The power of standing wave is higher than that of standing wave

[rf2020]

Hi, everyone! I'm an electronic enthusiast from China. I'd like to ask you a question. Recently, I made an RF power amplifier with 2sc1971, and connected the transmitting antenna to adjust the standing wave to a minimum of 1.05. Instead, the transmitting power is not as large as when the standing wave is greater than 1.05. Why? Thank you!

 

[FvM]

Not clear which two cases are compared. How do you measure power and SWR, how do you adjust matching respectively SWR?

 

[vfone]

The reason is your Power/SWR meter.
Its internal coupler (used for sensing the forward and the reflected power) has very poor directivity.
So, a lot of the reflected power is added to the measured forward power, showing that the output power on the meter is higher than should be, when the SWR is high.

 

[rf2020]

Thank you for your reply! Sorry, I do n’t speak English. I use Google Translate. Maybe you do n’t understand what I mean. I ’m comparing the FWD power when SWR 1.05 and SWR> 1.05. Match between the transmitter and the transmitting antenna, and adjust the length of the transmitting antenna element.

 

[rf2020]

Thank you for your reply! My power / SWR meter is a newly purchased NISSEI GD-503. It seems that it is not possible to send pictures here, that is, my power / SWR meter is not qualified.

 

[betwixt]

I think the misunderstanding is of how these devices measure power. The SWR is calculated by measuring the forward and reflected currents and calculating their ratio, it does not have any specific units, one is relative to the other. For power they only measure the voltage at the input or output socket, either one but not both. The power is calculated using standard Ohms law (V^2/R) but assuming that the antenna load impedance is purely resistive and 50 Ohms. That means that the power measurement is only accurate into a 1:1 SWR, if it deviates, the power measurement is less accurate. You really need a pure resistive dummy load to measure power accurately.

You can test this easily by disconnecting the load completely and running low power to it. The power will appear to be unreasonably high considering nothing is actually being fed to the load at all.

Brian.

 

[rf2020]

I think the misunderstanding is of how these devices measure power. The SWR is calculated by measuring the forward and reflected currents and calculating their ratio, it does not have any specific units, one is relative to the other. For power they only measure the voltage at the input or output socket, either one but not both. The power is calculated using standard Ohms law (V^2/R) but assuming that the antenna load impedance is purely resistive and 50 Ohms. That means that the power measurement is only accurate into a 1:1 SWR, if it deviates, the power measurement is less accurate. You really need a pure resistive dummy load to measure power accurately.

You can test this easily by disconnecting the load completely and running low power to it. The power will appear to be unreasonably high considering nothing is actually being fed to the load at all.

Brian.

Thank you for your reply! I connected a 50 Ω dummy load and the power / SWR meter showed SWR 1.23， FWD 6W， REF 0.05W. I connected the transmitting antenna to adjust SWR to 1.23. The power / SWR meter showed FWD 5.1W， REF 0.05W

 

[vfone]

Most of the cheap Power/SWR meters doesn't calculate anything. They just sense the sides of a directive coupler, rectify the two signals (FW and Reflected) with a diode, and drive a galvanometer.
If the directivity of the coupler is bad (and most of the time is bad in this kind of meters) you get these reading problems.
https://forums.qrz.com/index.php?attachments/img_3957-jpg.358782/

Using a professional Power/SWR meter as the Bird series, you get accurate measurements.
https://birdrf.com/Products/Test an...s/Wattmeters-Line-Sections/RF-Wattmeters.aspx

 

[rf2020]

Most of the cheap Power/SWR meters doesn't calculate anything. They just sense the sides of a directive coupler, rectify the two signals (FW and Reflected) with a diode, and drive a galvanometer.
If the directivity of the coupler is bad (and most of the time is bad in this kind of meters) you get these reading problems.
https://forums.qrz.com/index.php?attachments/img_3957-jpg.358782/

Using a professional Power/SWR meter as the Bird series, you get accurate measurements.
https://birdrf.com/Products/Test an...s/Wattmeters-Line-Sections/RF-Wattmeters.aspx

I bought the Fumei DG-503 Digital LCD 3.5 "SWR / Watt Meter HF 1.6-60MHz & VHF / UHF 125-525MHz 1-200W for Two-Way Radio. This is not cheap. The price is $ 165 on Amazon. You recommend this Power meter cannot measure SWR

 

[vfone]

Unfortunately adding an LCD display and higher price, doesn't make a device better, when is about RF performances.

Myself I never heard about Fumei company, but this doesn't mean anything.
But if the rating site Eham.net doesn't mention any Fumei products, that means something:

https://www.eham.net/reviews/view-category?id=40

 

[rf2020]

Unfortunately adding an LCD display and higher price, doesn't make a device better, when is about RF performances.

Myself I never heard about Fumei company, but this doesn't mean anything.
But if the rating site Eham.net doesn't mention any Fumei products, that means something:

https://www.eham.net/reviews/view-category?id=40

Fumei is not the trademark of power / SWR meter, but the name of the distributor. The trademark of power / SWR meter is NISSEI. This company manufactures digital power / SWR meter for mfj company. The digital power / SWR meter manufactured by NISSEI for mfj company is the same as the model I bought

 

[rf2020]

Unfortunately adding an LCD display and higher price, doesn't make a device better, when is about RF performances.

Myself I never heard about Fumei company, but this doesn't mean anything.
But if the rating site Eham.net doesn't mention any Fumei products, that means something:

https://www.eham.net/reviews/view-category?id=40

This is the power / SWR meter I bought

 

[betwixt]

The name on it means nothing, the same model is also sold as :
Nissei DG-503
Pomya DG-503
Ashata DG-503
Pro Power (no model number)
and several 'unbranded' versions too. All at different prices.

Brian.

 

[ktr]

What a terrfying thread to read.