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When did ignorance overcome education, for the correct plural is, "antennae"?
In article , rickman
wrote: What is wrong with pacemakers that they can't make one that functions around radios and microwaves? They can and do, for most situations. But an end-fed antenna will have common mode currents and I'd just as soon not have such flowing in my pacemaker wiring. My pacemaker is located just to the right of, and below, my left shoulder. There are two wires leading out of it which go to my right atrium and right ventricle. These wires bring signals up from the heart to the pacemaker telling the pacemaker when the heart is beating, and, when necessary, take much stronger signals down from the pacemaker to the heart to get it to behave correctly if it is not doing so. There must in effect also be a "ground" connection with the metallic pacemaker case embedded in tissue in front of my ribs. Common mode current from an unbalanced antenna sysem flowing in my body might do the pacemaker no good. I'd just as soon not find that out 10 miles from a trailhead. So I use balanced antennas, balanced feedline, and a good 1:1 current balun at the bottom of the feedline. The tuner inside my KX3 then makes the KX3 final amplifier happy. I have also used a coax-fed balanced antenna with balun at the antenna feedpoint, but a doublet with open-wire feedline and balun at the bottom is a better multi-band solution. Magnetic fields are also bad for pacemakers. When my cardiologist tests things he brings a magnet close to my chest while monitoring the pacemaker performance. This is done in a controlled way with observation of the effect. I have been warned that high intensity magnetic fields are very dangerous. At airports I present a card indicating I have a pacemaker, and I get gone over by hand instead of magnetically. I definiely don't put my iPod earbuds in my shirt pocket! The iPod itself is often placed there when I use it while walking or hiking. I have been warned that I must never have an MRI done on me. There is no reason to believe that the very low power cordless phones used with our landline telephone, or cell phones, pose a problem. The magents in them are up by my ear, not near the pacemaker, and aren't all that strong. The electromagnetic fields aren't all that strong either. But consider how much RF current flows, especially in a low-resistance high-reactance short unbalanced antenna, even at QRP levels. That's why I want what comes out of my KX3 to be radiated and do some good, not generate common mode current that may make my pacemaker, and then me, unhappy. Maybe I'm being too cautious. But better safe than sorry. And anyway, I suspect my balanced doublet fed with open wire is a lot more efficient than the typical random length end-fed wire connected to a transmitter through a possibly very lossy Unun. It does require two trees, unless erected as an inverted V, whereas an end-fed wire gets by with one. Doing it the safer way is correct from an engineering standpoint too. It would be a shame if one had to choose between efficiency and safety. David, VE7EZM and AF7BZ -- David Ryeburn To send e-mail, change "netz" to "net" |
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