When we have restrictions in mounting the station antennas, we start to make experiments with other alternative ways. It was what I did.
Fist of all, what is a balun?
The word Balun is probably the junction of two other terms, "Balanced" and Unbalanced". In earlier times, the junction between the antenna to the location where was the radio, it was made with a twin-lead cable, also known as ladder line, with two parallel conductors. This twin-lead is a balanced line. The current flows equal in magnitude in the two conductors, but it's offset 180º in direction from one another, so they cancel each other and there is no radiaton out of the twin-lead (that means very low losses). If the antenna is also a symetric antenna, as a dipole (do not need an image in earth of the other phase), they are ok to one another, but there is another problem. The antenna resistence Ω or even better the impedance Z, the opposition of an electrical circuit (resistence Ω, inductance L, capacitance C) to the current flow, when subjected to a voltage1. If you work in one band and the antenna impedance is the same as the twin-lead, good, no problem! (there are several twin-leads with several distances between conductors, which means several impedances 75, 300, 450, 600 Ω). If Z in the twin-lead is different or the antenna is a multiband, you'll need to adjust this impedance with a balun and of course you have to adjust the twin-lead to the radio output Z (50 Ω). With a coaxial line, you'll have to adjust the Z but also transform a balanced antenna with an unbalanced coax line. You will not need to adjust the coax to the radio 'cause they have the same Z and are both assymetric.
So the balun mate a balanced or symmetrical system with an unbalanced or asymmetrical system and adjust the impedance.
Here's an example of an 4-1air core balun to mach with an antenna with an impedance near 200 Ohm (200/50).
At first I made several ferrite core baluns and have obtained good results, but after some time of experiments I found that the ferrite core can become saturated with the increasing power and I focused my researches in air core baluns. Here are some pictures of my baluns experiments:
The black sealed box in the left side of the first picture, has a ferrite core with this thin phone wire used in ancient phone boxes centrals and is one of the baluns who give me better results. however become saturated with power, so I started to research that ones with an air core.
MY QRP ANTENNA END FED TUNER PROJECT
In my researches and experiments, I used an old balun coil to make an end-fed tuner. It seems to work at least in 40 and 20 meters band and maybe in other, but I didn’t test with correct wire length It was just an experiment, a promising one.
In internet, I also found a simple LED circuit to see when the antenna is well tuned. So decided that I intend to do by myself this little QRP antenna tuner with a LED indicator.
Here is my project:
- 1,7 cm diameter PVC pipe with 10 cm lenght;
- Two 3 mm different color insulated wire (1,5 mm without insulation), coiled 13 times side by side at the same time as shown in the folowing picture:
- Polyester variable capacitor ≤10 pF to ≥200 pF. If you find a better one use it. The good old air variable capacitors are hard to find and when you find it are expensive. For QRP purposes the polyester capacitor will do. Find one that holds at least 50V.
- It’s recommended a counterpoise wire 0,05 wavelength single wire. For 20 meters band 300/14,100x0,05=1,064mt.
- Now you need to add a circuit device that indicates when the antenna is tuned. I found the following one. It’s very simple, you just have to connect it on the output of the tuner to the antenna and certify yourself that the anode and the cathode are in the wright position. The cathode is the shorter leg:
- I already tested it. Verified that when you tune the variable capacitor and the RF can pass through the tuner circuit to the antenna, glows like this (almost like Janis Joplin song):
- A couple of days after and some more tests, I saw that this LED method of tuning the antenna to the radio was not so precise as I expected. The maximum bright it does not always overlap with minimum SWR indication. I remembered that I had in my junk box a charge meter indicator, VU meter or something like that and I had this idea of put it instead of the LED. The pointer touched hard in the end of the scale when I made the connections and experimented it. So I needed some resistor to reduce it and used a 10 kΩ resistor. It wasn’t enough, but with two 20 kOhm resistors in each side of the connections in the circuit, where before was the LED, worked good. The pointer almost enter in the red part of the scale using QRP when is tuned. So, I think it's better this way and if you have other ideas test it, be creative!
