Featured: Daiwa CN-501H2 Cross-Needle SWR/Wattmeter at DX Engineering.
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Tuning a linear amplifier without a meter in the line is guesswork. You need to know how much power is going forward, how much is coming back, and whether the match is getting better or worse as you turn the knobs. The Daiwa CN-501H2 cross-needle SWR/wattmeter shows forward power, reflected power and SWR together on one meter face, which is why this style of meter has lived on so many amateur benches.
How a cross-needle meter works
Inside the meter, a directional coupler samples the RF traveling toward the load and the RF reflected back from it. One needle shows forward power and the other shows reflected power. The printed curves on the scale are laid out so that the point where the two needles cross falls on the SWR reading for that combination.
The advantage over a single-needle meter is that you never flip a switch between forward, reflected and SWR. As you adjust an amplifier's tune and load controls, you can watch forward power climb while making sure the reflected needle is not climbing with it.
Choosing the right power range
The CN-501H2 has forward power ranges of 20 W, 200 W and 2 kW. The range selection is about power level, not band. Pick the lowest range that keeps the needle on scale, so the reading sits toward the upper part of the scale. The listing gives accuracy as plus or minus 10 percent at full scale, so readings near the top of the selected range are the most meaningful ones.
Two more listed specifications matter when you are testing an amplifier:
- SWR detection sensitivity is 4W minimum. At very low drive the SWR indication is not reliable, so bring the power up enough to get a usable reading.
- Power rating is 2.0 kW from 1.8-60MHz and 1 kW at 144MHz. Coverage is listed as 1.8-150 MHz, so it serves HF work and extends into VHF.
Peak reading for SSB
Voice SSB does not produce a steady carrier. Output rises and falls with speech, and an ordinary average-reading meter barely moves on it, which makes an amplifier look weaker than it is. The CN-501H2 includes a peak reading function for SSB operation. The listing notes that peak reading and the meter backlighting require external DC power, a 12-14 volt supply, so plan a power lead for it on the bench.
Peak readings are also where amplifier limits show up. If a build is being pushed toward the edge of what its devices can handle, peak power on voice is the number to watch.
Where it goes in the line
The input and output are SO-239 connectors at 50 ohms. On the bench, place the meter between the amplifier output and a dummy load, so every adjustment is measured without radiating. On a licensed amateur station it goes between the amplifier and the feedline. Either way, keep the jumpers short and well made, because a poor jumper can show up as SWR that is not really coming from the antenna or the load.
As with any amplifier discussion: external linear amplifiers are not legal for use on CB radio. A meter like this is for amateur stations operating within their license limits and for honest bench testing.
Where to get it
Check current price and availability: Daiwa CN-501H2 Cross-Needle SWR/Wattmeter at DX Engineering.
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