I am trying to run a harmonic regression on a quarterly series.

The software gets confused by the frequency of the time series when I try to do the above.

For example, I wish to know if there is a cycle of length 6 quarters:

plot(fourier(ts(1:100,frequency=4),K=6))

Error in …fourier(x, K, 1:NROW(x)) :
K must be not be greater than period/2

I have to trick the software and do :

fourier(ts(1:100,frequency=100),K=6)[1:3,]

it works.

My question is : How do I tell the software that I am looking for a cycle of length 6 or more quarters?