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23.01.2011

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Antenna and related topics

This page is ment to fill with antenna experiments and antenna system related topics. Some of them are not more than a picture and a few lines of text, are more extensive.

Z-match

The Z-match is a very popular antenna tuner in the world of QRP hams. Surfing the internet pages you can find kits like the NorCal's Balanced Line Tuner and the commercial ZM-2 sold by Emtech™. A lot of information about the working of the Z-match is described by Charlie Lofgren, W6JJZ.
Now my NC20 project is finished, I was looking for a antenna tuner which fits in the compact line of QRP my gear. Inspired by Charlie's work I have build a prototype combining a few designs. The schematic shows what I have done. Unfortunately I could not find a T130-6 toroid, so I used a T130-2 which would do the job too, I read. The variable capacitors are 7-345pF polyvaricon types from the junkbox.
The primary winding of the transformer consists of 30 turns 0,9mm enameled wire, tapped at 7 turns from the bottom (ground tap) and at 15 turns. On the secondary side there are two windings, both centered on the 7-turn ground tap. A high impedance link of 14 turns and a low impedance link of 6 turns, both 0,7mm enameled wire. With an extra switch, one end of the balanced output is connected to ground while the other end is lead to a BNC connector for connecting coax fed antennas. The series input capacitor is switchable with a center-off toggle switch. (a. one section, b. two sections, c. both sections plus an additional 470pF padder.)

The final cabinet will include the well known N7VE LED SWR indicator circuit. Dan Tayloe introduced it several years ago, and it has been proven a great addition to the QRP fraternity. The circuit is an absorptive bridge, which means that the transmitter sees a 50 ohm load when tuning the antenna. For the 50 ohm resistors I used two 100 ohm/1Watt types in parallel, so the bridge is rated at 5 Watts.

Below are three pictures of the prototype tuner. The prototype was also made to determine the dimensions of the final cabinet I still have to make. I used the BLT cabinet sold by the AM-QRP Club as a guideline.

Z match 1 Z match 2 Z match 3

The schematic of my balanced line tuner. I need to experiment with the series diode whether to use a 1N4148 (Si) or a AA117 (Ge) type. The LED is a transparent bright red type.

Schematic

Altoids Longwire Tuner

When I visited the "Four Days In May 2005" (FDIM) QRP event in Dayton (Ohio), Doug Hendricks from Hendricks QRP Kits sold this funny little antenna tuner, designed by KD1JDV, for about US $25,- I believe. It is build in a non-painted cabinet with the same dimensions as the famous Altoids boxes. The circuit is build up around a few switchable Amidon toroid cores and a tuning capacitor as used in transistor radios from long ago.

Altoids Longwire Tuner

The tuner model is of L-type with capacitor at the end (antenna side) and a inline switchable SWR circuit using three resistors and a FT37-43 transformer to feed a led when the circuit is disbalanced (= antenna not 50 Ohm). A short piece of RG174 coax with BNC connector for the input and two pieces of wire with alligator clip at the end completes the tuner, all fitting in the Altoids box.

This kit is followed by the "Switched Longwire Tuner Plus" where real switches are used to change the inductance instead of simple 2-pin SIP jumpers as used on many computer boards. For more information see QRP Kits.

Schematic

Elecraft XG1 test oscillator

This mini-kit was the 2nd price in the FDIM Building Contest in Dayton 2005. See the NC20 page for more info on that.
The Elecraft XG1 is a 7040 kHz crystal oscillator with accurate 1 uV and 50 uV output levels. It's ideal for receiver test and alignment. The 1uV output can be used to measure a receiver's sensitivity, while the 50uV output is useful for S-meter calibration. The XG1's unique oscillator stage runs at an extremely low level, eliminating the need for interstage shielding. A power-on LED (green) is provided, as well as a low-battery warning LED (yellow), and a red LED to alert the operator to accidental transmit into the XG1's output.

Elecraft XG1