Re: Random Wire Aerials for AM BCB
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Tony Germanotta
Longer wires pick up more signal but also more noise. So once you clear the noise floor, you can just add low noise amplification to get the same effect, according to US Navy research. You really don’t get much benefit with longer wires on MW until you hit a quarter wave length. That gives you some resonance that tends to amplify the signal rather than the noise. At medium wave, that can be hundreds of feet. And it gets longer as you tune lower on the dial. Resonance helps a little on receiving antennas but it really is important for transmitting, so the signal from the amp actually radiates out into space. That’s why hams are so focused on it. And most antenna books are written by and for hams. I find it overkill on receive antennas. A random wire won’t be directional. You can make it directional by adding a ground and resistor to create what is called a Beverage antenna, named after the guy who invented it, not a drink. But those also need to be long, preferably a half wave or longer, if I recall correctly. With the wire running head high or so in a direct line towards your target. At the end, you slope it down to your resistor and ground. The resistance needed to make it favor one direction varies by frequency. The most effective Beverages have resistors that can be adjusted remotely at the receiver end. They are very sensitive, low noise antennas. Especially if the ground is good. And if you have a farm with a wooden fence that points in the right direction. Loops are even lower noise, bi-directional and much easier to construct and transport. Also, unlike the old tube radios that had antenna posts, our ultralight radios are designed to match the impedance of loop antennas. If you go longwire, you will likely need a matching transformer or the radio could have trouble tuning. You could also overload the radio’s front end creating images across the bands. Communication receivers that are designed to use long wires have a lot of circuitry up front to offset that. The best can tune a faint signal a few hz away from a powerhouse transmission. Those who use long wires on portables often build antenna tuners and attenuation boxes to block out of band signals from saturating the RF stages. But antennas are often mysterious and magical, and sometimes things work and we don’t know why. So try putting up as much wire as you can, as high as you can, and as far from noise generators in the house as you can, and see how it works. Buy some cheap wire and string it. You don’t need thick wire or litz for receive only random wire antennas. Bare wire, single or multi strand, insulated. It all works. Enjoy the quest. As our resident FSL experts can testify, it can also be addictive.
On Jan 15, 2019, at 7:33 PM, Russ Edmunds <wb2bjh@...> wrote:
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