Modifying the AMT3000 for Driving the Base-Loaded Vertical Antenna

(Updated July 1, 2009)

 

 

 

 

 

 

 

 

Building a Base-Loaded Vertical Antenna

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Why Do You Need to Modify Your AMT3000?

The AMT3000 is designed for ease of tuning with a non-critical antenna setup for the casual user who wants to broadcast to radios within a house.  The standard AMT3000 antenna output circuit is designed to match a high-impedance, short wire antenna.  The base-loaded vertical antenna presents a low-impedance to the transmitter. The simple transmitter changes described here enable the AMT3000 to drive the low impedance antenna efficiently. The AMT3000 internal inductors are bypassed, and instead, the external base-loading coil provides the required inductance.  The antenna has a very high Q, which means that harmonic radiation will be highly attenuated (just as is true for the standard AMT3000). The transmitter modifications do not change the FCC-mandated 100 milliwatt RF input power specification.

If You Have Not Yet Assembled Your AMT3000

If you have not yet assembled your AMT3000 kit and want to use it with the base-loaded vertical antenna, follow these instructions.

  1. Skip the assembly step that calls for installing resistor R18 (820 ohms). This component location will remain vacant.
  2. At the assembly step that calls for installing capacitor C23 (820 pF), install instead, a 560 pF, 50VDC ceramic capacitor.  Both the 820 pF capacitor and the 560 pF capacitor are included in the kit.  If you have misplaced the 560 pF capacitor, you may order one by sending an e-mail request to mailto:orders@sstran.com.
  3. Install a jumper plug across the two jumper pins at location S7.
  4. Set all four switches to the ON position on the S5 DIP switch pack. These switches must always remain ON. They are not used as part of the tuning procedure for the base-loaded vertical antenna.
  5. Follow the tuning instructions in Building a Base-Loaded Vertical Antenna in place of the tuning instructions in the manual.

If You Have an Assembled AMT3000

If you already have an assembled AMT3000 and want to modify it for driving the base-loaded vertical antenna, follow these instructions.

  1. Remove resistor R18 (820 ohms).  You can clip the resistor out with wire cutters or unsolder it. This component location will remain vacant.
  2. Unsolder capacitor C23 (820 pF) and insert in its place a 560 pF, 50VDC ceramic capacitor. If you have misplaced the 560 pF capacitor that was included in your kit, you may order one by sending an e-mail request to mailto:orders@sstran.com.
  3. Install a jumper plug across the two jumper pins at location S7.
  4. Set all four switches to the ON position on the S5 DIP switch pack. These switches must always remain ON. They are not used as part of the tuning procedure for the base-loaded vertical antenna.
  5. Follow the tuning instructions in Building a Base-Loaded Vertical Antenna in place of the tuning instructions in the manual.