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Tuned MFJ-1024 Active Antenna
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David - Huh ?
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Hello Group
Thank you for trying but I finally found what I was looking for. I just needed the right combination of words in my searching. Richard |
David - Now that is a revelation ~ RHF
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"Telamon" wrote in message ... In article .com, wrote: Hello Group I have been wondering if it might be an advantage to remotely tune this active antenna using a varactor? The remote unit uses a 40673 dual gate mosfet followed by a 2N5109 npn vhf/uhf transistor which feeds a 30 turn bifilar wound 4:1 balun. Then rg58 to the control unit. The whip is 58"/1473mm in length. Any thoughts on this? Yes. People have done this to design a tunable antenna so you can do a Google search on it. The antenna element of the right type is inductive and the varactor diode capacitance across it can be used to resonate it at some frequency. -- Telamon Ventura, California A short whip will not be inductive at frequencies below 150 MHz or so. At HF they are entirely capacitive probes. Dale W4OP |
"Telamon" wrote in message ... In article zUcMe.6912$rR4.2379@trnddc08, "Dale Parfitt" wrote: "Telamon" wrote in message ... In article .com, wrote: Hello Group I have been wondering if it might be an advantage to remotely tune this active antenna using a varactor? The remote unit uses a 40673 dual gate mosfet followed by a 2N5109 npn vhf/uhf transistor which feeds a 30 turn bifilar wound 4:1 balun. Then rg58 to the control unit. The whip is 58"/1473mm in length. Any thoughts on this? Yes. People have done this to design a tunable antenna so you can do a Google search on it. The antenna element of the right type is inductive and the varactor diode capacitance across it can be used to resonate it at some frequency. -- Telamon Ventura, California A short whip will not be inductive at frequencies below 150 MHz or so. At HF they are entirely capacitive probes. He can use an inductor along with the varactor diode for a resonate circuit, to which the whip can be directly connected. -- Telamon Ventura, California This is true, although perhaps obvious.. Now you have the problem of a nonlinear device at the antenna input. If he lives in an urban area, it is an invite to horrible IM and crossmod problems. Usually not considered good engineeering practice. A better solution might be to forget tuning the whip input and use a high IP3 amplifier as in the Amrad project, or use a simple stepper motor to turn an air variable cap. Dale W4OP |
This is true, although perhaps obvious.. Now you have the problem of a nonlinear device at the antenna input. If he lives in an urban area, it is an invite to horrible IM and crossmod problems. Usually not considered good engineeering practice. A better solution might be to forget tuning the whip input and use a high IP3 amplifier as in the Amrad project, or use a simple stepper motor to turn an air variable cap. Dale W4OP ----------------------------------------------------------------------------- Hi Dale I am presently constructing the Amrad Active Antenna with the specified parts. I am kind of in the Burbs but I can see a small mountain about 10 miles airline that has an FM, 93.1mhz, station, UHF tv, channel 40, station and who knows what else. I found plans on building a MW phaser using loop and/or active antennas. I suspect the Amrad will be the better active antenna. This will bre a learning process for sure. Richard |
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