I am using spec.pgram(tsobj,spans=6, plot=TRUE) to obtain a periodogram for my univariate time series of monthly observations which were sampled over 86 years (so I have 1032 observations in total). It's defined as an object of class "ts" with frequency=12. The periodogram nicely depicts the seasonality for 1 year and has further peaks on following years. However, it stops at 6 years. I have reason to believe that there might be a seasonality with longer phase, say 8 years. How can I change the command to obtain values for longer seasonalities?
I am sorry, it sounds like a stupid question and possibly it is, but starring further on the help files, playing around with the available options and googling around didn't brought me any inside. Thanks alot for your help!