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Richard Clark wrote in
: On Sat, 19 Dec 2009 09:14:46 -0600, Lostgallifreyan wrote: What I really want to know is whether the ATS-909 will work ok with a long(ish) wire outside feeding a 50 ohm coax via a 9:1 transformer, or if that would cause more bother than connecting a wire directly to it and putting up with local noise picked up from nearby buildings. Hello Dr, Well, I've been on the East Coast for several weeks and am just catching up with your particular problem (which is not too uncommon for SWL'ers who post here). Many suggestions have been useful, but some contra-indicate others, while some merely rely on lore and superstition. One of the last suggestions, from JIMMIE, is probably the single greatest boon for signal strength: Ground. Ground is always the least appreciated component, and is always the single most important one. Ground for RF is not always the same as the ground for safety. Worse yet is that mixing them can sometimes introduce grief (AKA ground loops). ALL grounds should eventually find their way to the service ground. This advice serves both safety and engineering. More can be said - but we move on. One antenna does not always work for all bands (not without a lot of work and the ability to change its polarization). As such, two or more antennas are necessary for SWL'ing. They don't need to be isolated to one band, but if any antenna is resonant for one, it will probably be difficult on another band that is twice or half that frequency. Thus you add another antenna that is half or twice the first's dimension. The benefit here is that they can be wired to the same feed point with little interaction between them. More can be said here too. Matching with a one-size-fits-all doohickey is pabulum for the masses. When it is tossed into the mix, it usually forces the user to add the components already described above that are responsible for most of the benefit attributed to the doohickey. Hosanna's are misplaced. More can be said here, to not good outcome. Matching with an antenna tuner (yes, I am aware of the irony in its name) satisfies all issues (except for the transmission line loss - if it matters) of matching. The tuner's responsibility is to see to it that an unknown source is matched to an unknown load (that is why it has so many adjustments). You can use any Ham grade tuner, get one without a meter to save the big bucks. Whatever product that is designed for the ham bands is satisfactory for the SWL bands. Of course, you could build your own (what a concept!). Matching with a preselector takes the antenna tuner one step further, and protects your receiver from the scourge of these "modern" designs: intermod. The SWL-monkeys who demand the ability to "quickly" tune up different bands/frequencies usually whine and squeel about the difficulty of tuners and preselectors (and in the same breath praise the doohickey's font of blessings). I let them indulge in their illusions and say no more. INTERMOD is the silent killer (as they used to say about high blood pressure). A strong station (a nearby AM transmitter in town) can easily close down your 31M listening experience by simply driving the AGC into overload without you being aware of it. Preselectors and Tuners will drive down that off-band signal, peak the selected frequency, and give you what you tuned for in that band. Beware of imitations that suggest they do the same without tuning (what a crock). As for that antenna impedance. Others have suggested a myriad of possibilities. The first active component's shunt reactance (often the base-emitter capacitance) is the limiting factor even when humongous resistance bridges those same points. Resistance is for bias folks. More can be said, but enough has been said here. Feel free to ask for more to be said. 73's Richard Clark, KB7QHC Thanks. ![]() underneath it. My neighbourhood problems recently were solved when a guy on the first floor ****ed off someone enough that said someone blew their flat door in with a shotgun! So the whole problem ended with a neat flameout a few days ago. I couldn't go out there rigging antennae while paranoid criminals were still active, it's seriously asking for BAD trouble. Right now I have the lesser problems of dental and other bills imminent, but I'll get a good 4' ground rod and rig up an 18' vertical whip as I learned of in details I posted about earlier. I understand that good reception depends on a good compromise between selectivity and sensitivity, and no doubt the antenna 'tuner' helps with that, though I'll mainly be concerned with good ground and local common mode noise rejection. My first attempt at the line between antenna and receiver will be a balanced line with a toroid at each end for current isolation and possibly the suggested Norton preamp on the receiver input, but I'll try without it first as I suspect I'll get enough signal strength to satisfy me for a while. If I have to use coax I will but I'll try the easier options first. This basic plan does involve a 10:1 ratio in windings on the far end toroid which should help smooth out peaks of resonance as described by John Doty and others as mentioned before, and if nothing else, drives a stronger current in the balanced line part of the system. I'm no longer much concerned about matching impedances, but I will be watching for results of changing antenna length if resonance seems to be an issue. My interest in the 'doohickey' or any other widget was mainly in what appeared to be a means of reducing the difference in signal strength extremes due to resonance. I understand that if I subsequently have to select the weaker of two close stations I'll either have to add some 'trap' for a specific offender, such as a trimmed lengh of unterminated coax (though as far as I know, that trick is usually reserved for much higher frequencies), or use a manually tuned system which I'll explore if it becomes a dominant concern. |
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