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.
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.
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. That bridge is still missing in the pictures.
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.
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.
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.
QRP SWR meter for field operation
Because I occasionally operate in nature with QRP radio equipment, there recently was a need for a compact QRP SWR meter. During my last activity in that context on the 'Vloeiweide' the well known Monacor FS1-5 meter also went along again. Of course it works, but it is a relatively bulky,heavy thing that does not really fit in my idea of beeing active with QRP stuff. There is another reason why I want something different and that is because I enjoy self-building. Various circuits can be found on the internet. Circuits with toroids, but also with resistance bridges. After reading everything I finally went for the 'Stockton bridge', described by David G4ZNQ in SPRAT, the periodical of the G-QRP Club. The circuit, or variants, are widely used given the number of schemes that can be found on internet. This type of circuit is also offered as a kit by Kits And Parts Dot Com.
As G4ZNQ describes on the internet, the circuit has good accuracy, is frequency independent, nothing needs to be adjusted, the 'insertion loss' is low and above all: it is easy to build. This design is a so-called 'inline bi-directional' measuring system. You can simply reverse the circuit, then it works too. I liked the implementation of KC8AON so I started making an experiment print like the one shown in his article. After the dimensions have been taken over on a piece of printed circuit board, I used a sharp knife and cut away traces of copper to create a similar print. In my junk box I found a number of 4C6 toroidal cores of 9 mm diameter, comparable to the FT37-61 ferrite material of Amidon. They may be useful. When the coils and other components are soldered on the PCB, it appears that the circuit works but the reversibility is not OK. A 75 Ohm dummy load shows a lot difference on the meter when the circuit is connected in reverse. That difference continues to grow on the higher HF bands. So there is a frequency dependence somewhere! Because others use toroidal cores such as the FT37-43 or FT50-43, I decided to use those too. A friendly amateur had some FT37-43 kernels for me. With those cores the inequality problem is not completely resolved although it already makes a big difference. With a 50 Ohm dummy load everything is 1:1 nevertheless.
and then ...
However, I do not like the experimental print for the QRP SWR meter. The pieces of Teflon coax are difficult to assemble and therefore possibly the reason for the inequality in the operation of the circuit. Not that it makes a big difference, but still, I don't like it. Therefore, I decide not to use this test PCB but building the 'Stockton bridge' between two BNC connectors in a small box. So, I need a suitable box where the -also available from the junk box- meter would fit in. But also a potentiometer and switch for switching between 'forward' and 'reflected' should be given a place. Enough boxes to be found on Ebay but not the one I wanted. I used to make boxes from printed circuit boards in the past Hence the decision to do that again so that the measuring device would be as I want, so compact as possible. Why? Just say it; less weight to carry (not very relevant) or just the challenge. It was certainly the latter and I knew that too! Although it is obvious that the jigsaw will be used again (which I have done a lot) I still choose another option. From a legacy I own a Proxxon KS230 mini table circular saw with 58 mm saw blade. The blade appears to cut effortlessly through the circuit board. This saves me a lot of filing to get everything neatly straight and square. Because the meter is the largest part, it sets the standard for the design. And yes, fitting and measuring takes most of the time!
The front plate is made of green plexiglass, again from old stock. Because the meter is inside the box, a rectagle of 44x25mm must be cut out of the plexiglass. So there we go with the jigsaw and No.1 for metal! Then gently file until everything fits and the meter remains in place by clamping. The final outside dimensions of this box are 63x52x27mm. Because the circuit is partially soldered to the bottom, the front is removable. For fixing that front I used countersunk head M3x8mm bolts. On the short sides of the box are two strips with nuts underneath. First drilled 1.5mm holes in the right places in the plexiglass. Then aligned the support strips underneath with tape and drill the 3.2mm holes to ensure that a correct fit is guaranteed. Thereafter solder the four side panels with the bottom plate in between as a reference. Fasten the support strips with bolts to the plexiglass front and turn it upside down on the table. Now (tag) solder the support strips to the side panels. Once the box assembling is ready you can no longer reach it! Remove the frontpanel and finally solder the bottom plate with which the mechanical work is completed.
When the box is ready, the components can be mounted. Because the dimensions of this housing aren't too big, it is a bit of fiddling with tweezers and soldering iron to get everything neatly in place. I replaced the four 100 Ohm composite resistors by two 49,9 Ohm metal film.
I slightly modified the circuit of this QRP SWR meter by using one rotary potentiometer and not two separate trimmer pots like KC8AON did. The meter can easily be set to full scale in the 'forward' position regardless the input power. Apparently, hams regularly opt for full deflection at 5 Watt with a trimmer potentiometer but I don't want that. Building in small boxes requires some fine motor skills. With a bigger box it all goes a little easier. On the other hand, however, I have a QRP SWR meter which is to my liking and unique! All that remains is to find a suitable button for the potentiometer.