- Now you just have to put all together in a small box and enjoy it. That was what I did. A couple of weeks later, after I bought some material that I did not had in my stock, I assembled the project and it is no more a project, it's reality. It's a piece of QRP equipment that works good. Started with the box, an aluminium case at the right measurements and after some mistakes, a few unforeseen scratches and some bad words, nothing that a rasp and a little sand paper would not solve, I made the holes.
- After holes made, I disposed and fixed the components. At the left side of picture you can see the little air variable capacitor with four sections. A section of 8.5-382 pF, a section of 10.5-328 pF and two sections of 5-20 pF, a total capacity of 750 pF. At the center of the picture a lever switch, to divide the total capacity in two parts. A part to take advantage of the lower capacities, using just one of the sections, and the other to take advantage of the higher capacities, joining all sections in just one. At the right side the charge meter indicator that at the maximum indication of the pointer will tell me that the wire is tuned and at the bottom, the air coil.
- To fix the components I used a mastik glue to paste metal, plastic and other materials. I left a space between the coil and the bottom of the case as you can see in below picture.
- With wires welded and all components disposed and fixed in their places, I assembled the back part with the PL plugs, linked them to the coil wires with tin and welded the ground wire of the variable capacitor to the chassis ground on one of the PL plugs. I also welded the two capacities wires that the capacitor is divided to the connections of the lever switch. After all that connections made, I made some more tests to see if readings on the reader coil was still working well and found that the pointer returned to play the full scale. So, I had to make some changes and more experiments to find values of the resistors and I had to put also a 20 kOhm resistor between the 10pF capacitor and the diodes. With the maximum of 10 Watts power, the reading coil works good. If I want to use more power I have to increase the value of the resistor between the 10 pF capacitor and the diodes.
- All tests and experiments made, I assembled the simple circuit on his definitely place, as can be seen on the next picture. The definitely reading coil circuit diagram, can also be seen on the right side.
- After all assembled and some use of the end fed tuner, I verified that in some cases I had some difficulties on tuning. After a few more tests I introduced two more possibilities in the tuning circuit, adding another little coil with an internal diameter of 10 mm and 9 turns and a toggle on/off/on switch. One side of the coil is connected to Tx PL plug input and the other side is connected to the middle connector of the switch (the "off" one). The conectors "on" are linked to the PL antenna plug, one to the middle of the plug and the other to the chassis as you can see in the below picture:
- Here it is the final product working with a QRP radio, a Yaesu FT-817 with another homebrew device, a cw morse paddle (I show you in the next opportunity). I was amazed when I found that it can tune the 70 cm, 2 and 6 meters bands with the same wire antenna that I was using also for HF, but it can not tune the 80 and 160 meters bands. I suspect that can tune from 40 meters to 70 cm bands. After box closed the initial 10 W that was calibrated the reader coil, now can't put more than 5/6 W of power. With this reader coil system I have to memorize, for the power I'm using, where usually the pointer marks, because if the point of tuning is lower than usually it should be for that power, it means that it's not tuned, don't have the desired income, and might have some SWR.
- I still can pounce some letters and figures or a scale around the knob and will be much better. It will seems made in factory.
You can download the pdf file on the bottom of this page. Please wait for the scenes of upcoming episodes... hihi
SMALL RECTANGULAR LOOP ANTENNA
ANOTHER VERSION OF MY BALCONY ANTENNA
- After many experiments trying to decide what is best, a rectangular slinky wire, a rectangular slinky loop wire, a rectangular wire, a rectangular loop wire or whatever geometrical configuration you can try for an urban antenna with all kind of restriction, now I chose to work on a very small rectangular loop antenna. The measures are 2,80 x 1,02 Mt, the length of the wire in the rectangular shape is 7,64 Mt, some more windings in the pole where is the fed and a 40 cm for connections, in a total of 10,04 Mt of 3 twisted thin phone wires. Why these measures? That's the measures I have in the balcony and λ = 71,25/7,1 λ = 10,04 Mt, a quarter of λ in 40 Mt band and a full wave in 10 Mt band. The wire instead the slinky because is more stealthy or less flashy to the eyes of some kind of folks that, if some mobile phone operator offers them money, they would immediately accept the installation of an antenna, but not yours, that´s the really truth!
