HFU HF Underground
Technical Topics => Equipment => Topic started by: RVA on March 07, 2022, 1739 UTC
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Ok, so here is my first home for my 22m blackcat kit (callsign RVA). Who hasn't built something in an altoids tin? I have been running it from a 9v wall wart and the only issue that I have had is that I have to occasionally restart it by putting my finger on/near the crystal. It looks like I've been creating RFI when I operate between ~ 3-7 MHz that causes it to stop. Cycling the power usually would not make it restart.
https://ibb.co/LQhHqTk (https://ibb.co/LQhHqTk)
(https://ibb.co/LQhHqTk)
So the mod that I did after learning from another beacon op was to place a resistor in C4. I used a 450meg that came from my junk box. Then I added another cap to R18 to bring it as close as possible back to the frequency I started on. After this mod, when transmitting on the same low end of HF the beacon continues to transmit (it gets chirpy but it is there) and starts right back up when the power is cycled. Of course - your mileage may vary / try at your own risk!
https://ibb.co/db2pkC6 (https://ibb.co/db2pkC6)
(https://ibb.co/db2pkC6)
Feedback / comments appreciated. This unit may go out in the wild soon so I am going to consider a new housing.
PS mods - the img tag would not display my two pics.
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You can try to GND the case of the crystal to the circuit board GND. I do that to reduce phantom phase MOD and even spurs, than can radiate from the crystal itself. Passing thought, but I've had pretty good success doing that, when ICM engineers suggested that this is a good practice to do. (Too bad that they are gone, now.) Anyway, thanks for the info!
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Looking at the photo, I see you have a single skinny wire making the ground connection from the PCB to the BNC connector. You might try using a metal standoff to fasten the PCB to the metal case making a better low inductance RF connection to the PCB ground plane. What is your earth ground connection outside the box?
One more thing to try.... Once you have your enclosure properly grounded, where the 9V enters the box, add a .001 mfd cap from +9v to ground to help with any stray RF on that DC line.
Ok, so here is my first home for my 22m blackcat kit (callsign RVA). Who hasn't built something in an altoids tin? I have been running it from a 9v wall wart and the only issue that I have had is that I have to occasionally restart it by putting my finger on/near the crystal. It looks like I've been creating RFI when I operate between ~ 3-7 MHz that causes it to stop. Cycling the power usually would not make it restart.
https://ibb.co/LQhHqTk (https://ibb.co/LQhHqTk)
(https://ibb.co/LQhHqTk)
So the mod that I did after learning from another beacon op was to place a resistor in C4. I used a 450meg that came from my junk box. Then I added another cap to R18 to bring it as close as possible back to the frequency I started on. After this mod, when transmitting on the same low end of HF the beacon continues to transmit (it gets chirpy but it is there) and starts right back up when the power is cycled. Of course - your mileage may vary / try at your own risk!
https://ibb.co/db2pkC6 (https://ibb.co/db2pkC6)
(https://ibb.co/db2pkC6)
Feedback / comments appreciated. This unit may go out in the wild soon so I am going to consider a new housing.
PS mods - the img tag would not display my two pics.
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Great ideas guys, thanks for sharing. Having a better ground can only help. I especially like the DC blocking cap. Will see what enclosure I come up with next.
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I completely endorse the ideas already mentioned: 1) connecting the metal can of the crystal to PCB ground, 2) improving the RF ground at the output (though more for output power reasons), 3) 0.001 uF at the DC input.
number 3) is, I feel, the most important. This will provide a low impedance to ground at 3-7 MHz and should help to shunt away the RF that is getting in.
Also, if there is not already a 0.001 uF across the two IC power pins VCC and GND, as close as possible to the IC power pins as possible, I recommend adding them somehow. One place to do this would be on the bottom side of the PCB, at the socket pins for VCC and GND. This is different than having a generic 1 uF across the IC power pins; the 0.001 uF is more likely to resonate better at 3-7 MHz and reduce the impedance at those frequencies across the ICs power pins. You want the impedance at those frequencies to be as low possible to inhibit RF from getting impressed upon the power rails of the ICs.
I recommend wrapping your DC feed wires on the left around a toroid core. Any type that you may have could be helpful: iron powder, ferrite, whatever, but ideally you want a ferrite RFI toroid. These are available from many sources. You can have the toroid outside the Altoids tin and you want to wrap the DC feed wires around the toroid core as many times as possible.
You could also use a 1:1 UNUN in line with coax of the beacon's RF output to inhibit the RF from your ham transmitter on the outside shield of the beacon's coax.
Good luck.
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Appreciate your thoughts! Would be curious to see more build outs from other beacon ops. I think the Altoids tin will be retired soon and I can make some other improvements to the station.
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I think the Altoids tin will be retired soon
Many sizes and shapes available:
https://www.amazon.com/altoids-tin/s?k=altoids+tin
:)