Getting Started on 630 metres

Part 4: Getting On the Air
15/02/2018
Introduction     Transverter     TX Driver Stages     TX Power Stage     Firing Up

Powering Up the Transmitter

It pays not to be impatient, as initial tune-up and operation needs to be done with care, and you need to have the right equipment.

Power Supply
First of all, you need a suitable power supply. This should have a low power 12 V 1A output for the transverter and driver stages, and a separate adjustable output (or at least a low and high voltage) for the power amplifier stage. This needs to be capable of 30 Amps or more.

The low voltage supply for operating the output stage at low power can conveniently be 5 V. It needs to be capable of 10 A. The higher voltage supply for operating the power stage at high power needs to be about 12 V, although up to 14 V can be used with care. This could be the same supply that operates the transverter and the low power stages of the transmitter.


Power Supply Arrangements

I use an old computer server power supply, which has 5 V 30 A and 12 V 30 A outputs. An ordinary computer power supply simply wonít have enough puff, and may not last very long. I use the 5V setting to tune up the amplifier Ė it will easily provide 25 W output at this voltage, and itís much safer than trying to adjust your antenna at high power, as the currents can be enormous, and could trip or damage the power supply. A 12 V car battery would work well, but you still need a low voltage supply for tune-up.

Initial Tests
Letís assume that you donít have a 150 Watt dummy load, but you do have or can borrow a 25 W 50 Ohm load. I use an excellent little power meter with built-in dummy load (GY561, about $50 - the picture on the right shows the GY561 indicating the Transverter output power and frequency). Connect your load to the amplifier output. Connect the transverter to the input.

Check all the connections before applying power. Centre the duty cycle trim-pot. When applying power first, itís not a bad idea to do so via a resistor. Thereís already one there for the low power stages. Apply 12V to the low power stages, and check that the +V rail is close to 12 V. If itís low, and the 15 Ohm series resistor runs hot, check your wiring again.

If all is well, apply 5V to the output stage. I recommend using a 20 or 30 Amp meter in the supply lead. To start with, also fit a 1 Ohm 10 W resistor in series. If you have not added the RF operated switch (the circuitry in red on the schematic), you MUST apply drive before applying the power. Life is a lot simpler with the RF switch circuit.

If all looks good (the drain current should be about 1 Ė 2 Amps), remove the resistor and try again. The current should be about 5 A with a 50 Ohm load, so 25 Watts output.

If the drive level is too low, the current will be very high (without the RF switch), or maybe the transmitter wonít turn on (with the RF switch). If the drive is too high, the duty cycle will be affected. At this stage, set the appropriate drive level, which is just a bit more than is required to turn the transmitter on, and adjust the duty cycle for 50%. The best way to do this is by looking at an output of the TC4428 with an oscilloscope, but itís also possible to use a DC meter, adjusting so both the TC4428 outputs are the same voltage. The drain current should also have a slight minimum at the correct 50% duty cycle.

If you change the drive level, you may need to readjust the duty cycle.

Connecting the Antenna
I am assuming here that the antenna has been already set up. If not, see information further down the page.

Replace the dummy load with the antenna, and on low power, adjust its tuning so that the amplifier draws about the same current as on the dummy load. You will probably find that the current is lowest when the antenna is resonant at the right frequency, and you can then change the matching until the current at resonance is what you expect. The output stage works rather like a power transformer, with a fixed ratio between the supply voltage and drain current, with this amplifier a value of about 0.9 Ohms. (9 V, 10 A or 5 V 5.5 A). If the matching impedance isnít 50 Ohm, this changes the ratio.

Make initial adjustments at low power, and make final tweaks very carefully on high power. The best way to achieve maximum output is to use an RF ammeter in the antenna Ďupí wire (above the loading coil). Adjust the antenna tuning for maximum antenna current, while at the same time making sure you donít exceed about 6 A drain current (low power) or 13 A (high power). Operating at higher current will reduce the reliability of the amplifier and overheat the transformers and snubbers.

Operating High Power
A comment needs to be made here about operating at high power. When the amplifier switches on or off, there are huge surges in the drain current: brief, but typically 35 A or more. Your power supply may fail or may trip. The most reliable solution Iíve found is to intentionally introduce resistance into the drain supply lead. I use a 100 W 0.23 Ohm wire wound resistor, but a metre or so of mains power cord with the three wires connected in series, also works well. The transmitter power is lowered slightly, but the power supply will be more reliable. If you don't have 5 V and 12 V supplies available, a larger resistor in the drain supply is another reliable way to control the power, but the resistor will get very hot. Something like 5 Ohm 25 W or a car headlight bulb would do.

