A while ago, I built the Octaplumb octagonal RX loop, made from heavy gauge copper wire and PVC plumbing parts (see earlier posts). The loop was tuned to 504kHz, since that was what we had at the time.
Very recently, we know got a slightly different range. Hence, the center QRG of the Octaplumb had to be changed. Some experimentation showed that adding 82pF to the 680pF which are in parallel to the butterfly configed poly-vary-con.