The Effect of Radiation Losses on High Frequency PCB Performance

John Coonrod
Rogers Corporation

This paper is an extension of an IPC paper [1] presented last year which addressed microwave insertion loss of common PCB transmission line circuits. Insertion loss of these circuits is made up of 4 components; conductor loss, dielectric loss, radiation loss and leakage loss. The previous paper focused on conductor loss and dielectric loss, whereas this paper will address radiation loss.

Radiation losses can be a disruptive force for many different reasons. Designs which are sensitive to EMI (ElectroMagnetic Interference) can be affected by radiation loss of a circuit and specifically how the radiated energy may corrupt neighboring circuits. Also the performance of loss-sensitive systems can be impacted with the addition of radiation loss when it is not fully considered. Finally, broadband high frequency RF and millimeter-wave applications certainly have issues with radiation loss and designers expend many efforts to account for these losses.

In summary, there are many variables which can impact radiation loss and only a few of the more easily determined aspects were evaluated in this paper. Equations 2, 3 and 4 have been found to be relatively accurate for predicting radiation loss for microstrip circuitry and at microwave frequencies. However, they cannot account for several real life scenarios. The equations are simple enough to enter into a spreadsheet and use as a quick reference in concert with a field solver or MWI-2010 which gives effective dielectric constant.

In cases where equations 2, 3 and 4 were found inaccurate, they have been shown to overestimate the amount of radiation loss which is a conservative error. As an example, throughout this study most results were within 5% or better between the measured value and the radiation loss predictions, with more error found at mmWave frequencies.