Manual 1:1 BalUn 150 Watt for dipoleantennas

Manual 1:1 BalUn for dipole antennas

If a dipole antenna is fed with a coaxial cable from the transceiver, it is always a good idea to use a“BalUn”. BalUn means: Balanced to Unbalanced. This enables us to adapt an unbalanced fed system(coaxial cable) to a balanced antenna system (dipole antenna). The main reason for using a BalUn isto ensure that the coaxial cable does not become part of the antenna system and therefore radiateswith it. This has all sorts of nasty effects, think of: interference, EMI, RFI disrupted radiation patternof the antenna, restless noise level. This last point is because not only does the coax cable’s shieldradiate when you transmit, but the shield also works as a receiving antenna.

A ferrite toroid is used, which prevents common mode currents. The main characteristic of a goodBalUn is maximum common mode current reduction and minimal loss of the differential current.There is a lot of information on the Internet and even in literature about making a BalUn, butunfortunately there are also a lot of bad designs that hardly work. This design is one that has provento work very good!

The enclosure

We start by marking and drilling the hole for the coax connector. The diameter of this hole should be16 mm. This is a large hole and therefore it is easiest to create it with a ‘sheet step drill’. (If youhave never heard of that, Google is your friend for looking up what you need)

After drilling the 16 mm hole, I placed the chassis part into the hole, to determine the location of thefixing holes. I picked a chassis part where I fix the part with 4 screws, but 2 screws could also work.These holes can be drilled with a 3,5 mm drill.

We continue by marking and drilling the holes for the antenna connections and the tensile stools.Especially for longer or permanent antennas it is advisable to use a tensile stool. The hole for thestainless steel eye is drilled with a 6 mm drill and the hole for the antenna connections is drilled witha 5 mm drill. The place where the holes are drilled is to your own liking.

The Ferrite Toroid

Now it’s time to start the most important part, the toroid! For this antenna we will work with PFTEshielded silver plated copper wire, which is included in the DIY kit. The advantage of this PTFEshielded wire is that it is relatively thin, so the windings make maximum contact with the ferritetoroid and to each other. Now there are also other alternatives, but they don’t have this excellentinsulation value (600-1000 Volt) and temperature properties of up to 200 degrees.

We start by securing two wires with the use of a cable tie. This makes it a lot easier. Then wrap thefirst 10 windings on half of the toroid as shown in the pictures below. Make sure that the windingsare tightly placed together, because the space at this toroid is limited. Then wrap 10 windings on thesecond half. Note that the wires are wrapped exactly as in the example. Do not change colors orwrapping directions. At the bottom of the ring core are now the two red wires next to each other andthe white wires on the outside. At the top of the ring core, this is exactly the other way around, thereare the white wires next to each other in the middle and the red wires on the outside. If this is notcorrect, start again because then something went wrong.

Finishing up

UPDATE: HF Kits created a solution to fix the toroidal core. Every BalUn or antenna kit in a 82×80mm enclosure includes a mounting plate including M3 bolts. With this mounting plate it is easy toconnect the toroidal core with a few cable ties. See pictures below

Connect the primary side of the ring core (bottom, where the red wires are next to each other) to thecoax connector. The two red wires at the center pin of the coax connector and the white wires on thechassis part with a cable lug. Remove the plastic parts from the cable shoe so that a solid solderconnection can be made. now connect the other side of the BalUn to the connectors for the wireantenna. The two red wires to one side and the two white cables to the other side. Use the suppliedM5 cable lug. Use the included toothed spring rings above and below the M5 cable lug, so the boltwill not rotate when you connect an antenna wire. These toothed spring rings are both mountedinside the enclosure. Place a flat washer on the outside and then attach a M5 nut. The picturesbelow have not yet been fitted with a toroid mounting plate.

The impedance transformer can possibly be tested by putting a resistor of approximately 50 Ohm(e.g. 47 ohm) over the two dipole connectors. In this way the SWR meter should show a standingwave ratio of about 1:1.5 or lower Testing with a half-wavelength wire (in case of a dipole twice aquarter wave) is of course also a good alternative!

Applications

There are many antenna variants possible with a 1:1 BalUn. Of course, the antenna should have an

impedance of approximately 50 ohms in the fed point. It is easily to create a single band dipoleantenna. There are also multi-band variants to be devised. This can be done by connecting multipledipole antennas to the same feed point (cat’s whisker). It is also possible to choose for traps or coils,or even a combination of previously mentioned possibilities.

Single band Dipole Antenna

If two wires of each a quarter wavelength are connected to the BalUn, a single band antenna ismade of a half wavelength in total.

