QUICK TUNING WITH A MANUAL TUNER: Theres been some debates from time-to-time about auto tuners-vs-manual. The FACT of the matter is that theres benefits to both. An auto tuners great if youre using an antenna thats at least in the ballpark with an SWR of 3:1 or less. For example, if youre operating mobile and have a HamStick type antenna, you can pre-tune it mechanically for the middle of the band youre on (except for 80 or 40) and then use the auto-tuner to bring it in to an acceptable range. The problem with autotuners is that many, if not most, are limited to that 3:1 mismatch. Some will go out to 5 or 6:1, but that still limits your ability to use a single antenna on more than one band if youre in need. Here at K4YZ, I have an 80/40 meter trap dipole with a 30 meter inverted vee, both fed at a common feedpoint. However I can operate anywhere from 3.5MHz to 54MHz using my roller inductor manual tuner. The chart attached is the preset chart I made by spending an hour finding the hotspots with the tuner. All-in-all it was a fun hour spent playing with the tuner, and a learning experience for me. I can now QSY from 80M CW at 3525KHz to 10M AM at 29.000MHz in just a few seconds. Mapping the settings for YOUR station isnt rocket science! Even if you use a tuner with a tapped inductor, the procedure is basically the same. First of all, set the radio to the middle of the band youre going to map. Set both the Transmitter capacitor and the Antenna capacitor to their mid-ranges. With the RF gain at max and the volume at a comfortable level, rotate the inductor until you hear max noise. With a roller tuner, (at least the MFJ tuners Ive owned) the lower the frequency, the lower the number in the turns counter. Start at A for tapped inductors. Once you find max noise with the inductor, peak the transmitter and antenna caps to get the most noise in the speaker or headphones. Making sure that youre not on a busy frequency, you can now apply some RF and check the SWR at that point. Once youve done this, make a chart like the one I have. For 80 and 40 meters, youll want to also find more precise settings for the low and high ends of the band. Youll find that on 20 and above, once you find the settings for the middle of the band, you can make pretty wide excursions without having to re-peak the tuner except at the far ends of the band. And guys...REMEMBER TO ASK QRL? BEFORE YOU KEY UP AND I.D. PROPERLY! Its not only the polite thing to do, its the law! To find the resonant frequency of the antenna without a noise bridge and if youre using an antenna thats specifically cut for one band, simply select bypass and slowly tune the radio through the band listening for that max noise spot. Once you find it, apply some RF (as little as you can and get a good positive needle deflection) and check the SWR. If you want to be REALLY thorough, get a piece of graph paper and make a chart showing SWR-vs-frequency to really see where your antenna is in the band. Ive made an example for your consideration. And before all the RF engineers jump on me for the very primitive methodology I used here, Yeah, its not the best way to do it, but lacking things like spectrum analyzers, Smith Charts, Noise Bridges, etc, its the EASIEST way for John Q Newguy to do some antenna testing and evaluations. 73 de K4YZ
Posted on: Mon, 29 Dec 2014 10:31:11 +0000
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