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I watch a lot of circuit building videos on Youtube and I can see that the "Manhattan" style of circuit building (placing small islands of copper clad board on a larger piece which acts as the ground plane, and soldering components across the "islands") is quite popular.

I can see that this method is used even in high frequency circuits such as small transmitters. But won't this introduce a lot of stray capacitance, as an "island" of copper clad board on top of the ground piece is essentially a parallel plate capacitor?

Let's say one island is \$ 0.005m\ *\ 0.005m = 2.5 * 10^{-5}m \$. I have some of such board and I measured its thickness as approximately \$1.3mm\ =0.0013m.\$ Therefore the capacitance between an island and the ground plane would be \$C=\epsilon_{0}\frac{A}{d} = 8.854*10^{-12}*\frac{2.5*10^{-5}}{0.0013m}=1.7*10^{-13}F.\$

At radio frequencies such as 100Mhz, the impedance would be \$\frac{1}{2\pi f C} =\frac{1}{2\pi*100*10^{6}*1.7*10^{-13}}=9400 \Omega\$.

This isn't a short circuit, but it isn't very large either. So why do we use this method of construction? What advantages do we get using it that makes it worth having these \$9400\Omega\$ extra impedances to ground? Wouldn't it be better to build on stripboards for example, that don't have that same problem?

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    \$\begingroup\$ It would be pretty nifty if you could edit this to include a photo of what you're talking about. Just grab an appropriate one and reference it well. I considered doing it myself, but it will better reflect what you're talking about if you do it yourself. \$\endgroup\$ – Scott Seidman Aug 15 at 13:36
  • \$\begingroup\$ Even worse, \$C=\epsilon_{0}k\frac{A}{d}\$ where k is about 4-5 for FR4 \$\endgroup\$ – glen_geek Aug 15 at 13:54
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The two main advantages of using the Manhattan style of assembly are

1) A solid ground plane underneath everything
2) A secure construction
3) You're not constrained by the geometry of stripboard
(the three main advantages ...)

Whereas for some circuit nodes it's important to minimise strays to ground, for many RF circuits it's rarely necessary, it's only important to manage them.

Two copper islands connected by a thin wire form a CLC network. For small enough lands, and short enough wires, the -3dB break frequency of that network can be in the GHz. Below that, you'll match the impedance of the filter to 50 ohms, so it's ideal for connecting your mixers, amplifiers, connectors, modules etc.

Bear in mind that a trace on FR4 that's twice as wide as the substrate thickness has an impedance of about 50 ohms. A trace above air needs to be three times as wide as the distance from the ground plane. Anything less than that width, and the connection looks inductive, and will benefit from a bit of extra C at the ends to match it in.

Consider a PIN diode switch. When it's closed, it looks like a piece of wire with a residual inductance. The correct size copper lands at each end will match it in to 50 ohms, improving its insertion loss. When it's open, it looks like a small capacitance. Large copper lands at each end will keep its isolation high at high frequencies. That's a win for copper lands for both diode states!

Finally, when you do have the odd node that must have low C to ground, then you'd 'air-wire' it, but supported by components that are solidly anchored to Manhattan islands.

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  • \$\begingroup\$ Thank you for the answer! \$\endgroup\$ – S. Rotos Aug 19 at 14:36

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