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IW5EDI Simone – Ham-Radio

A Cost Effective Current-mode 1:1 Balun

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describes a 1:1 balun.

Introduction

A cost effective current-mode 1:1 balun can be constructed from a length of coax and a rod typically used for a broadcast antenna loop-stick, some electrical tape, cable ties, a length of PVC water-pipe and some connectors. The balun is formed by winding several turns of coax on the ferrite rod.

Principle

The operating principle is that the inner conductor and the inside of the braid act as two opposing bifilar windings with substantial inductance inserted in the outside of the braid. Differential current passes through such a transformer with little insertion loss as the opposing windings of the transformer mode effectively eliminate the winding inductance. If you want to run an unbalanced differential current through the transformer then substantial inductance will be present. Thus the current balun suppresses common-mode current. Since current flowing on the outside of the braid, is referenced to ground it must flow through the impedance resulting from the winding inductance formed by the outside of the braid and the core. This inductance will reduce the current if the impedance is high enough.

The same principle applies in the common-mode choke where two or more wires pass through a ferrite core. A typical example is seen in the ferrite chokes clamped on the monitor cable of computers.

High permeability cores can be used for current-mode baluns or common-mode chokes as there is not net magnetic field around the bifilar winding even though substantial currents are flowing.

Construction

A ferrite rod is easier to wind and cheaper than a toroid. At 160m I found that I needed 30 turns of RG58 U/C to ensure that I obtained equal, but opposite current, in each leg of an asymmetrically mounted dipole. To place 30 turns you will need to wind more than one layer across the core. The turns can be held by insulation tape and by applying two cable ties on the ends of the last layer. One end of the coax is terminated in a connector while the braid and centre conductor are split out and used as the balanced feed at the other end. You should use coax with adequate breakdown voltage to avoid damage when operating into mismatched loads.

Housing

The balun can be housed in PVC water pipe. Cut a section large enough to make two end pieces which can be flattened with the aid of the hot air from a hair-drier or heat-gun. The circular end-sections can be cut with tin-snips. I drilled a hole for a panel-mount connector in one end and used banana connectors for the balanced feed on the other end. The end sections should be inserted inside each end of the pipe and held in place with the PVC glue. I have found hot-melt glue adequate and easily removable. Extra protection is obtained for the ends if you leave an overhang by inserting the ends further into the pipe. My balun has survived several four-wheel-driving desert trips and is still intact and operating after five years.

Ralph Holland 1996

4m Slim Jim Antenna

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This 4m Slim Jim is cheap and easy to build yet it greatly out performs the more usual dipole due to its low angle of radiation. An SWR of 1:1 is obtainable across the 4m FM band with simple adjustment. The photograph and PDF diagram show the construction, however a brief description is also given. The spreaders are made from plastic knitting needles; two small holes are drilled about 10mm in from each end of the spreader to accommodate the aerial wire.

Suitable sized holes are then drilled in the fishing pole to mount the spreaders. The spreaders are then glued in position using a two part epoxy. The telescopic fishing pole sections are also glued to each other using epoxy. Once the glue is set the antenna wire is then threaded through the spreaders according to the diagram.

RG-58 is connected to the matching section but not soldered yet, just wrap the braid and inner around the aerial wire. Raise the antenna at least 3m above ground and clear of any objects (walls etc), apply a few Watts of RF and adjust the feed point to give a 1:1 SWR (slide the RG58 up and down the matching section). When the correct feed point has been found, solder the feeder connections.

I dabbed some yacht varnish over the connections and the first couple of inches of the coax to stop water ingress down the feeder.Mount the aerial as high as possible; due to the construction materials the wind resistance is very low so it should survive the worst of any storms. N.B. I did try making a 4m slim Jim out of 450 ohm ladder feeder from a diagram posted on the web, however i could not get it to work correctly (maybe the two conductors in the ladder line are too close for 4m use.

Barry Zarucki M0DGQ

Article originally available at http://www.m0dgq.co.uk

Rewinding Power Transformers: Considerations and Step-by-Step Guide

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Transformers are essential components in many radio projects, often accounting for a significant portion of the overall cost. However, since they can usually be reused in various ways, salvaging power transformers can be a cost-effective approach.

Assessing Power Handling Capability

The weight of a power transformer provides a clue to its power handling capability. For unpotted transformers, those weighing 15 pounds (6,8 Kg) or more generally fall within the kilowatt (KW) range. For example, an 800-volt, 20-pound transformer in a voltage tripling circuit should be able to intermittently power 1,000 watts of RF.

Testing Salvaged Transformers

When testing salvaged transformers, always use a variac or place a light bulb in series with the primary. Wiring color codes can be found in the appendix of most electrical and electronic handbooks, including Orr’s great reference.

