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Security (anti-spam) Question: How many dits in 72?
Wed, 22 Nov 2006 09:32:00 +0000
Antennae: The antennae are two 33 foot high vertical radiators spaced 33 feet apart. The masts are made of telescoping aluminum tubing that I bought from Texas Towers, 800-272-3467. The type is 6083-T832 draws tube ranging from size 2.125 thru .625; Each section is 1/8 inch smaller in diameter than the next section. I bought the 6 foot sections which will ship UPS. All 13 pieces of tubing were cut in half (3 feet each) and connected to form the two vertical elements. The masts are supported by a 2 inch PVC pipe 5 feet long. Structural strength is added by inserting a 1 1/2 inch iron pipe into the center of the PVC pipe. Buried 3 feet into the ground, the mast is inserted over the pvc pipe, held in place with hose clamps. There is a 2 1/2 inch coil between antenna mast and ground for lightning protection. Pipe is buried with 2 - 60 lbs bags of concrete mix. The antennae are fed with 2 equal lengths of RG-8X and connected to the balanced antenna tuner. Tuning System: This tuning system enables a radio station to feed a single antenna in either balanced (twin lead) or unbalanced (coax) fashion. It also permits feeding dual antennae using unbalanced coax and the insertion of phasing lines. A 50 ohm RF source is applied directly to an LC matching network (L1, variable cap & switched fixed cap). The inductor is a T106-2 or T130-2 toroid wound with 16 turns of (evenly spaced) #18 gauge magnet wire. The outside exposed wire on the toroid is scraped with a sharp tool to remove the insulation coating on the wire and then the wire is tinned. Solder 12 pieces of #20 or #22 gauge tinned wire to the toroid windings. The other end of the wires are soldered to a 12 position switch (single pole 12 throw waffer switch from RS, see picture). Switch SW2 is used to configure the LC network to select either high or low impedance transformation. Switch SW3 is used on 30 and 40 meters if the variable capacitor is of insufficient value. L2 is a T200-2 toroid with three parallel windings of 12 turns of #14 - #18 gauge insulated wire. (I used #14 house wire; black white and copper). The schematic shows all 3 windings in-phase. One winding connects to switch SW4. This is used to switch a phasing line to either antenna connector (J2 or J3) vai switch SW5. Normally, switches SW4 and SW5 would only be used in a multi-feed antenna system. If you intend to feed only a single antenna, then wire the windings from L2 dirctly to the output antenna connectors J2 & J3. J2 & J3 should be female SO259 type. They are perfect for connecting a balanced line; solder standard banana plugs onto the end of the balanced line wires. They fit perfectly into the SO259/UHF jacks. If the feed line is only one coax cable, L2 and J3 are not required. connect J2 directly to the right side of L1. SWR Indicator: It is nothing more than a pair of 2 milliamp high intensity LED`s connected across a balanced bridge network. The bridge if formed by three 50 ohm resistors and the antenna. Each 50 ohm resistor was made from two 100 ohm 2 watt carbon resistors connected in parallel. The swr indicator is activated by switch SW1. The worst bridge imbalance would be if the antenna was disconnected or shorted to ground. In that case, the voltage across the bridge (without the LED`s out of the circuit) would be half that supplied by the transmitter. If the antenna were exactly a 50 ohm impedance, there would be ZERO voltage across the bridge. As the voltage across the bridge approaches 2 volts (SWR is increasing), LED D1 starts to conduct current through the 240 ohm resistor. As the SWR increases further, the 240 ohm resistor will eventually turn on LED D2 as the voltage drop across the resistor reaches 2 volts. This dual LED action has the effect of creating course and fine tuning indicators. I should note here that any power in excess of 5 watts may damage the LED`s with a high SWR present. With 5 watts of power, I have found experimentally that the LED D1 turns off when the SWR is about 2:1; manipulating the tuner, I can usually interpolate where the minimum swr would be. I have verified this with a commercial swr meter and was always able to tune the swr to less than 1.5:1. After tuning for lowest swr, throw switch SW1 to remove the swr bridge from the circuit.
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