Tony - a built-in tuner, or an external autotuner, might not be able to cope with the SWR of something like the G5RV at some frequencies, especially the lower ones you're specifically asking about.
Say your antenna is adjusted for best performance at 3890 KHz and you want to operate at 3975, for an example. That's 85 KHz difference. With a good manual tuner it would be no problem, but it might give an autotuner a headache.
On higher frequency bands, like 20 meters, an antenna cut for the middle of the band (14.200 MHz) will cover the full band (14.000 to 14.350) without complaint from most built-in tuners because that 350 KHz is much less of a percentage at 14 MHz than the 85 KHz in the earlier example is a percentage of 3.5 MHz.
Lower frequency = longer antenna, with narrower bandwidth.
Say your antenna is adjusted for best performance at 3890 KHz and you want to operate at 3975, for an example. That's 85 KHz difference. With a good manual tuner it would be no problem, but it might give an autotuner a headache.
On higher frequency bands, like 20 meters, an antenna cut for the middle of the band (14.200 MHz) will cover the full band (14.000 to 14.350) without complaint from most built-in tuners because that 350 KHz is much less of a percentage at 14 MHz than the 85 KHz in the earlier example is a percentage of 3.5 MHz.
Lower frequency = longer antenna, with narrower bandwidth.