A newbie wanting to measure a CB (27MHz) antenna with a UHF plug when his nanoVNA has an SMA connector sought advice of the collected experts on groups.io.
One expert advised that 100mm wire clip leads would
work just fine. Another expert expanded on that with
When lengths approach 1/20 of a wavelength in free space, you should consider and use more rigorous connections.
At Antenna analyser – what if the device under test does not have a coax plug on it? I discussed using clip leads and estimated for those shown that they behaved like a transmission line segment with Zo=200Ω and vf=0.8.
So lets model the scenario of a perfect 50Ω load at 30MHz measured through λ/20 of clip lead with Zo=200Ω and vf=0.8.
The InsertionVSWR of the clip lead fixture is 3.
Note that Zo in this case is quite low because the wires are quite close together, Zo would easily exceed 200Ω with wider spacing and the transformation would be even worse.
Another expert opined
as long as the lead is much shorter than about one tenth of a wavelength, it won’t matter much. (Zo=200, VSWR=6.7)
In fact the InsertionVSWR of a λ/120 section exceeds 1.2 With a λ/120 fixture (80mm @ 30MHz), if the actual VSWR of the DUT was 1.5, you might read anywhere from 1.2 to 1.9 on the nanoVNA.
The outcome is ridiculous, the advice is ridiculous, this is the way we help beginners… or is it principally about inflating the egos of the experts?
At the time of writing this, the advice goes unchallenged… possibly because it is unsociable to call out BS.
A fair question is
can accurate VSWR measurements be made of a CB antenna with UHF connector?
The article A check load for antenna analysers with UHF series socket
describes a low cost UHF 50Ω load that will outperform most other types of UHF load. Lets use that and a SMA-UHF(F) adapter to measure insertion loss of the fixture and load adapter.
Above the SMA load with UHF(M) adapter, and a UHF(F) to SMA(M) adapter. UHF connectors do not have a constant through impedance of 50Ω, but how bad are they in this test?
Above, a sweep from 1-200MHz suggests that the adapters are quite suitable for modest accuracy measurements to 200MHz. This is a lot better than improvising with clip leads.
A word of warning, using large connectors and heavy cables might subject the nanoVNA on-board jacks to damaging forces… be careful.