Some time ago I have investigated the frequency adjustment of my Diamond discone antenna with the results in the below picture.
| Frequency [MHz] | 35 | 50 | 70 | 75 | 80 | 85 | 90 | 95 | 100 | 105 | 110 |
| Length [mm] | 1270 | 820 | 380 | 310 | 257 | 215 | 185 | 163 | 122 | 100 | 82 |
Standard there is a top rod with dimensions for 50 MHz operation, but it can easily be tuned up or down across a wide range.
There is a lot of D-130N discones around, and I think the information will be helpful for others who want to use this popular antenna on 4 m!