Considerations for Rewinding Transformers

Power transformers can be used directly or rewound to power tube filaments or provide various specific voltages. When deciding whether to rewind a transformer, consider the following factors:

  1. Cost of purchasing a replacement: If the required secondary is readily available and inexpensive, it may not be worth the effort to rewind the transformer.
  2. Complexity of the secondary: With experience, practically any secondary combination can be rewound. However, the time required for a casual builder to reach that level of experience may not be practical. It is encouraged to limit ambitions to single secondary windings.

Methods for Rewinding Transformers

There are two methods for rewinding a transformer, both based on the same principle:

  1. Complete disassembly method: This involves completely disassembling the transformer, replacing the secondary winding, and then reassembling the laminated core around the coil form. This method produces tighter windings and more predictable results.
  2. Partial disassembly method: This requires replacing the secondary winding without disassembling the laminated core. It is limited to situations where sufficient space exists between the windings and the metal core, and when only a limited number of secondary turns makes this method practical.

Step-by-Step Guide for Complete Disassembly Method

  1. Remove the mounting bolts and carefully pry the laminates apart using a knife or narrow-bladed screwdriver.
  2. Keep track of the manner in which the laminates were assembled to facilitate reassembly.
  3. Remove the coil, wound upon a phenolic or treated cardboard form, without disturbing its size or shape.
  4. Carefully remove the secondary winding one turn at a time, keeping a written record of the total turns.
  5. Calculate the turns per volt ratio by dividing the total turns by the secondary voltage.
  6. Consult a wire table to ensure the selected wire size can handle the required current.
  7. Wrap the new secondary turns in the same direction as those removed, using craft or wax paper between layers.
  8. Reassemble the laminates in the same order as when disassembled, using C-clamps or a vise to aid the process.
  9. Insert the mounting bolts, apply a coat of varnish, and compress with C-clamps until dry.
  10. If the core seems loose, shim it with small wood wedges or cardboard and apply a coat of varnish to hold them in place.

By following these steps and utilizing the wealth of information available in handbooks related to wire capacity and projected performance, you can successfully rewind transformers for your radio projects. Happy winding!

J-vertical Antenna for 10 meters band

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Article by EI9GQ

My first J vertical was a Slim Jim for two metres. This simple aerial performed better than a centre fed dipole at the same height. I compared it with an end fed half wave and an end fed 5/8 wave vertical. The Slim Jim always gave better results.

SLIM JIM

When I built my first 10 metre transceiver, I decided to make a Slim Jim for 10M. The 2M Slim Jim was made from Aluminium tubing. I soon realised that it would be quite difficult to make a 10M version.

J VERTICAL

I abandoned the idea of using a folded half wave vertical, like the Slim Jim. I used a half wave vertical section made from three different sizes of Aluminium tubing. The quarter wave matching section was made from co-axial cable. I used 75 Ohm TV co-ax, for both the feeder and the matching transformer. To calculate the correct length of the 1/4 wave matching section, the velocity factor of the co-ax cable must be known. The well known formula for converting frequency to wavelength is 300/F in MHz = Wavelength in metres. The free space wavelength at 28.500MHz is 300/28.500 = 10.526Mtrs. One quarter wave = 2.631Mtrs. This must be multiplied by the velocity factor of the co-ax, 0.8 for the low-loss 75 Ohm TV co-ax that I used, giving a length of 2.105Mtrs (6ft 11.5in.) I used a 7ft length of co-ax with the feeder tapped in at 1ft from the bottom (see drawing.) If you use standard 50 Ohm co-ax like RG8, RG58, RG213, the velocity factor will probably be much lower (about 0.66.)

J VERTICAL WITH CO-AXIAL MATCHING SECTION

J VERTICAL USED AT EI9GQ

The original Aluminium J-vertical was dismantled when I moved QTH in 1996. I recently built a new one from fibreglass, (Thanks to John EI7BA for the very nice tapered fibreglass pole.) I ran a length of insulated wire through the fibreglass tube, for the 1/2 wave vertical. Design information below.

Frequency 28.8 MHz

Wire length = 16Ft 3In

Matching section = 5Ft 8In of RG213 co-ax. VF 0.66, short circuit at botton, feeder tap at 9 In from bottom.

The fibreglass pole is fixed to the wooden support with vinyl tape. Don’t be mean with the tape. If you tape it up well, it will be strong enough to withstand the winter storms. My original J Vertical survived the 110 MPH winds in Feb. 1987. I cut the cable and soldered the feeder in at 9In from the bottom. A much neater method would be to use three PL259 plugs and a T connector. Take care to waterproof the connections. At the top of the quarter wave transformer, where the half wave wire is connected to the co-ax, I put some PVC tubing over the end of the co-ax, the PVC tubing was filled with Silicon grease.

