On 8/1/14, 9:22 AM, Richard (Rick) Karlquist wrote:
FR-4 dielectric is quite lossy, comparable to the copper
loss.
At HF?
I worry about it "a little" at 3GHz, but not much, but then, I'm doing
fairly casual low performance designs.
I found one reference that gives 0.008 loss tangent at 100-3000 MHz.
An article has a graph that shows something like 0.25 dB/inch at 3GHz,
but roughly linear with frequency, so at 30 MHz, it would work out
0.0025dB/inch (or is that 250 microBel?)
http://www.parkelectro.com/parkelectro/images/leysfinal.pdf
I've also seen loss tangent numbers like 0.05, which is pretty bad, but
I don't know that loss at HF would be all that big a deal. I'd bet that
ohmic loss would dominate. (just like in coax, at HF the ohmic loss
dominates over dielectric loss).
Also, there's a significant variation in FR-4 dielectric properties from
mfr to mfr and lot to lot, as well as substantial variations with
humidity. Then again, folks do build things like interdigital stripline
filters and microstrip patches on FR-4
Making the trace wider neither increases nor decreases
dielectric loss, but it does spread the heat over a larger
area.
And change the Z, if you care about that.
What many designers do is print the trace on the
bottom of the PC board in an area with no ground plane
and then use the chassis underneath the PC board as the
ground plane. This eliminates most dielectric loss
(there is still a little fringing within the FR-4).
Dielectric loss is proportional to frequency while
copper loss is proportion to the square root of frequency.
Having air dielectric also approximately halves the copper
loss.
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