antenna-theory.com

[Sekutma]

I know that SWR doesn't directly correlate with gain but what would cause peak gain at a high SWR and a low gain at low SWR?

For example, the antenna I am tuning in has an SWR < 2:1 from 1.9 GHz to 2.2 GHz, however gain is less then 1 dbi that entire band. However at 2.6 GHz it peaks at about 4 dbi. I have had this several times before but I was wondering what is causing this? At 2.6ghz the SWR > 4:1.

[admin]

Antenna Gain correlates to antenna efficiency, not VSWR.

If you take a 50 ohm resistor and measure the VSWR, it will be close to 1. However the gain will be negative infinity dB

[helix]

How are you making you gain measurement?

[Sekutma]

We have an anechoic chamber to test antennas in. I've run several of our regular antennas and the gain numbers come out just fine. Sounds like it's an efficiency issue.

So any tips on improving efficiency? Some type data I could use to help the design?

[hagster]

I presume because the antenna is electrically longer at the higher frequency. Antennas with a larger aperture have more directivity.

[Parna]

When you are measuring your antenna gain in an anechoic chamber, VSWR matters because the power that gets reflected back at the antenna terminal won't be picked up by the probe (realized gain is what the text book calls it).

To have an accurate gain measurement you probably want at least 10dB RL. With the VSWR of 4:1 (only 4dB RL) your gain measurement won't be accurate (you would measure an even higher gain if you had better RL i.e. less reflected power).

Also, antenna losses tend to increase with frequency, so the reason for the higher gain is probably the higher directivity at the higher frequency.

[S^3]

@ Sekutma,
Antenna gain is related to the directivity of antenna in the required direction. VSWR is related to Return Loss and is a function of input matching with the circuitry near the input ( e.g. Balun, Circulaters etc. ) plus of course the antenna but is not related with gain alone. Therefore, better VSWR does not necessarily imply good gain of the antenna.

You may try to load the actual antenna connection port with a proper termination instead of the antenna and measure VSWR. This will give the practical characteristics of interface between actual antenna port and the input.

BTW, as pointed out by Parna, your VSWR/RL needs to be improved a lot.

Also getting a better gain at 2.6 GHz may not be the characteristics of antenna directivity but the intermediate circuitry may have a better transmittivity at 2.6 GHz so it provides more power to antenna which you may mistake for a better antenna gain