Re: July 2010 Oregon Beach Ultralight DXpedition Report
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Tony Germanotta
Stephen, you keep asking the same question in different ways, but the answer can't change. If you are in an area where your portable radio is being overwhelmed by RF splatter from other stations, it's pretty much impossible to hear the, by definition, faint signals from DX stations. It's like trying to DX under the noise of a bad fluorescent bulb. I have one in a back room that just wipes out the entire band when I forget to turn it off.
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Can you build a preselector? Sure. What will it do. Well, there are types that peak up just one narrow band of frequencies. That helps sometimes, and others that block out either everything above its designed frequency or everything below. And those can help sometimes and some that just notch out one slice of the bandwidth. Again, it can help in some conditions. But if your area is buried by off-frequency splatter, which seems to be the case from your various postings, then none of those will help at all because the offending frequency will be on the same frequency you are trying to DX. If you filter it out, your DX is also blocked from the receiver. The FCC in recent years has become pretty lax on keeping AM stations to their specs, I have discovered. There just aren't that many folks like us out there who get offended by the local talk station bleeds over 10KHz, since there is usually no other local station on the air there. And the days of clear channel stations designed to cover wide sections of the country are over. Those frequencies are now shared by several stations with national coverage now something ceded to the internet, satellite radios or just not deemed a national priority anymore. You keep searching for more sensitivity when your problem is probably too much signal already. Changing out the internal ferrite won't help if it makes the radio more sensitive in an area already over washed by RF. What you might want to do is cut sensitivity some. That's easy on radio's with a DX/Local switch that puts an attenuator between the antenna and the front end. You can do that yourself on shortwave or FM with your radio, but the AM circuit doesn't have that kind of outside input unless you go to the little one with the external antenna jack. On that one, you could put a variable resistor, or Pot, in line and use that to cut signal. There are better ways to do it, with resistance boxes, if you find the rough method works. With your radio, the antenna is always in line. So you can try reducing signal by putting the radio in a grounded metal can, for instance, with just a small opening for RF to get through. That might reduce the RF to below the point where it is overwhelming the chip and causing it to do weird things by going what they call non-linear. Before you do that, though, try to borrow a radio with an attenuator on board and see if it does anything for you. Again, if the locals are broadcasting over adjacent frequencies rather than your radio just being overwhelmed and reproducing them on those frequencies, attenuation won't help. It will just reduce both the DX and the local pest equally and the DX is going to be the loser, since it is much, much weaker. If the attenuation helps, then you know it's overload causing the majority of your problems. And you can build your attenuating can. Of course, you won't be able to fit that in your pants pocket. Then again, DXing isn't something you do while taking a walk. Moving a couple feet inside a house can make a ton of difference in DX sometimes because of the way the radio waves are being reflected and blocked by stuff in your walls an attic. That said, sometimes heading to the far reaches of the back yard can improve things by getting away from near field RF noise. Then again, you may just be moving closer to the neighbor with the bad bulb, aquarium heater or touch lamp that causes all sorts of problems. The first thing I would do is look for noise sources. Find a spot on the radio that just crackles with noise and try to track down the source. My plasma TV is a real nuisance when I try to DX. Some computers are just noise generators. The solution, shut off the problems when you start a session. One method that works is to turn off all the circuit breakers in your house and then flick them on, individually, and listen for the noise. When you find one or more that has problems, then you DF in the areas those breakers serve. You will be surprised what kinds of things can cause problems. The wall wart charger on a portable phone or cordless razor, for instance. Light dimming switches are notorious. Microwave ovens or the digital displays on appliances have been known to radiate noise if you're too close. And remember, all of this can vary from frequency to frequency. On some, the radio may just be overloaded. On others, the local may be operating out of its assigned bandwidth. I'd do that kind of work first before trying to gut the 380 and replace anything. You have to identify the problem before you can engineer the solution, I've always found. Good luck. And don't just give up the radio. You might find a lot of fun taking it on summer vacation trips or discover a world of DX if you and your family go camping. And it's still a great SW and FM receiver and there's lots of DX fun available on those bands as well.
On Aug 4, 2010, at 12:57 AM, Stephen wrote:
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