The wavelength and antenna length can be found in the table below, but can also be calculated byusing the following formula:

u λ = --- f

In this formula λ is the wavelength in meters, u the light velocity in meters per second, and f thefrequency in Hertz. The following example determines the wavelength of the 20 meter band. Thefrequency of 14.1 MHz is used in this case. First, some zeros are striped away against each other.

u 300.000.000 300λ = --- = ------------- = ------ = 21.28 mtr f 14.100.000 14.1

We have now calculated the full wavelength. To get a quarter wave, the result must be divided by 4.To get a half wavelength, the result must be divided by two. Antenna wire always has a certainshortening factor. This depends on the conductive material, the diameter and the isolation.Generally the shortening factor will be about 95%. So the total length of our 20 meter dipoleantenna (half wave) will be calculated as follows:

u / 2 300.000.000 / 2 150λ = ------- * 0,95 = ----------------- * 0,95 = ------ * 0,95 = 10,1 mtr f 14.100.000 14.1

Wavelength table

Band FrequntieMHz

Golflengteλ

Halve golflengteλ/2

Kwart golflengteλ/4

160 meter 1,81 MHz 165,75 Meter 82,87 Meter 41,44 Meter80 meter 3,6 MHz 83,33 Meter 41,66 Meter 20,83 Meter60 meter 5,35 MHz 56,07 Meter 28,04 Meter 14,02 Meter40 meter 7,1 MHz 42,25 Meter 21,13 Meter 10,56 Meter30 meter 10,1 MHz 29,70 Meter 14,85 Meter 7,43 Meter20 meter 14,15 MHz 21,20 Meter 10,60 Meter 5,30 Meter

Band FrequntieMHz

Golflengteλ

Halve golflengteλ/2

Kwart golflengteλ/4

17 meter 18,1 MHz 16,57 Meter 8,29 Meter 4,14 Meter15 meter 21,2 MHz 14,15 Meter 7,08 Meter 3,54 Meter12 meter 24,95 MHz 12,02 Meter 6,01 meter 3,00 Meter10 meter 28,4 MHz 10,56 Meter 5,28 Meter 2,64 MeterNote that the shortening factor of the wire is not included in the table. So the antenna will be about5% shorter in reality.

Multi Band Dipole Antennas

Cat’s whisker Dipole Antenna

It is also possible to pair multiple half-wave dipole antennas to the same feed point of the antenna.The lengths are chosen in the same way as with the above single band dipole antenna. This way thedipole antenna can be used on multiple bands. The distance between the two dipole antennas can bequite small, this has no negative effect on the SWR. The radiation pattern of the antenna is slightlyaffected. If you like to prevent this then make the distance (angle) as large as possible. It is alsopossible to place one part horizontally and one part as inverted-V as shown on the image below. It isalso possible to add another dipole antenna to this two band dipole antenna. There are severalexamples of 4 or 5 band cat’s whisker antennas available on the Internet.

Fine tuning

Place the complete cat’s whisker dipole antenna with some extra wire length at the desired spot.Always start by tailoring the longest dipole for the lower frequencies. Always remember: don’t cuttoo enthusiastic. If the longest dipole shows an acceptable SWR on average, it can be started totailor the short dipole. In practice, the antenna will be slightly shorter than a single band dipole.Always make sure you cut the same length of both sides to keep the antenna symmetrical.

Dipole with coils

By applying coils in the dipole antenna something special happens. Because the coil for the higherfrequencies is a high impedance, the last piece of wire (after the coils) does not count for this higherfrequencies. The total antenna including coils only participates for the lower frequencies. In the caseof the 20 and 40 meter band variant, the piece of 10.1 meters of wire works for the 20 meter band.The entire antenna is mechanically 13 meters long but for the 40 meter band, the antenna includingcoil has an electric length of 20 mtr. This means it creates a half wave length for the 40 meter band.A disadvantage of working with shortened antennas is the limited bandwidth on the low frequencybands.

Examples

80/40/(15) Multi band Dipole antenna

A 100 uH coil is made by using a 19mm PVC pipe. Make 150 windings tight against each other.

The coils

Try to wrap the coil as tight as possible, so there will not be any space left between the windings.Then fix the entire piece with some tape. Continue by drilling two small holes, right next to the coil,where you can stick the ends of the wire in.

The next step is to drill two more holes as shown in the photo above. These holes will be used to putthe wire through, which will be used as the strain relief. Finish this step by tying a knot in the wireand pulling it tight.

The excess wire can be cut off from the winding wire and the antenna wire. Leave just enough sothere will be enough material for soldering. Make sure that the winding wire is completely strippedof the enamel layer. This can easily be done with a sharp knife or some sandpaper. Now solder thewire to the coil and put this into the pipe.

Now apply the heat shrink and heat evenly until the heat shrink fits nicely.

Fine tuning

Place the complete dipole antenna with some extra wire length at the desired spot. The first step isto then tailor the wire for the higher frequencies. These are the wires directly connected to theBalUn. Always remember: don’t cut too enthusiastic. If the upper band shows an average of anacceptable SWR, it can be started to tailor the pieces of wire after the coils. Always make sure youcut the same length of both sides to keep the antenna symmetrical.

Dipole with traps

to be continued….

More info

The BalUn and impedance transformer What about exactly? (Dutch)

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