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We present the low-frequency radio spectra of 9 high-redshift quasars at $5.6\leq z \leq 6.6$ using the Giant Metre Radio Telescope band-3, -4, and -5observations ($\sim$300-1200 MHz), archival Low Frequency Array (LOFAR; 144MHz), and Very Large Array (VLA; 1.4 and 3 GHz) data. Five of the quasars inour sample have been discovered recently, representing some of the highestredshift radio bright quasars known at low-frequencies. We model their radiospectra to study their radio emission mechanism and age of the radio jets byconstraining the spectral turnover caused by synchrotron self-absorption (SSA)or free-free absorption (FFA). Besides J0309+2717, a blazar at $z=6.1$, ourquasars show no sign of a spectral flattening between 144 MHz and a few GHz,indicating there is no strong SSA or FFA absorption in the observed frequencyrange. However, we find a wide range of spectral indices between $-1.6$ and$0.05$, including the discovery of 3 potential ultra-steep spectrum quasars.Using further archival VLBA data, we confirm that the radio SED of the blazarJ0309+2717 likely turns over at a rest-frame frequency of 0.6-2.3 GHz (90-330MHz observed frame), with a high-frequency break indicative of radiative ageingof the electron population in the radio lobes. Ultra-low frequency data below50 MHz are necessary to constrain the absorption mechanism for J0309+2717 andthe turnover frequencies for the other high-$z$ quasars in our sample. Arelation between linear radio jet size and turnover frequency has beenestablished at low redshifts. If this relation were to hold at high redshifts,the limits on the turnover frequency of our sample suggest the radio jet sizesmust be more extended than the typical sizes observed in other radio-brightquasars at similar redshift. To confirm this deep radio follow-up observationswith high spatial resolution are required.

No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6