A 700 Watt Ferrite Core 500 KHz 600 Meter RF Matching Transformer

by W5JGV

July 23, 2010

After installing my vertical antenna for 600 Meters, I was faced with the problem of matching the antenna to the 50 ohm output of my transmitters. Tuning the antenna to resonance was easy enough; a large loading coil took care of that problem easily enough. Since I could remotely adjust the loading coil from inside the shack, I thought I was all set. Wrong. As the ground conditions change with rainfall, dry weather and the change of seasons, both the resonant frequency of the antenna and the ground resistance tend to change.

As I just mentioned, I could retune the antenna to resonance easily enough, but what about the change in ground resistance? As the resistance changed, the loading on the transmitter also changed, causing problems with the solid state amplifier. Of course, I could walk out to the antenna matching network at the base of the antenna and reset the impedance matching taps on the loading coil, but that was not too much fun when it was cold or raining. I needed an easier way to readjust the loading.

Since my RF amplifiers have fixed output impedance and loading, I needed something else. Since it was going to be quite difficult to remotely adjust the loading taps in the antenna matching network, so I decided to work on a way to adjust the loading inside the shack.

Now, purists will cry foul here, because what I decided to do was to simply use a transformer in the shack to match whatever load impedance happened to appear at the shack end of the transmission line. That meant that the VSWR on the transmission line between the shack and the antenna would be worse than 1:1. Horrors!! Not to fear, however, as the large foam-filled transmission line I am using has very low losses at 600 meters. Even a 10:1 VSWR would cause rather insignificant losses. Anyway, the worst VSWR I expected to see on the line was between 1.6:1 to 2.5:1.

Because an air core RF transformer would be quite large at 600 meters, I though that using a set of ferrite "E" cores would be the way to go. In addition to being small in physical size, the wire required would be a lot less than if I were to use an air core coil, hence the wire losses would be reduced. In addition, the "E" core design tends to be fairly good at magnetically self-shielding, I figured that in-shack stray RF should be reduced considerably.

The final design turned out to be a transformer using a single layer winding of 20 turns of #14 AWG solid copper wire. Taps were placed on the first five turns from the top of the winding. Using two 6 position switches, I am able to adjust the transformer taps in 31 steps so that an input resistance of 28 to 89 Ohms is transformed to a 50 Ohm load for the transmitter.

Using half of a Bud Box, the transformer is held in place by a large plastic Tie Wrap fastener. The left switch is for the input from the transmitter, and the right switch is for the output to the antenna.

Switch position 1 is connected to the top of the transformer winding, and position 6 is 5 turns further down on the winding.

An overall view of the switch and transformer assembly. Layout was done to minimize connecting wire length to avoid excessive RF losses at high power.

A top view of the transformer. No coil form is used; the winding supports itself. This allows for better cooling air flow around the coil and the transformer core.

The taps on the transformer winding were easy to make. I wound the first six turns of the coil, then made a pair of lines slightly offset from each other end-to-end on the coil. Then I unwound the coil and carefully removed the insulation where the marks were placed, being sure to the alternate gaps in the insulation from one set of lines to the other. That way the taps do not short to each other. Next I soldered the tap wires to the places where I had removed the insulation, and then rewound the coil.

Inter-switch wiring using short leads.

The finished transformer installed in the RF network for the WD2XSH/7 600 Meter Station.

Note that the bar graph VSWR meter indicates 200 watts forward power and zero watts reflected power.

Click here for a PDF copy of the Schematic Diagram


E-80 - EE80 Ferrite E-Cores

Permeability = 3000

B(sat) = 500 mT

Dimensions: 80mm x 38mm x 20mm each half.

Magnetic Path length: 18.43 cm

Eff. Core Area: 3.93 cm ^2

Eff. Core Volume: 36.20 cm ^3

Power Handling capability index (WaAe) 44.69 cm^4

Inductance (AL): 8910 nH/N^2

Material: ASTM P7070 ( TSF7070 ) ferrite

Because these cores are rated for frequencies slightly lower than 500 KHz, I used more turns of wire than would normally be used. I did this to reduce the magnetic flux through the core to reduce core losses. Since this is an autotransformer rather than a 2-winding transformer, this works well. The transformer easily handles 700 Watts CW with very little heating.

An early test used 5 turns of wire as a primary and 5 turns of wire as a secondary. At the 700 watt power level, the core became fairly warm after 10 minutes of CW operation. As a result of this and other tests, it was determined that at least 12 turns of wire were required at the 700 watt power level for 50 Ohms. In the design presented here, switch position 6 has the minimum number of turns (15) in use, which is more than required for proper operation.

The following chart shows the impedance values available for the various switch settings.

The "INPUT Left" switch is for the input from the transmitter, and the "OUTPUT Right" is the switch to the antenna.

73, Ralph W5JGV


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