Despite the fact that I used 75 Ohm low-loss satellite TV co-ax as the feeder, the SWR is 1.1 : 1 at 28.6 MHz and less than 1.5 : 1 from 28.3 to 29 MHz.

6m/10m Dual Band mobile whip

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This antenna is made from either an old CB whip or a stripped-down 2m 5/8 whip. When you strip down and re-shrink an antenna, be careful because some heat shrink tubes can change the antenna’s resonance, especially with helical windings. You can use a piece of 20mm conduit for the former of L2 with a few adjustments. Start by building the antenna roughly to size and adjusting the 6m section below L2 first, then move on to the 10m adjustment above L2.

Article by VK5AH

2m Folded Dipole

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This folded dipole has a 200 ohm connection point, which matches well with 50 ohm coaxial cable using the 4:1 coaxial balun shown. It’s made with one side using thinner wire (2.5mm square building wire) to achieve the right impedance. The SWR (standing wave ratio) is very low across the band.

Article by VK5AH

6m 1/2 wave Melrose Vertical

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This vertical antenna is made from coaxial cable. You can use either RG8 or RG58. You pull out the inner part of a piece of RG8 cable and replace it with a piece of RG58 cable. You can adjust the coil to change the antenna’s center frequency. To increase the frequency, take some turns off the coil, and to decrease it, add more turns.

Article by VK5AH

VK5AH – HF 4 Bander Antenna Construction & Adjustment Tips

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In this article, VK5AH shares valuable insights and tips for constructing and adjusting an HF 4 Bander antenna. Covering construction techniques, adjustment procedures, and performance characteristics, this guide offers practical advice for maximizing the antenna’s effectiveness across various frequency bands.

Construction Tips:

None of the dimensions could be said to be critical except perhaps the 80m Tips (B) and the G/H spacings. Build the Antenna complete before adjusting or trimming anything. The Antenna was erected at about 25 feet in reasonably free space in a horizontal plane. Dimensions would differ in a sloping configuration and extra ropes may be needed from the bottom of the spacer conduits to the ends of the inductors perhaps. Feed the antenna with a 1:1 balun and be careful the common feed points dont get twisted up near the balun. The coils are not critical. Half a dozen turns here or there wouldnt be critical but would tend to affect the 80m tips a little. The resonant frequency will tend to drop typically 20-30 Khz on both 80 and 40m when the coils are soaked in rain. They could be sealed perhaps with polyresin but be careful they dont become too heavy. The spacer conduits were drilled through and threaded onto the A section and held in place using cable ties to stop it from sliding along the wire. The coils were wound with fine stranded insulated wire of about 2mm total thickness (insulation included) . The important thing is to be able to get that many turns onto the former close spaced.

Adjustment:

Start by adjusting the 40m sections (A) and then the 80m tips (B) then adjust the 20m section followed by 15m lastly. Be careful to make all adjustments so as to keep the antenna symetrical particularly the 80m tips. As a guide the 80m tips will adjust at about 40Khz per inch and the A section will adjust 40m by about 10Khz per inch. The 20 and 15m legs have little effect on 80 or 40 but the 20m legs will effect the 15m section. Do not be tempted to bring the 15m sections up closer to the 40m section as strange things start to happen on 15m. Dont be decieved adjusting the 15m legs. The SWR will appear to drop nicely at about 20.8 Mhz but this is in fact the 40m section coming into play. If problems are encountered making it work on 15m then disconnect the upper section of the antenna (A) from the balun but leave it physically in place. Then adjust the 15m section and finally reconnect the upper section. The SWR will rise slighly on reconnection and the best you will get is about 1.6:1 SWR . My solid state radios seem quite able to still deliver 100 % power forward into this sort of load.

Performance and Characteristics:

The antenna performs as a standard dipole on all bands except 15 and 10m. It has a bit of end fire on 15 and 10m making the antenna work almost omnidirectional. Particularly so on 15m due to the 1.5 wavelength section from 40m The Bandwidths of the 2:1 SWR points is as follows on each band.

80m – 40 Khz, 40m – 250 Khz, 20m – 500 Khz, 15m – 21.0-21.350

Other Bands:

I have not tried the WARC bands but the antenna seems to work reasonably well on 10m using a tuner. The SWR without is about 5:1 .

160M

Try adding about 6.9m to the 80m ends and it works on 160 . I used some aligator clips near the 80m end Egg insulators to clip/unclip the 160m sections as i can get my antenna ends up and down fairly quickly on pulleys. The antenna adjusts on 160m at about 3Khz Per Inch.

Article by VK5AH