Total posts : 45366
The connection point from the coil to the copper pipe or other radiator is a very high impedance point and any connection made here for measurement will load the antenna and the readings will be meaningless. Even a 10X scope probe will present a resistive load which will pull the voltage down and a capacitive load which will change the resonant frequency of the antenna.
Using the measured Z of my antenna system and the measured input current at the base of the loading coil the voltage at the bottom of the radiator is calculated to be approximately 100 Volts with 1.5 Vrms at the coil input. This implies that the 1.5 Vrms is transformed up to approximately 100 volts by the coil/radiator combination which is to be expected since this is a series resonant system. This voltage for other antenna systems can vary greatly since the ground loss and the coil Q affect this voltage.
I have found that a very good way to assess what is happening with the SSTRAN type antenna is to measure the feed voltage, current, and phase angle. This makes adjustment relatively easy since at resonance the phase angle is 0 and if not resonant then the angle indicates which side of resonance the antenna system is set so the appropriate adjustment can be made.
For the current measurement I use a home made RF current transformer which displays the current waveform on the scope. I’ll try to put up the details of this in the next few days as a blog post.
If a SSTRAN AMT-3000 transmitter is used then the antenna coil inductance is set off resonance so there is a “residual” inductance which functions with the transmitter internal parallel capacitance to transform the antenna impedance up to about 800 ohms required by the transmitter final. If this is the case it is best to tune the antenna according to the SSTRAN instructions rather than by field strength.