Getting Started with the VNWA
June 3, 2014 Leave a comment
I just got a vector network analyzer (VNA) from SDR Kits and after performing the calibration procedure, I decided to try it out quickly on a few VHF and UHF antennas. The version I got is called VNWA 3E. The antennas I tried were the 144-148MHz rubber ducky that came with a Kenwood TR-2500 (some 30 years ago), an 434MHz rubber ducky that came with the Texas Instruments (SPWH24433TI made by Pulse Larsen), an 144/430/1200MHz Diamond antenna (SRH815), short VHF and UHF antennas that came with a Baofeng UV3R, and a 145MHz PCB magnetic loop that I built a while ago. You can see the VNA and the antennas (ordered from left to right) in the picture on the right.
The Kenwood antenna performs reasonably well in its design frequencies, but at least when mounted directly on the VNA (as opposed to being mounted on the transceiver it was designed for), it resonates a little higher than its design frequencies. When I placed my hand on the VNA, the resonance frequency dropped significantly, to well below 144MHz. Placing my hand on the VNA puts the antenna in a fairly similar environment to the one it was designed for, namely being on top of a handheld transceiver. Either with my hand or without it, the SWR between 144 and 148MHz was not much worse than 3:1 (return loss of -6dB).
Staying on the same band, the Baofeng antenna performed a little worse. Without my hand the return loss was as low as -4dB. With my hand on the VNA, resonance moved into the band and return loss improved.
The Diamond performed poorly with or without my hand on the VNA. This is a pretty expensive antenna (about $30), so this is disappointing.
The Diamond performs better at UHF, where its length is about λ/4 at 430MHz and about 3λ/4 near 1300MHz.
The Pulse UHF antenna, which is also a λ/4 whip, is the best performer of the lot. The SWR is less than 1.4:1 from 430 to 450MHz.
The short UHF antenna that came with the UV-3R is okay at 430MHz, but pretty bad at 450MHz. Not too bad.
Finally, the loop antenna. It is very narrow band, as expected from a small loop. The useful bandwidth is about 1MHz.
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