I’m the proud owner of a MiniVNA for quite a few years already. One of its features is that you can use it as an RF signal generator. However, the output power cannot be adjusted, so I needed an attenuator. I could build one myself of course, but for about the same amount of money you can buy a kit at the dutch web shop of Kent Electronics. I never tried their kits before, so I decided to give it a try.
The kit is a classic attenuator with six sections, each section containing a Pi-circuit of selected resistors and a switch. Each switch allows you to include or bypass the corresponding Pi-section. The sections of this kit are 1, 2, 3, 4, 10 and another 10 dB, which makes up a total of 30 dB. The impedance remains 50 ohm, whatever attenuation is selected. Of course you can skip the included resistors and select your own values.
Attenuators can be used to:
- measure RF amplifier characteristics without overloading/damaging the input of your VNA or spectrum analyser;
- calibrate S-meters (6 dB per division);
- determine least required power during a QSO (for the best miles-per-watt rating);
- regulate the output power of a signal generator;
The kit contains a single-sided circuit board, a bunch of resistors and switches, and a single sheet of paper with some information on the kit. The circuit board is not drilled; case and connectors are not included (as stated in their catalog). There are no building instructions. However the documentation includes a circuit drawing, represented in the same layout as the circuit board.
After drilling the holes in the circuit board I placed all the resistors. Next I put the switches in place, but did not solder them. First I determined the distances between the switches and drilled the case to ensure all switches will fit. Then I put the print with switches inserted into the case., aligned all switches and soldered them on the PCB (while in the case). Finally I fixed the switches to the case using the included nuts.
I used my MiniVNA to measure the transmission loss (S21) of the attenuator at different configurations:
|Selected attenuation||3.5 MHz||7 MHz||14 MHz||28 MHz||50 MHz||70 MHz||144 MHz|
|0 dB (none)||0.0||-0.0||-0.1||-0.3||-0.8||-1.3||-1.1|
|1 dB (section 1)||-0.8||-0.8||-0.9||-1.2||-1.7||-2.2||-2.1|
|2 dB (section 2)||-1.8||-1.8||-1.9||-2.2||-2.7||-3.3||-3.6|
|3 dB (section 3)||-2.8||-2.9||-3.0||-3.3||-3.8||-4.4||-5.0|
|4 dB (section 4)||-3.5||-3.6||-3.6||-4.0||-4.5||-5.1||-5.9|
|10 dB (section 5)||-9.4||-9.4||-9.5||-9.8||-10.3||-10.8||-11.7|
|10 dB (section 6)||-9.4||-9.4||-9.5||-9.8||-10.3||-10.8||-11.5|
|30 dB (all sections)||-29.2||-29.2||-29.2||-29.4||-29.5||-29.7||-28.7|
As you can see, the attenuation is not very precise. This is not a big surprise, since the resistors do have a 5% tolerance. If you want a more precise attenuator, forget the packaged resistors and select your own values.
There is some strange behaviour: when adding all the individual attenuations together for a certain frequency, the sum is less than the attenuation with all sections enabled. For example, at 14 MHz the sum of all individual sections is -28.4 dB, with all sections enabled the attenuation is 29.2 dB. I’m not sure if this is caused by the attenuator itself, or by the MiniVNA (which is a great tool but definitely not the best/most accurate VNA available).
Although the kit is meant for HF purposes only, I also measured the attenuation on 50, 70 and 144 MHz. As you can see the results get worse. The circuit board allows you to shield the different stages by inserting grounded sheets between the switches. This will make it suitable for VHF as well, as stated by the documentation.