February 13, 1998

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Part 2

Getting back to the subject of tuning radios . . . 

You may have noticed the number of times that I have referred to a dummy load as being 50 ohms NON-REACTIVE. What this means is that to have a low SWR and appear to the transmitter as a perfect load with no reflections, the load must be as pure as resistance as possible. By reactance we are referring to any stray unwanted capacitance or inductance that may be part of the circuit. 

Any stray capacitance or inductance in a dummy load makes it appear to be something other than 50 ohms resistive. The effects of stray reactance on the load worsen as frequency increases. Thus it is very easy to build a good low SWR load at HF frequencies and very hard at the microwave bands. Lucky for us that at our bands of interest, two meters and 70 cm, low-cost loads are possible. 

I had considered a 50 watt dummy load kit project for the club. It is based on the new Caddock non-reactive resistors mounted in a TO-220 power transistor type case. The special resistor and a real 50 watt heat sink costs out to about $25.00. Recently at hamfest a considerable number of 100 watt dummy loads designed for cell phone tower sites have come on the market. These are excellent microwave loads equipped with an "N" connector. I've seen them priced as low as $20.00. For that kind of money, you're silly to build your own load! 

Most professional surplus loads you'll find at a hamfest will have "N" series connectors on them. Don't expect to find an SO-239 on a real dummy load! If the "N" connector bothers you, buy an adaptor for $2.00 to UHF or make up a jumper cable with an N on one end and a UHF on the other. You're going to need one anyway. 

Before you buy any hamfest dummy load, check it with an ohm meter. If the seller objects, walk away, if you're like me, you don't need another useless door stop. The load should read between 45 and 56 ohms. If it does not, chances are its been overheated and it is worthless. 

On rare occasion you'll see a device that looks like a dummy load but has TWO RF connectors on it. Look close at the builder's plate, and you just might have a power attenuator! Most I have seen surplus are in the range of 150 watt dissipation and are 20 dB units. What that means is that they have a total power dissipation of 150 watts. Since they are a 20 dB attenuator, if you apply a 100 watt signal to the input, you'll get one watt out. 

As your RF test bench grows, you'll find these attenuators very useful for a variety of off the air testing. For example using two in series, I can connect a 100 watt transmitter directly to my spectrum analyzer and eliminate a lot of hand effects and haywire on my test bench. These power attenuators can also be used as dummy loads as well. 

Lacking a hamfest bargain or the special order Cadwell RF resistors, you can still homebrew a small useful dummy load. The one presented here is made from ten three-watt metal oxide film resistors. These are available from Digi-Key, ten sell for $2.30 and will make a 30 watt load. All you need extra is a RF connector and a scrap of PC board or brass to mount the resistors on. Note that ten 510 ohm resistors in parallel equals 51 ohms. 

Obviously the schematic for ten resistors in parallel with a connector is very simple. The key to getting the load to work correctly is its layout. The shorter the leads, the lower the inductive reactance of the load and the SWR will be in the VHF/UHF range. To do this, mount the connector in the center of a metal plate as shown in the drawing. The resistors are than mounted in a star pattern symmetrically around the connector with as short as leads as possible. 

The load will be able to dissipate up to 30 watts continuously and maybe up to 50 watts for short periods without damage. You can increase the loads power handling by immersing the resistors in oil. Ordinary drug store mineral oil will work perfectly in this application. The resistors have to be completely submerged in the oil (at least a pint) to give sufficient cooling. This is the principle used by Heath on their very popular "Cantenna." 

When you finish the load, check the resistance with your ohm meter and log the value you measure. If the resistance climbs above 55 to 60 ohms, you've damaged the resistors by over heating. Considering that it costs only $2.30 to repair the load, damage is not much of a problem. 

With a little more effort you can make your dummy load a wattmeter. Figure 2 shows a -20 dB resistive attenuator used as a load. For example, a 25 watt signal applied to the attenuator comes out as .25 watts. This signal is further attenuated by the 10 K resistor and then detected by the diode. The resulting DC current is displayed on the micro- ampmeter. 

Note that the response of a diode detector is logarithmic. Thus, the wattage cannot be read direct off the linear scale of the meter. Probably simplest solution to this problem is to calibrate your home brew wattmeter against a known meter such as a friend's Bird 43. You could then make a chart to convert the readings of your meter to watts. If you're really ambitious, you could scan your meter face and make a new logarithmic scale for it. In any case the result would be very affordable wattmeter suitable for tuning or setting power output for most amateur transmitters up to 70 cm. 

NEXT MONTH: More on power measurement, SWR and antennas. 

David Metz, WA0AUQ

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