Varying power levels depending on antennas...

[SvenMarbles]

Well I'll admit, this is something that kind of puzzles me and I'd like to understand why this happens.

I've experienced many instances where your radio's output power seems to be influenced by different antennas on the other end.

For example,.. You have a mobile/base radio, coax, in-line power meter, coax, antenna. It's a 1.1 SWR and it reads 40 watts output. You then don't change anything except for the antenna and the end of the line, but now you're getting a 1.1 SWR and 32 watts on your power meter. How does the power level between the radio and the power meter get influenced by the antenna at the end of the equation? No additional reflection shown on the meter...

Does anyone else notice this happen?

 

[SteveShannon]

10 minutes ago, SvenMarbles said:

Well I'll admit, this is something that kind of puzzles me and I'd like to understand why this happens.

I've experienced many instances where your radio's output power seems to be influenced by different antennas on the other end.

For example,.. You have a mobile/base radio, coax, in-line power meter, coax, antenna. It's a 1.1 SWR and it reads 40 watts output. You then don't change anything except for the antenna and the end of the line, but now you're getting a 1.1 SWR and 32 watts on your power meter. How does the power level between the radio and the power meter get influenced by the antenna at the end of the equation? No additional reflection shown on the meter...

Does anyone else notice this happen?

 

Are you confident that your SWR/wattmeter is correct and sensitive enough?  Many inexpensive ones do a poor job of differentiating between very slight reflected power levels, especially at UHF. 
What does an antenna analyzer show for the two different combinations?

[SvenMarbles]

1 minute ago, SteveShannon said:

Are you confident that your SWR/wattmeter is correct and sensitive enough?  Many inexpensive ones do a poor job of differentiating between very slight reflected power levels, especially at UHF. 
What does an antenna analyzer show for the two different combinations?

No, it's the common inexpensive Surecom 102. But, I do look at it with a Nano VNA when I put up new antennas. There's never that much of a discrepancy.. But assuming there was perhaps a .1 deviation in reflection, would that account for a 20% difference in output level? Because that's the sort of thing that I encounter..

[SteveShannon]

28 minutes ago, SvenMarbles said:

No, it's the common inexpensive Surecom 102. But, I do look at it with a Nano VNA when I put up new antennas. There's never that much of a discrepancy.. But assuming there was perhaps a .1 deviation in reflection, would that account for a 20% difference in output level? Because that's the sort of thing that I encounter..

It really depends on the radio, but it could.  If the Surecom 102 is right at the edge of its ability to detect reflected power (which it very easily could be at an SWR of 1.1:1) it really might not be able to accurately tell the difference between 1.11:1 and 1.39:1.  Even Bird meters are only represented as reading +/-5%.  Most inexpensive meters are more likely 20%.

Are these readings all taken on the same transmit frequency?  Does your radio have an ALC circuit that adjusts to SWR?
Show us a picture of your sw-102 for both antennas so we can see all of the readings, please.  There’s a lot more information than just SWR and power.  Can you spot the errors in the following photo from the Radioddity site?:

[tcp2525]

1 hour ago, SvenMarbles said:

Well I'll admit, this is something that kind of puzzles me and I'd like to understand why this happens.

I've experienced many instances where your radio's output power seems to be influenced by different antennas on the other end.

For example,.. You have a mobile/base radio, coax, in-line power meter, coax, antenna. It's a 1.1 SWR and it reads 40 watts output. You then don't change anything except for the antenna and the end of the line, but now you're getting a 1.1 SWR and 32 watts on your power meter. How does the power level between the radio and the power meter get influenced by the antenna at the end of the equation? No additional reflection shown on the meter...

Does anyone else notice this happen?

 

What you are experiencing is the way different antennas are resonating. If you look at this using a VNA you will see even though the SWR is showing 1.1:1 for a given frequency, or relatively close to it, you will see the resistance can vary anywhere between 40-60 ohms.

The other main consideration is American watts versus Chinese watts. This is why I always use the famous Bird watt meter that accurately displays American watts and has the capability to covert Chinese watts to American watts with a high degree of accuracy. Most Chinese SWR/Watt meters simply can't handle the conversion properly.