Where is the best location to place the tuning capacitor? I have a capacitor and found the "hot spot on the coil" for the antenna. I am confused because the diagrams I have seen on line have the capacitor located between the "ground end" of the coil and ground, or between the antenna end and the gound end of the coil. Can anyone offer ideas? Do I need to experiment?
Bill Harms
Where is the best location to place the tuning capacitor? I have a capacitor and found the "hot spot on the coil" for the antenna. I am confused because the diagrams I have seen on line have the capacitor located between the "ground end" of the coil and ground, or between the antenna end and the gound end of the coil. Can anyone offer ideas? Do I need to experiment?
Bill Harms
Bill, I am not sure what setup you are using. If it is a typical base loaded antenna with taps and the transmitter ground is at the base in the earth, I would recommend that the cap be placed at the top of the coil in series with the antenna. If you connect it to the bottom of the coil, the transmitter will be feeding the series LC circuit in the middle and the cap will shunt the RF to ground. I have read articles about this where the cap is in the ground lead. It will do no harm to experiment with either connection and choose the one that works best.
Here, I use a tuning cap in parallel with the loading coil forming a parallel resonant circuit and link to the transmitter with a coil wound around the cold end of the loading coil. Edit I neglected to mention that the bottom end of my coil is connected to ground.*** I built the coil and antenna for another purpose but it works well on part 15 AM and gives a really sharp peak at resonance. It is so sharp (high Q) that I had to connect a resistor (R 18 in the SSTRAN) to lower the Q so the audio sounds right.
Check this link for some more information.
http://lpam.info/index.php?page=antennas
Neil
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
http://www.rhemaradio.org