -30dB.
The problem with measuring with a long transmission line in the system is that if there is enough loss, the VNA's receiver doesn't see enough signal to make an accurate measurement. If you're measuring -20dB reflected power with a short transmission line, and I put 10dB of loss in, the receiver is now seeing -40dB (which it will "calibrate" to -20dB), but the lower you go, the more the SNR starts to affect the measurement accuracy.
The other place where a long cable bites you is in the large phase shift - you calibrate at a particular frequency and temperature, and if the temperature changes, the length and the propagation speed in the cable change, and with lots of "cycles" in the cable, that could affect the phase measurement. This is more a problem with microwave frequencies - at 10GHz, a 10 meter long cable is 500 cycles. A 0.1% speed change is 180 degrees of phase.
There are techniques to deal with this (put a deliberate small mismatch at the end of the coax, and *measure* the propagation constant, then use that to adjust the measured values)
I just bought, haven¡¯t installed yet, the MyAntennas.com EFHW-8010-2k
I also want to connect the vna to it and document it¡¯s performance on each band using Saver. I¡¯ve done this on small antenna¡¯s (HT¡¯s...) connected directly with no feed lines. But I wonder how to perform this task with long feed lines after the antenna is installed. Thanks for the help...
