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We are using a low cost (sketchy looking datasheet) external 915 MHz monopole antenna in our product and we're seeing a large variance in RF performance across the 50 pieces that we built.

2D Antenna Drawing

Here's what I was thinking to better understand this variance:

  1. Use a spectrum analyzer to measure the conducted emissions from the PCBA. If the signal strength is fairly similar across the units, then the variance is not from the PCBA
  2. Use a network analyzer to measure the impedance of the antenna

For (2), what is the correct way of measuring the impedance of the monopole antenna? Would it work to connect the monopole directly to the VNA? Monopoles require a good ground plane to work well, so is it safe to assume that the VNA has a good ground plane? Or would we have to measure its impedance when it is connected to the PCB? Does the VNA output a balanced or an unbalanced signal? Does it matter?

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I have one of those antennas somewhere. It was used screwed into a PCI wireless card.

That antenna intrinsically will vary in impedance. Impedance will depend on how much it is bent and will depend on the exact nature of the board it is screwed into, AND what that board is attached to, AND how good the electrical contacts are...

In other words, expect lots of variation. But then, antennas sort of kinda work even when not matched and not particularly close to resonance. They just don't work well.

It would be instructive to take a piece of metal, put an SMA connector on it, and connect a VNA in back of the metal sheet.

Direct attachment to the VNA means that the VNA itself, and whatever ill-defined stuff is connected to the VNA, provides a sort of ground plane. Such a measurement would not be very helpful. It would tell you if the antenna is approximately resonant at 915 MHz, but I think you know that already.

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The antenna is a shortened helical antenna that do not require a significant ground plane. They are commonly used on hand held radios. A good discussion by Igor Grigorov here. The small single wire section (shown in the circle labeled 4) may indicate some top loaded performance. It would be interesting to mount a connector to a variety of ground plane sheets with different areas to compare measurements. It is not a λ/4 monopole.

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