- I do not have a suitable measuring device and as you can see in the picture, I only have a 1:9 balun made with a ferrite toroid Amidon FT 240-43 with 8 windings of three color multifilar wire, that does not conform to the requirements for impedance adaptation of this kind of antennas. The characteristic resistance of a full wave loop in free space is ± 120 ohms and a half wave
loop is expected to be ± 60 Ohms. This loop should be near 80 or 100 Ohm, the ratio is about 2:1, it must be a 2:1 balun. Some time later I saw in the internet an antenna for 10 Mt band very similar to this one. The shorter sides of the described antenna were to the top and bottom and is said that the input impedance is near 50 Ohm, no need a transformer device. The feed point of my balcony antenna is not in a center but at a corner, almost as a rhombic, the working position is different, the antenna is practically inside doors, which means that the building iron will influence the antenna behavior and the antenna wire has some windings in the pole on the feeding side to complete a full wave in 10 meter. I fed it just with a 50 Ohm coax and the SWR in the different bands was constant, pretty much similar to the ones achieved now with a balun, a little higher. So what to do? Experiment other connections and join a ferrite rod with some more windings. After some tests and measures I achieved a configuration that seems to result. I used a ferrite rod of an old transistorized radio receiver and a wire (blue) winding on it with 20 turns and welded from the braid connection of coax to the other wire of the loop, passing the ensemble through the Amidon hole.
- The SWR measurements made with 5 Watts gave the following results, which increase with more power but, are acceptable without a tuner in the 80 till 30 Watts, 1.15 and 10 meter band till 60 Watts, 1.12:
- The best results are achieved in 80 meter band and in the higher portion of the 10 meter band. The antenna efficiency it will be pretty much the same of the previously slinky antenna, hopefully a little more because of better coupling to the coaxial cable.
A SMALL UNNOTICEABLE IAMBIC CW KEY FOR BACKPACK MOBILE OPERATION
You have some in the market, but you can easily make a less expensive one with the satisfaction that was you who made it. You just have to acquire the following materials:
- 2 arm micro switches
- Welding glue
- 7,5 cm of a rigid plastified copper wire with a big section
- 120 cm of a shielded thin cable (two conductors and mesh)
- A 3.5 mm metalic stereo audio jack (with a plastic internal insulator)
- Tin weld and and an welding iron
- Heat shrink sleeve at measure
- Two flat polyester capacitors (doesn't matter the values, see your junk box)
With a welding glue, join the two arm micro switches back to back. Shape a "U" with the 7.5 cm of the rigid plastified copper wire and weld it in the lateral contacts of the switches. The big section of this rigid copper wire will also create a support structure on the center bottom of the "U" where should be glued the shielded cable with the welding glue. The twisted shielding mesh of the cable will be welding on one of the lateral contacts of the switches. Cut the wires of the polyester capacitors and the arms of the switches at the desired size. Glue the capacitors on the switches arms in the way they don't pass the limit line of the switches bodies, as the pictures.
With a multimeter in condutivity test, find the contact of the two bottom contacts that has no condutivity with the lateral contacts of the switch but, when you press the arm switch, will make the contact. After you find the contact, tin and weld one of the wires of the shielded cable on that contact and do the same to the other wire in the other switch on the opposite side. Open the screw capsule of the stereo audio jack and insert it back on the shielded cable together with the plastic insulator. This way, after all weldings made, you will not verify that you forget it. Twist, tin and weld the shielding mesh on the exterior contact of the stereo jack and the other two wires in each of the the remaining two contacts.
Now test the CW key. The wires in the switches can still be changed in accordance with your one preferences and with pliers bend the center of the switches arms to adjust them. After all tested put a heat shrink sleeve in the key contacts and that's all, it's done!