How to construct a quarter wave test antenna.


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A picture says a thousand words. In this case you will be able to see what vswr looks like. Actually, it is only the reflected power (return loss) you'll see, but since swr is relative, it gives you an idea of what is going on.
The antenna is just a quarter wave ground plane antenna. Nothing special, though it is only 28.5mm high in this case. I designed it as a test antenna for 2.5GHz, so it looks very small compared to what people are used to seeing. It is perfect for wireless lan 802.11 stuff. The ground plane was slightly bigger, about 30mm radius. I found it only made any real difference if it was smaller than 1/4 wavelength.
The signal that is injected is 1 mW (0dbm).
This is the second line down on the graph.
Each square on the graph is 2db. (-3db = 1/2 the power)
The top of the graph is +4dbm (2.5mW).


1mW is sent down the coax at all frequencies. The plot is looking only at what is coming back from the antenna.
If you look at 1.3Ghz, you'll find that -0.79dbm is coming back. The that is 834uW. If you scale it up.. if you put 100W into this antenna at 1.3GHz, 83 Watts would be coming back. (most of the power, and this would be enough to break you amplifier. ouch ! I don't care to rember the formula for vSWR, but I'd guess that this was worse than 5:1.

But at 2.5GHz, it's a different story. The antenna was designed for this frequency and you can see the big dip not far from it. At 2.5GHz the return loss is -12.95dB. This is almost 20 times less power. (50uW) Again, translate this into big power...

If I connect up my 100W microwave oven magnetron at 2.5GHz to this antenna, only 5 Watts makes it back to the magnetron. I'm just guessing (too lazy to work it out) that 95% is an SWR of about 1.5:1 which is ok for most situations.

An interesting side effect of this antenna is that is is also very well tuned at 7.35GHz. This works out to be 5/8 wavelength antenna. The return loss is -16dB. This is 40 times less power, so if 100W was sent to it at 7.35GHz, only 2.5 watts would be coming back down.

If there is a coax feed back to the transmitter that has a 3db loss, only 50W of your original 100W makes it to the antenna, where 1.25W would be reflected back to the transmitter, and half of that is lost again, so of your original 100W, only 750mW (0.75W) makes it back to the transmitter.
On your VSWR (swar) meter if it was back at the transmitter, this would read 1:1 and look like a very good antenna !





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