I'm answering my own quote here to keep things more peaceful... A typical suppressor coil will have a fairly squat L/D ratio and for maybe 100 years it has been possible to use this ratio and a classic graph to predict the so called self capacitance of a solenoid. In the case of a suppressor coil I'd expect this self capacitance to be about 0.6pF or so. This would mean the resonant frequency when measured on a classic s11 measurement (against ground) would be up around 600MHz or so if the inductance was just over 100nH. However this is fed against ground and when the solenoid is against ground it will show a lower self resonance compared to the case where it isn't grounded at either end. This is because the solenoid is really a complicated transmission line and when it sees a short at one end to ground you get a free 180degree phase shift in the reflection at that end of the solenoid because I think the coil (or transmission line) treats this as a short circuit there. At the other end it will obviously look like a fairly decent open circuit at resonance and many people refer to this as the 1/4 wave resonance mode. However, if you place it in free space as you would normally do with a dip meter then it's best to consider the length of the wire when trying to predict the SRF. In this case both ends are unterminated so it is tempting to think the SRF will now leap up to 1.2GHz because the 180degree shift (from the short circuit at one end) is gone. But in reality a regular form factor coil like this would probably show a SRF close to the 1/2 wave frequency of the wire length. 15cm is a 1/2 wavelength at 1GHz. Therefore the SRF doesn't double in frequency for various reasons but if you had a dip meter that could work to 1GHz you would see this resonance mode with the coil in free space and it would be somewhere around 1GHz. This is miles away from 100MHz. I'll try and get a few 47R Ohmite OY resistors and see how this affects the SRF when the coil is wound around it. I'm not expecting to see anything dramatic in terms of a change in SRF when in free space. Rather than use a GDO I'd recommend a VNA as it can mimic a GDO and cover a huge frequency span in a fraction of 1 second. Try it... the coil will have a 'first' freespace SRF up around 900-1000MHz assuming you make a reasonable copy of a classic suppressor. I'll try and get several 47R resistors and try various coil form factors including a hairpin type. it might not be for a week or so but I'm due to place a Farnell order early next week There will be higher order SRF modes displayed on the VNA as well and these will be in the 1.5-3GHz region but I guess we aren't concerned about those.