Thursday, October 8, 2009

N2PK VNA - It is a "Lab in a Box"!

I love my new toy! It is so nice to be able to perform all kinds of measurements and tests without begging "that guy with the HP VNA" and be left at his mercy (and free time).
(btw. Here is a link to a very informative document about the VNA basics by Agilent)

I tested a transmitter low-pass filter (C-511-T by Bell Industries) which claims 80db attenuation at 54 Mhz on its label.

As one can see - attenuation is only 63 dB at 54 Mhz with maximum attenuation of 66.5 dB at 57 MHz - that is pretty far from the claimed "80 dB". This screen-grab shows the true beauty of the Dual Detector setup - for example being able to plot the VSWR curve in reflection mode at the same time as the attenuation curve in transmission mode.

Its not only a VNA - it is a handy signal generator - myVNA software allows you to use it as adjustable signal generator. This picture shows a rough frequency measurement (my scope has current calibration), 14 Mhz - same as the value set in the software. (the oscilloscope of course can't measure with such great resolution as what the software allows you to adjust but gives an idea about the clean sine wavefrom generated by the DDS).

Among other things - there is a vector volt-meter available in myVNA. This image shows the result of -10 dBm output measured thru a variable precision attenuator (set to 8dB) at 6 MHz : -18.0026 dBm. I'd say pretty accurate! The vector voltmeter can use both ADC at the same time for measurements and display the phase difference as well.

Plot of the Return Loss of my LMR-600 coaxial feedline going to the SteppIR vertical. The length of the cable is exactly 100 ft (+/- 1 ft). The graph shows a Return Loss of approx. 1.2 dB @ 50 MHz. The actual loss is one half of the measured RL (just one way) or 0.6 dB @ 50 MHz. Times Microwave specs for LMR-600 are listed as 0.5 dB @ 50 MHz per 100 ft. A 0.1 dB difference is insignificant (I have connectors on both ends and I used a BNC-to-N adapter for the measurement). The cable is under ground in a PVC conduit (hopefully watertight) but I can keep an eye on the losses with the VNA.

Here is another interesting plot. This graph shows the Return Loss of my entire station - from the transceiver antenna connector to the antenna feed-point connector 100 ft away. The signal path includes a RF patch bay, 2 coaxial switches, 2 directional couplers, gas-discharge lightning arrester, ACOM 1000 amplifier, common-mode choke, well over 100 ft of coaxial cable (in transmission line and jumpers) as well as a large amount of UHF and N connectors.
Detailed RF signal path in the AE1S station can be seen in an old post - here
The loss is approx 1.9 dB on 6 meters (again RL/2) and between 0.3 dB and 1.25 dB on the HF bands. Not bad at all, keeping in mind what is on the signal's path!
The two distinctive "peaks" on the graph are caused by the ACOM 1000 amplifier. These peaks represent the self-resonance of a built-in choke in the amp. The good news is that they are located outside of the amateur bands.

... and I am just scratching the surface of this great instrument!

1 comment:

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