A Suitable Antenna

Configuration
Itís not possible to use variable capacitors or conventional tuners at LF and MF. Very few hams would have enough space for a half-wave dipole for 474 kHz (over 300 metres long!), or tall enough masts (a mere 100 metres or so).

The normal and most practical arrangement is a Tee antenna, with a series loading coil at ground level, then a vertical wire as high as you can manage (the 'up' wire), and at least two Ďtopí wires, as long as you can manage. The more wire you have up top, the smaller the loading coil can be, so the loss is reduced.

You also need an extensive radial field under the ground connected to the bottom of the loading coil. The wires need to be similar lengths to the 'top' wires, and use as many buried wires as possible, at least eight or so. Run the longest wires in the same direction as the top wires.


A typical MF antenna, 'Marconi' type, showing sources of loss

Managing Losses
The picture above shows where the main sources of loss are. There is also loss in the loading coil due to wire resistance and skin resistance. Hence bigger coils would with heavy wire, preferably Litz wire, will have less loss.

Parallel losses can be due to unwanted capacitance from any part of the antenna to objects such as the house or trees and other vegetation. Keep the antenna clear of these. You need capacitance to ground, especially to the earth radials, as that comes with little loss. By using really large insulators you will minimise leakage resistance loss.

Ground resistance loss will be the biggest factor. The radiation resistance of an electrically short antenna such as this may be 1 Ohm or less, while ground resistance could be 10 Ohms or more, making it quite obvious where improvements are best made: lots of radial wires, and heavy wire and loading coils. It doesn't matter if you only run low power: especially at low power, best efficiency is aboutkeeping the losses down.

Think also about the voltage on the antenna: if you are running 100 W, and have a loading coil Q of 500 (ideal), there could easily be 50,000 Volts on the antenna above the loading coil! I recommend all insulators used should be of the 'dogbone' type, either glass or ceramic, 200 mm long or more. If the loading coil is indoors, you will also need a 50 kV 'feed-through' insulator. I use an insulator off an elecrical supply pole transformer.

   
My tuner hut (left) - note 11 kV feed-through insulator! Inside the hut (right)

A Typical Setup
My tuner is in a grounded metal hut, and tuning remains stable no matter the weather. Note how in the left picture the feed wire to the upwire goes down from the feed-through to a strain insulator connected to the house (stops drips running into the feed-through), and also prevents lateral strain on the feed-through insulator and hut. The insulator securing the 'up' wire to the house is a 250 mm ceramic 'dogbone' type. The feed wire and the 'up' wire consist of the same three lengths of heavy appliance wire, and continue on 9 metres upwards to become the three 'top' wires.

The 'up' wires are pulled away from the house and the mast by a further 'dogbone' about 4 metres up. At the top of the mast is a 400 mm long glass insulator, and the three wires pass through this unsecured, meaning that they are individually tensioned at the remote ends. The antenna is like three Inverted 'L' antenna in parallel, with all the wires identical length.

The picture on the right shows the tuner hut door open. You can see the red wire coming down from\ the feed-through is further held away from other parts by a glass insulator secured behind the door. The loading coil for 630 metres is the black one at the bottom behind the panel. Only half the coil is used. The coil is 400 mm diameter and about 500 mm tall. It has a variometer inside the base, which is adjusted by the knob on the panel.

The coils above are used on the 2200 metre band, connected in series with the 630 metre coil.

Losses can be a lot higher in wet weather, and the antenna tuning may well change with weather conditions if your setup is sub-standard. It also plays to wipe down the insulators regularly, especially after winter, to remove mould and dust which not only increase loss, but can cause high voltage tracking and damage to the insulators.

Antenna Matching
There are three main adjustments on an antenna such as this: coarse coil taps; a fine-tuning adjustment (usually using a variometer), but maybe finer taps; and a tapped or adjustable link for matching to 50 Ohm. Sometimes this is achieved using taps near the bottom of the loading coil, or with an L Network at the bottom of the loading coil.

The purpose of all these adjustments is to maximise the antenna current, while at the same time keeping the transmitter drain current within bounds. This is much more easily and safely achieved by making initial adjustments at low power.


