[TowerTalk] Circuit Board Question

[Mike Fahmie via TowerTalk]

It's a pleasure to find a list that produces such knowledgeable responses to tech questions!

To elaborate on Rick's response,  losses go down in the FR-4 because the RF voltage distributes itself inversely proportional the the dielectric constants of the materials involved.  Air being 1 and FR-4 being about 4 or 5 means that the voltage will mainly be across the air gap thus lowering the loss in the FR-4. Be sure that the source and load have intimate contact with the chassis so that the image currents have a return path.


Increasing the  gap between the stripline and it's ground plane combined with the decrease in the effective dielectric constant of the substrate allows the stripline to be wider, not only lowering its losses, but providing more capability to dissipate heat.

 -Mike-





>________________________________
> From: Richard (Rick) Karlquist <richard at karlquist.com>
>To: Steve Hunt <steve at karinya.net>; towertalk at contesting.com 
>Sent: Friday, August 1, 2014 9:22 AM
>Subject: Re: [TowerTalk] Circuit Board Question
> 
>
>
>FR-4 dielectric is quite lossy, comparable to the copper
>loss.  Making the trace wider neither increases nor decreases
>dielectric loss, but it does spread the heat over a larger
>area.  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.
>
>Rick N6RK
>
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