Reducing output power

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SA4DHT
 

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Joined: 10 Jan 2022, 14:13

Reducing output power

Post by SA4DHT »

When using a mag loop antenna it would be great to be able to reduce output RF power for tuning. Is there a way to do so in the menu?

72, de Dirk
DL2MAN
 

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Joined: 30 Dec 2021, 19:18
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Re: Reducing output power

Post by DL2MAN »

No, but you can either reduce Input Voltage or run tuning from USB Power Supply (=0,5W Out).

73 Manuel; DL2MAN
Ohwenzelph
 

Posts: 207
Joined: 01 Jan 2022, 03:47

Re: Reducing output power

Post by Ohwenzelph »

Most of the QRPGuys tuners and such, include a "N7VE LED absorption bridge circuit for sensing SWR". I think the "absorption bridge" part can be replicated with 1)a double pole/double throw switch and 2)a 50 and 3)a 100 ohm resistors. With a slide switch you would slide up to tune and slide down to bypass it. Rig on the left, tuner on the right, in the tune position the rig should not see less than 33 or more than 100 ohms, so never worse than the equivalent of a 2:1 swr. (IF I figured it right...)This assumes the ground is connected straight thru, and I tried to draw BNC connectors (ok, laughing permitted...) but it could be half a coax jumper cable at each end or whatever. I don't know if that's good enough to protect the (tr)uSDX but if you tune for short intervals it ought to help.
Grounds connected
Grounds connected
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Last edited by Ohwenzelph on 28 Apr 2022, 16:50, edited 1 time in total.
SA4DHT
 

Posts: 19
Joined: 10 Jan 2022, 14:13

Re: Reducing output power

Post by SA4DHT »

DL2MAN wrote: 26 Apr 2022, 07:56 No, but you can either reduce Input Voltage or run tuning from USB Power Supply (=0,5W Out).

73 Manuel; DL2MAN
Yes, that is a good idea!

73 de Dirk
David Mangnall
 

Posts: 8
Joined: 31 Dec 2021, 11:13

Re: Reducing output power

Post by David Mangnall »

Oh … Right!
I thought I was reducing rf power by reducing
8.2 PA Bias max, from 141 (gives 5.16 W) to between 1 and 2.
What have I actually been doing then?
Am intrigued!
Thanks for this wonderful resource which has got me back after a 40 year break-and wow-how stuff has changed.
G8 SNR
DL2MAN
 

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Joined: 30 Dec 2021, 19:18
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Re: Reducing output power

Post by DL2MAN »

David Mangnall wrote: 28 Apr 2022, 14:54 (...)
I thought I was reducing rf power by reducing
8.2 PA Bias max, from 141 (gives 5.16 W) to between 1 and 2.
What have I actually been doing then?
(...)
Reduced your Output Power while your Input Power almost stayed the same ;)
In other words: You reduced your efficiency.

73 Manuel; DL2MAN
David Mangnall
 

Posts: 8
Joined: 31 Dec 2021, 11:13

Re: Reducing output power

Post by David Mangnall »

AHAAAYes I think I understand now
Reducing eff. would appear to degrade/lessen pwr o/p against i/p pwr. I then have 30db 10watt att into tinySA.
It has not blown SA(yet!).
These are, I know, very basic rf questions and it has been over 40 years since college and use of rf trickery.
Is there please a resource(s) you might signpost me to?
I think I have viewed most, if not all your vids(I have tiny memory so that means I remember only 6 of..!), as well as others. My mind just gets clogged, so any assistance to choose a good road to follow down will be brilliant.
Hopefully I will transmit and marvel about your slice of wonder up top of a nice hill. So far my UK built tr(u)Sdx seems to test all ok.
Again many thanks Manuel/Guido and community.
tnx
Dave
G8 SNR
John
 

Posts: 49
Joined: 19 Feb 2022, 08:56

Re: Reducing output power

Post by John »

A more long-winded reply. Some hams believe bias voltage for either bipolar or FET transistors sets the power output. They are wrong. To the degree it does affect average power output it is a side effect, and typically a bad one. The bias voltage/current is meant to get the base/gate transistor voltage/current up to a point where even a small incoming signal will immediately induce a linear output. This is necessary because transistors are imperfect devices and need some help to get them to a functional voltage/current starting point on their gate/base. If the bias is set correctly an incoming signal will generate a fairly clean output signal with few harmonics, and that point is where it should be adjusted to. If it is set incorrectly it will cause an output without an input, (wasted energy dissipation,) and/or audio distortion, and/or other undesirable, and possibly illegal harmonic content. With the (tr)uSDX as with many other transmitters the power output is set by both the input drive signal level and the DC voltage applied to the final amplifier.

The (tr)uSDX is set up so that it can be powered either off of a 5V USB cable or a 12V/13.8V power supply connector. When a rig that is set up to output 5 Watts using the 12V DC supply is powered by the 5V USB cable (+12V power removed,) it will still operate, since most of the radio normally (always) runs on 5 Volts, but the DC voltage available to drive the final amp stage has dropped to 5V from 12V, and thus the rig's output will drop to around .5 Watt. That's perfectly acceptable for very low power operation, as well as for safe tuning of antennas, as the 3 FET's can easily dissipate that power level internally if the SWR is high.
David Mangnall
 

Posts: 8
Joined: 31 Dec 2021, 11:13

Re: Reducing output power

Post by David Mangnall »

Thankyou.
Perfect answer, I get it now.
I hadn’t twigged that this is what occurring when I changed values. Its like what one does with valves(ish).
Learning/re-learning rf electronics is both rewarding and frustrating at the same time, so many thanks for your kindly nudge along the way.
Cheers
Dave.
G8 SNR
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