Power adjustment question

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kt4lh
Posts: 272
Joined: Sun Jan 12, 2020 4:27 am

Power adjustment question

Post by kt4lh » Fri Oct 08, 2021 6:30 pm

I got a SW-33 power meter (says it'll do .1 to 100W) today with a 5W dummy load and have been of course measuring everything. My GD77 (analog mode of course!) seems to do the following. VHF is fine, but wow UHF is way down on expected power on the low end. I don't really care about 5W vs 4.4W or anything, but would be nice for the sub-1W to be somewhat closer :) Is this something if I really wanted to tweak I'd do in the Calibration? I took some screenshots of my calibration screens here: https://imgur.com/a/20xjZrt

VHF
50 - Doesn't register
250 - .1
500 - .6
750 - 1.13
1W - 1.4
2W - 2.53
3W - 3.55
4W - 4.55
5W - 5.33

UHF
50 - Doesn't register
250 - Doesn't register
500 - .03
750 - .33
1W - .68
2W - 1.75
3W - 2.65
4W - 3.5
5W - 4.4

For giggles, I changed UHF 1W 445MHz from 99 to 110 and it did bring 1W up to 1.25W, 750 is now .86, 500 is .38, 250 and 50 still don't register. Interesting thing is I use a local repeater at "250mW" quite a lot, and it appears to be <100mW in the original calibration :D
Last edited by kt4lh on Fri Oct 08, 2021 9:00 pm, edited 1 time in total.

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Power adjustment question

Post by VK3KYY » Fri Oct 08, 2021 8:42 pm

kt4lh wrote:
Fri Oct 08, 2021 6:30 pm
I got a SW-33 power meter (says it'll do .1 to 100W) today with a 5W dummy load and have been of course measuring everything. My GD77 seems to do the following. VHF is fine, but wow UHF is way down on expected power on the low end. I don't really care about 5W vs 4.4W or anything, but would be nice for the sub-1W to be somewhat closer :) Is this something if I really wanted to tweak I'd do in the Calibration? I took some screenshots of my calibration screens here: https://imgur.com/a/20xjZrt

VHF
50 - Doesn't register
250 - .1
500 - .6
750 - 1.13
1W - 1.4
2W - 2.53
3W - 3.55
4W - 4.55
5W - 5.33

UHF
50 - Doesn't register
250 - Doesn't register
500 - .03
750 - .33
1W - .68
2W - 1.75
3W - 2.65
4W - 3.5
5W - 4.4

For giggles, I changed UHF 1W 445MHz from 99 to 110 and it did bring 1W up to 1.25W, 750 is now .86, 500 is .38, 250 and 50 still don't register. Interesting thing is I use a local repeater at "250mW" quite a lot, and it appears to be <100mW in the original calibration :D

The power levels below 1W set to a proportion of the value at 1W

So unless you calibrate the 1W value all the other values will be wrong.

Likewise. the values between 1W and 5W are interpolated between the 1W and 5W calibration points.


Power levels below 1W can never be super accurate, becasue the calibration data format only has 2 data points. 1W and 5W, and the PA FETS in each radio will have slightly different gain.

You would also need to make sure your power meter is itself accurate, especially at low power levels.

Looking at the price of the meter, i.e sub $50, I would be cautious about guaranteeing its accuracy.

kt4lh
Posts: 272
Joined: Sun Jan 12, 2020 4:27 am

Re: Power adjustment question

Post by kt4lh » Fri Oct 08, 2021 9:04 pm

Yeah I'm not assuming perfect accuracy by any means, but just comparing relative values seems reasonable, although it might also not be linear at all. Really no way for me to measure otherwise.

Even after my tweaks, 1W is now 1.35, 250 and 50 still don't measure, but the others are pretty close, 450mW at 500mW setting and 980mW for the 750mW setting. I tweaked 440 and 445 from 99 to 112 I think is where I landed. Close enough I suppose. :)

Interestingly my Anytone 878 also gives pretty close values at Medium, HIgh, and Turbo, but won't read anything on Low. Yet it seems like the meter can do something pretty low, given it was showing .03W on UHF 500mW at first.

Ah well, it's probably tweaked a little closer to the value now, didn't mess with any high settings, nor User Power in the radio, don't see the need to output half a watt more on High or anything.

VK3KYY
Posts: 7478
Joined: Sat Nov 16, 2019 3:25 am
Location: Melbourne, Australia

Re: Power adjustment question

Post by VK3KYY » Fri Oct 08, 2021 9:41 pm

Despite these radios having calibration data where they appear to have the frequency and other settings calibrated

The factory calibration of the power is often wildly incorrect
I doubt they calibrate the power level at all as part of the factory calibration process.

If you want vaguely accurate power output, even on 1W and 5W on 2m and thoughout the 70cm band, you will have to calibrate the 1W and 5W values for all the frequency ranges you intend to use

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F1RMB
Posts: 2518
Joined: Sat Nov 16, 2019 5:42 am
Location: Grenoble, France

Re: Power adjustment question

Post by F1RMB » Sat Oct 09, 2021 2:55 am

Hi,

The SW-33 Mark II can't make power measurements below 100mW (I guess that value is way above):
sw-33-part-specs.png
sw-33-part-specs.png (16.71 KiB) Viewed 1913 times
https://www.surecom.com.hk/sw33markii


Cheers,
---
Daniel

kt4lh
Posts: 272
Joined: Sun Jan 12, 2020 4:27 am

Re: Power adjustment question

Post by kt4lh » Sat Oct 09, 2021 1:53 pm

F1RMB wrote:
Sat Oct 09, 2021 2:55 am
Hi,

The SW-33 Mark II can't make power measurements below 100mW (I guess that value is way above):
sw-33-part-specs.png

https://www.surecom.com.hk/sw33markii


Cheers,
---
Daniel
Yep, had that in the original post. Interestingly it did register .03 originally on 70cm, but who knows about that. Everything is closer now, although honestly, it was working fine into the local repeater which is at 400 feet (450 total above me), moto repeater, perfect line of sight at 2 miles. I can see the antenna. So probably only takes a few milliwatts to hit it really.

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