SWR Analysis of a Cheap eBay WiFi Yagi Antenna

While perusing amateur radio equipment on eBay recently, I came across some incredibly cheap high-gain Yagi wireless network antennas from China. This type of directional antenna is very useful for accessing WiFi networks which would normally be too far away, or triangulating the position of WiFi access points. I usually find commercial antennas to be ‘surprisingly expensive’, but this 25dBi 16-element Yagi I found on eBay was ‘surprisingly cheap’ at a mere £6 delivered. Perhaps I should have trusted my gut instinct and kept on surfing, but for such a low price I thought I’d give it a go.

I usually have quite good luck buying cheap electronics from China on eBay but this antenna took almost three months to arrive. It either got lost in the post or the eBay seller took ages to make a trip down to the Shenzhen electronics market to get their stock. When it did arrive, it looked alright:

The cheap eBay 25dBi 16-element WiFi Yagi antenna.

While waiting for the antenna to arrive I had read some pretty poor reviews of these cheap WiFi antennas including one on the asdfghjkl.me blog (in my excitement at the low price I neglected to check any reviews beforehand!). I thought it would be a good idea to check the standing wave ratio measurement (tuning) on the antenna before connecting it to my trusty Alfa USB wireless adapter. A poorly tuned antenna will have a high SWR, resulting in a significant amount of the transmitter’s radio energy being reflected back from the antenna down the feeder. If the SWR is too high, this reflected power can damage or destroy the radio transmitter electronics.

Antenna analysers that are capable of measuring 2.4GHz equipment are rather expensive, so I asked a friend to use his MRS miniVNA Tiny 3GHz analyser on the Yagi. The miniVNA produced the following SWR and impedance measurement:

SWR analysis plot for the WiFi Yagi antenna, produced by the miniVNA.

The blue plot is the antenna impedance, which should ideally (in this case) be a relatively constant 50 Ohms. The light blue horizontal line shows the 50 Ohm target. The red plot shows the SWR measurement, which in an ideal antenna would be at a ratio of 1:1 (no return power), but conventionally anything below 1.5:1 is fine. The yellow box highlights the miniVNA results which are within the 2.4GHz WiFi operating frequency range.

What was measured is a pretty poor result, showing adversely high (> 1.5:1) SWR across much of the WiFi channels. The SWR curve’s dip seen around channel 13 should be as wide as the WiFi frequency range (the yellow box). Moving the antenna around and placing it on different objects during testing did make some differences to the results, but one thing was consistent: this antenna was not properly engineered for 2.4GHz operation. Although it performs pretty well around channel 13, it’s only marginally better than a piece of damp string on the lowest channels.

The only saving grace for this cheap antenna is that normal WiFi adapters don’t put out much power – less than 500mW – so in a practical sense the poor SWR performance is not too much of a concern (although it’s still not going to perform as well as this style of antenna could). My ALFA Network external WiFi adapter (AWUS051NH) does work with the antenna and I do get better range with this Yagi vs a large 8dBi ‘rubber-ducky’ omnidirectional antenna.

I will hold on to this antenna and make use of it, but I certainly won’t be recommending it. I don’t think it will be very easy to attempt retuning it either. If you are looking for a proper WiFi Yagi antenna, the MFJ-1800 is well worth a look. It’s not much more expensive and will be properly tuned by people who aren’t in the business of selling cheap jewellery and dog toys on eBay.

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