Methods used to connect to a Marconi Antenna

Although not shown in the diagram, the antenna connects to a point near the top of the coil, which makes the coil and antenna capacitance series resonant at the frequency of operation. Fine tuning is best achieved using a variometer (not shown in the diagram). Once resonant, the antenna is adjusted for 50 Ohm feed impedance by adjusting a feed tap or link, or transformer tap.

The simplest arrangement is that shown on the left in the above diagram, but all of these methods are valid. I use the fourth method, with an auto-transformer. This provides the smoothest matching adjustment, made by taps on the transformer. The first two require some fiddling with taps but work well. The third method is widely used commercially, but only suitable for a fixed frequency. Finding suitable high value mica capacitors would be a challenge almost exceeding the necessary calculations, as nothing is adjustable with this method.

Initial antenna adjustment can be done with an RF Bridge, but a simple signal generator and RF probe or field strength meter waved near the 'up' wire should do the trick. Once you have the antenna resonant on the appropriate frequency, you are ready to connect the transmitter and tune up for real!

Connect up the transmitter to the antenna, and run it on low power. Adjust taps (or preferably a variometer) to make the antenna exactly resonant and minimise the transmitter current. The antenna current meter should also peak at this point.

Is the transmitter drain current higher or lower than when it was tested on 50 Ohm? If so, adjust the matching (tap, link etc) until it is about right. At each step readjust the resonance. It might take some time to get these adjustments correct.

One of the problems with amplifiers like this is that if the antenna load is not 50 Ohm, you may not get the result you expect, since the output filter transforms the antenna impedance to something else at the FET drains. The filter is designed to provide a 50 Ohm match to the FETs via the output transformer, which steps up the ~ 5 Ohm FET load up to 50 Ohm (1:9 impedance). If the amplifier doesn't behave as you expect, check the impedance matching at the antenna.

Finally, apply high power to the transmitter, and check that everything is still operating as it should do. Make sure the drain current is within the correct range. Watch (and listen) carefully for arc-over in the tuner or antenna system. Nothing above the coil should be anywhere near a metallic or wooden object - it's really easy to start a fire! If there is an arc-over, stop transmitting immediately, as the transmitter will draw heavy and destructive current.

Notice that in the above drawings, the RF ammeter is shown in the 'cold' end of the loading coil. It works just as well here, and means that the meter is not exposed to high voltage. If you use a variometer, it should be connected in series above the ammeter, often in the middle of the main coil.

If you don't have a suitable RF ammeter, you can make one with a current transformer. These have the advantage of being relatively indestructable. Another simple trick is to connect a small neon lamp (NE2 type) to the very top of the loading coil. Connect just one wire, leaving the other free. The lamp will glow (even at 10 Watts), and will glow brightest when the antenna is exactly tuned.

Connecting Everything Up

What I have described in these articles is exactly what I use at home. My SSB exciter is an old Kenwood TKM-707 marine transceiver, and the computer uses a conventional digital interface to control it. The PTT is operated by a small reed relay from a serial port. I generally operate WSQCall or WSPR modes, but the arrangement will also operate CW, QRSS modes and Hellschreiber.

The transceiver, transverter and amplifier are connected together and powered up all the time. When the PTT operates the transceiver, audio from the computer internal sound card drives the transceiver, RF drive at 10.474 MHz goes to the transverter and then to the amplifier at 474 kHz. The RF-operated circuit then enables the transmitter.

I use a dedicated transmit antenna, and a separate receiving antenna. So the only switching involved is the PTT transmit control of the transceiver at 10 MHz, and thus everything is controlled from the computer. With no drive, the transverter and amplifier draw virtually no current, and cause no noise on 630 metres.

Keying is quite fast enough, in fact fast enough for semi-break-in Morse and even Hellschreiber. I use the IZ8BLY Hell software for both these modes. With WSQCall, automated replies can take place unattended.

To avoid using an antenna relay, I use a separate receiving antenna (PA0RDT Active Whip), and a separate receiver (Drake R8A), rather than using a receiving path through the transverter. Itís simpler and more flexible that way. I also use a separate USB sound device for the receiver, as my old computer internal sound card isnít full-duplex. Without full duplex, receiver sounds would end up being transmitted, causing feedback. Itís not a problem with modern computers or USB sound devices. The arrangement is so flexible that I can operate cross-band 80 metres to 630 metres, using digital modes on both bands on the same computer!

Introduction     Transverter     TX Driver Stages     TX Power Stage     Firing Up
Copyright © M. Greenman 1997-2018. All rights reserved. Contact the author before using any of this material.