Years ago, I modified NMO coils for HF operation. I started with a stacked Maxrad coil arrangement and wound telephone wire externally to make a coil for 75 meters along with a base capacitor, and used it to check into nets. Yes, it's only good for about 30 kHz without a tuner - so I have a MFJ autotuner under the seat. Eventually I had NMO antennas for almost all the HF bands 75-10 meters with 4 foot whips, and they all are good for real communications. The main reason for this is I drive vans with no room for a long antenna and bumper mount. Now I have an Elecraft KX2 for mobile/portable use, and I'm winding even smaller coils inside old CB coils along with base capacitors for the work van. I actually got the 75 meter coil to fit inside a single Maxrad housing. I start with 180pF across the base for 10 meters, 360 pF for 20, and keep increasing as I go down. Internal coils are close wound, and around 11 MHz the single winding maxes out and I have to start overlapping a second layer of windings. Somewhere around 3 MHz would involve a third winding if I ever get that far and decide to do a 160 meter antenna! Has anyone else tried this?