Here is an example with the popular Sirio 5000 mobile antenna scanned with a Rigexpert AA-54 analyzer to show bandwidth and why I tune antennas to bandwidth rather than the tired old way to even out channel 1 and 40.
Here I scanned for the lowest SWR and moved the cursor arrow to near the lowest point in the SWR curve. This was done with whip inserted about midway into the antenna for an initial check. It's hard to see in the pic but the Frequency was 26.320 Mhz which is now displayed after moving the cursor arrow to the lowest point..
The analyzer bandwidth was set for +/- 1000 Khz. So you can now see the SWR curve at 500 KHz per side centered at 26.320 Mhz.
The bandwidth you can see is about 500 Khz just below a 2.0 SWR on the left, and it's just over 500 Khz before it would exceed a 2.0 SWR on the right.
Total bandwidth to operate this antenna under a 1.8 SWR would be about 800 Khz. That's plenty of bandwidth to operate on.
You can now see to operate on the CB band that the antenna is too long since it's resonate at 26 MHz, so I have to shorten it some for 27 Mhz.
With this kind of bandwidth on the Sirio 5000 antenna, If I make channel 20 the lowest SWR at near flat, I should see channels 1 and 40 at a 1.4 or less SWR. More than ideal!
Here are the numbers with this scan.
Here I scanned for the lowest SWR and moved the cursor arrow to near the lowest point in the SWR curve. This was done with whip inserted about midway into the antenna for an initial check. It's hard to see in the pic but the Frequency was 26.320 Mhz which is now displayed after moving the cursor arrow to the lowest point..
The analyzer bandwidth was set for +/- 1000 Khz. So you can now see the SWR curve at 500 KHz per side centered at 26.320 Mhz.
The bandwidth you can see is about 500 Khz just below a 2.0 SWR on the left, and it's just over 500 Khz before it would exceed a 2.0 SWR on the right.
Total bandwidth to operate this antenna under a 1.8 SWR would be about 800 Khz. That's plenty of bandwidth to operate on.
You can now see to operate on the CB band that the antenna is too long since it's resonate at 26 MHz, so I have to shorten it some for 27 Mhz.
With this kind of bandwidth on the Sirio 5000 antenna, If I make channel 20 the lowest SWR at near flat, I should see channels 1 and 40 at a 1.4 or less SWR. More than ideal!
Here are the numbers with this scan.
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