Phil, My lab "simulated antenna" consists of a dummy load and a Johnson Matchbox. I can connect a dummy load to the Matchbox and twist the dials to create just about any reasonable impedance that you want. I set it up using my MFJ259B if I am looking for something specific.
The helpful parameter is that it is frequency sensitive (just like an antenna), while a resistive dummy load is not sensitive to frequency. I do normally use Caddock Thick Film resistors to create dummy loads. I have several 50 ohm loads, but also have a 25 ohm (2 50 ohm resistors in parallel) and a 100 ohm (2 50 ohm resistors in series) that I use for setting the 2:1 SWR point while I am calibrating wattmeters. These are 1% tolerance loads, and if mounted to the connector (and heat sink) with short leads present a flat response up to 500 MHz. Note well that the Caddock literature says only that the 50 ohm resistors are non-reactive - that may be true for other values, but I have taken that information at face value and use only the 50 om resistors for dummy loads. 73, Don W3FPR On 3/9/2012 8:42 PM, Phil & Debbie Salas wrote: > I've been making tuner loss measurements for an upcoming QST review of some > remote autotuners. My set-up is similar to the ARRL lab, but I've made a > few changes. I have two different load boxes. One is a resistive load box > that lets me measure loss with loads from 12-800 ohms. The second load box > simulates different types of electrically short end-fed antennas - like a 43 > footer on lower frequency bands, or an 8-footer like you might have mobile, > and other combinations. I use Caddock thick-film 30-watt resistors for the > resistive portion of both test boxes. For the short antenna simulator, I > use series silver mica capacitors with shunt Caddock resistors. > > Basically, I feed the 40 watt output of my test transceiver through a high > power 6dB pad, through an Array Solutions PowerMaster, then to the tuner. > So my test power is 10 watts. The 6dB pad helps keep the power relatively > constant, but primarily ensures that any reflected power from a non-perfect > tune (the tuners have a target of 1.5:1) is attenuated 12dB more if > re-reflected by the transmitter. The output of the tuner feeds the load > box. The load box has an output that feeds a 50 ohm attenuator/Minicircuits > PWR-6GHS+ power sensor (that output is shunted or seriesed with Caddock > resistors to give the required test impedance). So I start with no tuner > in-line and adjust the Minicircuits offset so it and the PowerMaster read > the same at 10 watts. They are both NIST-traceable cal'd, and were within > 3% of each other, but I adjusted the offset so they are within 1%. Then I > insert the autotuner, hit it with RF and let it tune. When tuning is > complete I adjust the input drive so it is exactly 10 watts, read the output > on the PWR-6GHS+, and compare that to the expected power under lossless > conditions. > > Phil - AD5X > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
