Sub-100MHz attenuator - ground plane or not? - UK Vintage Radio Repair and Restoration Discussion Forum

6SN7WGTB · 11th May 2022, 6:58 pm

Am planning to make a small simple RF attenuator, which will be used sub-100MHz.

It's a 2,4,8,16dB Pi/T design which will be made with SMD components on a custom PCB, built into a die-cast Hammond box, BNC each end.

Switches will be pin type, directly mounted to PCB. They will be slide switches - I seem to read as much pro as con so have concluded it won't be a needle mover on this design/frequency range.

My question is:

Should I used a double-sided PCB - with the reverse as a petty much complete ground plane?

My intuition says this isn't necessary in a fully-shielded box at these frequencies. I could see it adding capacitance across the whole PCB...

Similarly not sure I need to have e.g. copper sheet shields between sections?

Appreciate any advice.

G6Tanuki · 11th May 2022, 8:57 pm

I wouldn't go full-on ground-plane in this instance; in times-past I built a similar sort-of thing from an old ARRL-handbook design, using Switchcraft dual-pole-changeover slide-switches whose cases were soldered to a 'box' made from soldered-together PCB offcuts, and using 2-watt wire-ended resistors; it worked just fine though my co-workers used to sneer at it.

It worked 'just fine' until I inadvertently transmitted 50 Watts into it...

Search "A Step Attenuator You Can Build - ARRL" for the design [Sept 1982]

6SN7WGTB · 15th May 2022, 3:25 pm

Made it.

Seems to work OK. 0.6dB down at 100MHz, 1dB down at 200MHz.

Will post in the 'successes' section!

11th May 2022, 6:58 pm	#1
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 443	Sub-100MHz attenuator - ground plane or not? Am planning to make a small simple RF attenuator, which will be used sub-100MHz. It's a 2,4,8,16dB Pi/T design which will be made with SMD components on a custom PCB, built into a die-cast Hammond box, BNC each end. Switches will be pin type, directly mounted to PCB. They will be slide switches - I seem to read as much pro as con so have concluded it won't be a needle mover on this design/frequency range. My question is: Should I used a double-sided PCB - with the reverse as a petty much complete ground plane? My intuition says this isn't necessary in a fully-shielded box at these frequencies. I could see it adding capacitance across the whole PCB... Similarly not sure I need to have e.g. copper sheet shields between sections? Appreciate any advice.

11th May 2022, 8:57 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Sub-100MHz attenuator - ground plane or not? I wouldn't go full-on ground-plane in this instance; in times-past I built a similar sort-of thing from an old ARRL-handbook design, using Switchcraft dual-pole-changeover slide-switches whose cases were soldered to a 'box' made from soldered-together PCB offcuts, and using 2-watt wire-ended resistors; it worked just fine though my co-workers used to sneer at it. It worked 'just fine' until I inadvertently transmitted 50 Watts into it... Search "A Step Attenuator You Can Build - ARRL" for the design [Sept 1982] __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk. Last edited by G6Tanuki; 11th May 2022 at 9:15 pm.

15th May 2022, 3:25 pm	#3
6SN7WGTB Hexode Join Date: Apr 2021 Location: Crawley, West Sussex, UK. Posts: 443	Re: Sub-100MHz attenuator - ground plane or not? Made it. Seems to work OK. 0.6dB down at 100MHz, 1dB down at 200MHz. Will post in the 'successes' section!