Limit power for TX

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papkie
 

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Joined: 25 Nov 2022, 11:38

Limit power for TX

Post by papkie »

My first post here, hey!

I wonder if there is a way to limit output power while transmitting. I have fried my BS170s this tuesday when I was transmitting FT8 with 5w (12v). Then it suddenly stopped transmitting (PWR 0) - probably my antenna SWR is too high (~2).
I checked FETs and all of them are broken.
I'm replacing my FETs today, and I wonder if there is a way to limit power to eg. 3W with software, to avoid burning anything again.
Maybe setting MAX PA Bias will do the job?

What do you think?
DL2MAN
 

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Re: Limit power for TX

Post by DL2MAN »

Pa bias max should be Set for max efficiency and Not be touched beyond that.
If you want to Limit Output Power: simply do so by reducing Input voltage.

73, Manuel DL2MAN
N6YWU
 

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Re: Limit power for TX

Post by N6YWU »

If you can't reduce the input power supply voltage, you can insert a 50 Ohm 5W rated 3 dB attenuator at the antenna output. The input power will be the same, but the power sent to a 50 Ohm antenna will be reduced by 50%, the difference being dissipated in the attenuator. The 3 dB attenuator will limit the worse case SWR seen by the transmitter to a max of 2:1, even with a completely open or shorted antenna on the other side of the attenuator. Thus, an inserted 3 dB attenuator makes connecting an unknown antenna (for testing, etc.) much safer for the finals.

Question: How much should the input voltage be reduced to reduce the transmitter output power by 50%?

73, Ron, N6YWU
DD5MA
 

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Joined: 11 Mar 2022, 21:20

Re: Limit power for TX

Post by DD5MA »

Reduce Input Voltage down to 9 V and you will get appr. 3 Watts HF out.
But even better ist toimprove your SWR. 2.0 is to high for key-down modes like FT8 if you transmit to long.
SWR 1.5 will probably do.
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