Total posts : 45366
The question you have raised has no simple answer. The reasons for tapping the coil and adding a tuning capacitor are:
1. The coil taps act as “coarse tuning” for the antenna system.
2. The capacitor, then, becomes the “fine tuning” for the system.
These components are inserted in the antenna for the purpose of removing, most or all of, the reactance in the antenna system so that the transmitter is happier.
Here is how the math looks: Xl + Xc = 0 reactance. (Xl=inductive reactance, Xc=capacitive reactance.) This would be defined as a resonant circuit at a particular frequency of RF energy. The narrow-ness of the circuit is called the”Q”. A high-Q circuit is very narrow banded and extremely efficient. To widen the bandwidth, you only need to add pure resistance (a shunt load). A near perfect circuit for a Part 15 antenna circuit would be 0 ohms reactance + 50 ohms resistance. However, to get most of the audio through the antenna, some value of shunt resistance may need to be added.
I would suggest you:
1. Start by adding the capacitor between the transmitter and the antenna coil (series connection).
2. Next, try the capacitor between the connection of the transmitter end of the coil and ground (shunt connection).
3. Connect the capacitor across the coil, between the transmitter and the antenna “radiator” (parallel connection).
All of the scenarios change the balance of capacitive and inductive reactance in the circuit. Remember, we add the coil to lower the value of several thousand ohms of capacitive reaction in the very short antenna. The capacitance is added to fine tune the reactive balance in the antenna circuit (resonance). Also remember, resonance will probably not be 50 ohms resistive. Short antennas are usually in the range of 2-5 ohms resistive.
Through a little experimentation, you should be able to find the right combination. Good hunting.
Marshall Johnson, Sr.
Rhema Radio – The Word In Worship