Checking the circuit I built.

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radio_waves

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Tonight I built this static drain/attenuator circuit for an external antenna for my SW radio. It's in a plastic project box, has a 1/8 mono jack for the antenna, and has a cable with a 1/8 mono plug to plug into the ext jack. When I check the resistance across the leads, it reads about 5k ohms(the pot is 5k)(see image below). Is this how it's supposed to be? I thought measuring across them would give indication of an open circuit.


 

radio_waves

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Here's the static drain/attenuator I built. The attenuator works great. The pot had a switch, so I wired it so you can disconnect the antenna by switching it off.








 

ab3a

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Caution

If you happen to be nearby a broadcast station of any significance, those diodes may hurt more than they help.

In fact, though it is unlikely, it is not impossible that you may be re-radiating harmonics or intermodulation from those nearby radio stations.

My advice is to use this with relatively short antennas or to keep signals attenuated as much as possible.

A better idea would be to put those diodes behind a selection of passband filters. This would help eliminate problems with nearby broadcast stations and provide your receiver with additional immunity from strong nearby broadcast stations.
 

bharvey2

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you are measuring directly across the POT in your circuit so the 5Kohm is to be expected. A Question/observation: The schematic shows four diodes but your breadboard only has two. Why the disparity?
 

radio_waves

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you are measuring directly across the POT in your circuit so the 5Kohm is to be expected. A Question/observation: The schematic shows four diodes but your breadboard only has two. Why the disparity?

Because I read the schematic wrong. Oh well, with the solderless bread board it can easily be fixed. I have extra diodes-they were 10 cents apiece so I got 10 of them.
 

Boombox

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If you happen to be nearby a broadcast station of any significance, those diodes may hurt more than they help.

In fact, though it is unlikely, it is not impossible that you may be re-radiating harmonics or intermodulation from those nearby radio stations.

My advice is to use this with relatively short antennas or to keep signals attenuated as much as possible.

A better idea would be to put those diodes behind a selection of passband filters. This would help eliminate problems with nearby broadcast stations and provide your receiver with additional immunity from strong nearby broadcast stations.

Although theoretically it's possible (as you and others have said) most other Sangeans (including their flagship radio the ATS909X) have protection diodes. And they are in some Sony radios as well.

If he's close enough to an AM broadcast station, he could get intermod just from being close enough to the station, diodes or no. The DX-390 is a great receiver but it is a bit hot and it's a fact that it can overload on strong signals.

Mine did it before diode protection and after diode protection was added -- on SW, on some occasions (20 meter CW showing up about 455 khz or 1 mhz below the band, strong local RTTY showing up in one of the SW bands from time to time, etc.).

In my case, the diodes made no difference in intermod/overload. But at least I don't have to worry about my radio's RF amp getting blown by static electricity.

The shorter antenna idea you suggested is a good one. Unless he's in a very low signals area, even a 20-30 ft antenna will work very well. I used a 100 ft antenna for a while and got good results, but I"m in a hole, in a low signals area. When I took my DX-390 to Louisiana a few years ago even 15-20 ft of wire plugged into the external antenna jack overloaded it. It worked just as well off the whip. Right now I use a 20-25 ft indoor antenna and get good results.
 

wyShack

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I would suggest that you solder the components and get rid of the breadboard. At radio frequencies the strips that 'jumper' the wires act like capacitors. Just for fun, you can move say one of the Black wire over one position and not notice the difference other than a small signal loss. At RF keep all leads as short as feasible and either wire point to point or use a printed circuit board with ground. Proto board is good for audio and digital-I learned the hard way when I tired to make a PLL running at 10 MHz on proto board. That was several years ago but the construction of proto board is similar today-If you have an oscilloscope you can 'see' signals all over the board.

If you have a DMM with a Capacitance function you can measure the capacitance between two points on the board-the readings will likely surprise you.
 
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