RF Sampler
This page was last updated : 14-Sep-23 18:05:27
This page is work in progress and will be updated frequently.
Updated : 12 December 2022 - added "Note to my self : read the article multiple times"
My OZ2CPU digitale RF mW-dBm-mV meter has a input range from 1nW to 1W which is good enough for QRPp rigs and my TinySA has a imput limit of +10dBm / 10mW. When you want to measure on equipment which gives more output power, you need a serious set of attenuators, or you could build yourself a 40dB RF sampler .
On the website of the OZ2CPU digital RF mW DBm mV meter the OM placed a section on how he build a one (scroll down to the section labeled "How To Measure High Power"). Which is also described on this page. Both seem to be inspired on Wes Hayward, W7ZOI, "Simple RF-Power Measurement," QST June 2001, pp. 38-43 (WayBackMachine link).
After reading these articles I decided that I want one in my shack and went to my local electronics store to get the parts. For the INPUT and OUTPUT connectors I have chosen BNC chassis part and for the sampling output a SMA chassis part. I used a double sided PCB of 1.6mm and on one side I cut out a 2.7mm path as a strip-line.
For the resistors I used 3 x 820 Ohm 1W resistors allowing it to handle 100W without a problem (if it's worth doing, it's worth overdoing).
After construction I calibrated my NanoVNA H4 and tested it up to 900MHz in pass-through mode.
Info : the image above shows a un-terminated Sampler output connector (SMA). For correct measurements it should be terminated with 50 ohm.
As shown below, the impact of the cables can be seen in the S11 LOGMAG measurement and the S11 SWR.
The S21 LOGMAG shows a kind of flat (minimal attenuation) over the entire span up to 450MHz.
Then I terminated the OUTPUT and hooked up the NanoVNA port 2 to the RF Sampler output.
As seen in the image below, for such a crude DIY rf sampler, the S11 LOGMAg is very acceptable.
The S21 LOGMAG starts from -40 dB at 100KHz but starts to decrease to -43.7dB at 433 MHz.
Because I want to use this RF Sampler for the amateur readio bands, I repeated the above measurment for HF up to 6m (100KHz-50MHz), the 2m band (144MHz to 146MHz) and the 70cm band (430MHz to 440MHz)
The results are shown below :
100KHz-50MHz overall attuenation aprx -40.1dB
144MHz to 146MHz overall attuenation aprx -41.2 dB
430MHz to 440MHz overall attuenation aprx -43.6 dB
3 labels from my label write to give it a more complete / finished DIY look.
Very happy with the result for now.
Note to my self : read the article multiple times
Although I was very happy with the result, I found out that I forgot a small thing.
To be exact, a small piece of wire which flattens the curve on higher frequencies.
So I installed it and use a small piece of shrink tube to hold it in place. I have experimented with the length and found out that for me this length was the best of all three worlds (HF, 2m and 70cm).
HF hasn't changed.
2m did. We have 0.5 dB better response.
70cm changed bigtime. We have 2.9 dB better response.
Of-course I needed new labels for 2m and 70cm.
So you see, it's worth reading the articles multiple times and it's never to late to experiment and improve.
What's next?
I want to add some photo's of test when using RF Sampler in combination with my OZ2CPU digitale RF mW-dBm-mV meter and TInySA.
To be continued . . . . . .
Youtubes about this device
Also there are some nice Youtube video's about this device like :
#61: Basics of RF Samplers, Sampling-Tees, RF-taps, etc. by W2AEW.
#150 RF Sampler RF Tap 3 in 1 Home Build by Tony Albus
How to Build an RF Power Tap by Level UP EE Lab,
How To Build An RF Sampler Box by K7AGE
DIY 40dB RF Power Tap for Ham Radio by TheSmokinApe
.
More interesting links about the RF Sampler
VK4HAT has build his own SMT RF Sampler and wrote some blog entries ( 1, 2, 3 ) about it. Nice work Rob.
When you want to go big time (50dB up to 1.5KW), check out this article.
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