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A carrier which is amplitude modulated carries the audio information in the sidebands, one above and one below the carrier frequency. The total bandwidth of the signal is twice the highest audio frequency in the modulating signal. For proper operation the antenna must radiate the signal including the sidebands.
A coil loaded antenna acts as a band pass filter which radiates energy within the band pass frequency range. A hi Q antenna has a narrower bandwidth than a low Q antenna. If the Q is high enough to reduce the signal at the edges of the sidebands then the effect is that the higher audio frequency information is lost and the audio will lack high frequencies at the receiver.
I don’t know about the Isotron so can’t predict if this effect is present but now you have some theory.