ANTENTOP- 01- 2018 # 022

The Inverted L Ham Antenna

Photo 2

Matching network is inside rural mailbox at base of antenna. Adjustments are accessible through door and back of mailbox. Switch is for lighting protection

The transmission line is buried about 6 inches in the sod and runs from the house to the base of the antenna where it enters the housing for the impedance matching networks.

Photo 1

Vertical portion of antenna made of copper water tubing, fastened by stand-off insulators

The ground radials are made of four lengths of # 12 AWG copper wire 35 feet long. They stretch out in four directions from the base of the pole and buried about six inches into the turf. They are securely bonded together at pole to reduce losses and are connected to the matching network by a copper braid. In dry, sandy soil the radials should be made no less than 60 feet long and their number increased to six. It is imperative that the antenna have a good ground system, securely bonded to reduce losses.

The efficiency of this antenna on 80 meters is better than that of the 33-foot and 44-foot verticals because the high current (and high radiation) portion of the antenna has been raised.

Almost any length of wire or antenna configuration can be made to look like 50 ohms or any other transmission line impedance. This is the job of the impedance matching network. Since an R.F. Bridge wasn't available, the impedance of the antenna was measured with a Q- Meter and calibrated SWR Meter. The results are given here as a matter of information only: 14.2 MHz., Z= 650-j100 Ohms; 7.2 MHz., Z = 5000- j100 Ohms; 3.8 MHz., Z= 26 + j0 Ohms.

As can be seen from the circuit diagram of Figure 2 and the Photo 3, band-switching was handled by three separate switches. If the builder substitutes a ganged switch and different parts layout, it is suggested that the coils be placed at right angles to each other to reduce mutual coupling.

All components were mounted on a plywood board which is slightly smaller than inside dimensions of the RFD- Type mailbox. The finished unit is slipped into mailbox which provides excellent protection from the weather.

In the author's unit, C2 was made up to 50- pF unit and 30- pF variable capacitor in parallel since the variable was on hand. The actual capacity needed is 70- pF. The 400- pF mica capacitor used as part of C3 should be of the high current type. The one used here is a surplus Sangamo Type A2LH (2500 Volts). The switches should also be able to withstand high current if high power is to be used because they will carry the full antenna current. The switches shown in the Photo 3 were taken from an army surplus antenna tuning unit.

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