How much I can trust to AIM4170?

[ve3ssr]

Last weekend I have checked 3-el 20M quad by AIM-4170. It showed minimum SWR at around 14.350. So I decided to climb on the tower and to make appropriate adjustments.

To be on the safe side I double checked the antenna with Daiwa NS 660A SWR meter. It indicated SWR minimum at 14.190
I used Daiwa for many years and have never had a reason to question its readings.
The difference between 14.350 and 14.190 is only 1%, but in real life it means a lot e.g. to climb on the tower or not to climb???

At last I checked the antenna with ICOM-756 Pro-III internal SWR meter. It measured minimum SWR at 14.190 that was identical to Daiwa's measurements. Finally I decided not to climb to the tower this time.
Now I am a bit confused. Can we use AIM-4170 for antenna measurements? If so, how much can we trust to it?

73, VE3SSR

[wa3off]

I've seen something similar and I wonder if it has to do with the amount of power used when checking the SWR. In my case, I have a 9:1 balun in the line, so I'm not too surprised to see some difference due to power level. However, I have yet to try different power levels from the same source, particularly since I don't have another source that can get measurable power anywhere near as low as what the AIM4170 uses.

Also, what might be more important than the frequency difference between minimum SWR points is the SWR difference measured by an individual device at the two frequencies.

Dave, WA3OFF

[Bob]

Be sure the value of Zo is appropriate for your antenna system. Sometimes the default value is 75 ohms. If you need 50 ohms this can make a difference. If you're using 53 ohm coax, try that value too. I just checked one of my antennas and the min swr shifted over 100KHz by changing from 50 to 75 ohms. Note that when you change the value of Zo, the same raw data is replotted using that new value, so there is no additional measurement noise involved.

If there is a balun in the system, you may be able to get an idea of any non-linearity by reducing the power level by 50% or 25%. If the balun is non-linear, the effect will change a lot when the power level is reduced even a moderate amount, so you may still be able to use the internal swr meter.

73/ Bob

[ve3ssr]

I would like to thank Bob for his comments regarding importance of entering correct value of Zo. After making adjustment from 75 to 50 Ohm the calculated SWR minimum shifted down to 14.190.

I BELEIVE WE CAN TRUST TO AIM4170.

73, VE3SSR