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QDX RF filter sweep fine for all bands but 20m... #10m #20m #qdx

 

Hi, I recently built the QDX (for 10m - 20m) and it went really well. So far, I haven't been able to test with an antenna because of the weather, but I have used the internal test tools.
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Doing an RF sweep, I noticed that most bands are good (or acceptable for some), but the 20m doesn't seem right.
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Here are measurements for 10m and 20m:
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RF Filter Sweep at 10m
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RF Filter sweep at 20m
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As you can see, sweep doesn't look great for 20m.
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My first question is, am I measuring this correctly? I didn't want there to be any spurious transmission signal, so I hooked up a 50? dummy load. Is that necessary or shouldn't I do that? (If this is mentioned in the manual, I must have overlooked it.)
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Secondly, given the response in the second image, what could be wrong? Perhaps something wrong with the toroid (number of windings), capacitors? I'm just trying to determine how I can get to the root cause quickest. I have some measurement equipment, like DVM and oscilloscope.
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Any input would be greatly appreciated!
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73
Sander [VK6PI]
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¿ªÔÆÌåÓý

Hi,

there are various reasons for this. It looks as if the transmission curve has shifted upwards. That could mean that there are too few turns on the coil, or the capacitors have the wrong value. I would first check whether all the soldering points are making contact and then re-solder them. You can use an ohmmeter to check whether the coil wire is making proper contact.

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73 Reinhard DL3AS

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Von: [email protected] [mailto:[email protected]] Im Auftrag von Sander [VK6PI]
Gesendet: Montag, 22. Juli 2024 15:17
An: [email protected]
Betreff: [QRPLabs] QDX RF filter sweep fine for all bands but 20m... #10m #20m #qdx

?

Hi, I recently built the QDX (for 10m - 20m) and it went really well. So far, I haven't been able to test with an antenna because of the weather, but I have used the internal test tools.

?

Doing an RF sweep, I noticed that most bands are good (or acceptable for some), but the 20m doesn't seem right.

?

Here are measurements for 10m and 20m:

?

RF Filter Sweep at 10m

?

RF Filter sweep at 20m

?

As you can see, sweep doesn't look great for 20m.

?

My first question is, am I measuring this correctly? I didn't want there to be any spurious transmission signal, so I hooked up a 50? dummy load. Is that necessary or shouldn't I do that? (If this is mentioned in the manual, I must have overlooked it.)

?

Secondly, given the response in the second image, what could be wrong? Perhaps something wrong with the toroid (number of windings), capacitors? I'm just trying to determine how I can get to the root cause quickest. I have some measurement equipment, like DVM and oscilloscope.

?

Any input would be greatly appreciated!

?

73

Sander [VK6PI]

?
 

Yes, you're measuring it correctly from the sounds of it.?
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What's wrong? The receive BPF on the QDX HB is shared between 20m and 17m, so compromises had to be made. On this model, the BPF sometimes has problems spanning both 14 and 18 Mhz, and there may be some interaction with the board, stray capacitance, etc around 14 MHZ as well, I suspect. So others have seen that 20M on the HB QDX often is less sensitive on receive.? It was discussed in some previous threads here... one of them: /g/QRPLabs/topic/102685749
?
Your options? First thing would be to try and squeeze windings on the BPF toroid, especially the 17/20M portion...you may see some improvement. You could try and add turns to that "last" portion as well. You might try and mess with capacitors if you want and have some on hand. Another option if you NEED 20m to receive very well is to bypass the BPF in that slot, but that comes with risks. There's also a possibility there's some LPF interaction that feeds into this, so bypassing the BPF may not solve it entirely.
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I attached pics from my QDX HB build, which came out the same as yours on 20m, built as instructed BPF-wise. In my case, I can use 17m-10m on this QDX, and have a low band QDX cover 80-20m, so I didn't worry about trying to optimize the BPF on 20m, so I'm not sure how easy/difficult it may be on the high band.?
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-Nate
N8BTR
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Aha! Well, at least it didn¡¯t come down to me making mistakes during building. I¡¯ve been very careful but mistakes can still be made¡­?

Maybe I should just go and see how it performs and truth be told, I¡¯m looking at using it (for now at least) on 10m only.?

73
Sander?


On Tue, 23 Jul 2024 at 20:57, natereik via <natereik=[email protected]> wrote:
Yes, you're measuring it correctly from the sounds of it.?
?
What's wrong? The receive BPF on the QDX HB is shared between 20m and 17m, so compromises had to be made. On this model, the BPF sometimes has problems spanning both 14 and 18 Mhz, and there may be some interaction with the board, stray capacitance, etc around 14 MHZ as well, I suspect. So others have seen that 20M on the HB QDX often is less sensitive on receive.? It was discussed in some previous threads here... one of them: /g/QRPLabs/topic/102685749
?
Your options? First thing would be to try and squeeze windings on the BPF toroid, especially the 17/20M portion...you may see some improvement. You could try and add turns to that "last" portion as well. You might try and mess with capacitors if you want and have some on hand. Another option if you NEED 20m to receive very well is to bypass the BPF in that slot, but that comes with risks. There's also a possibility there's some LPF interaction that feeds into this, so bypassing the BPF may not solve it entirely.
?
I attached pics from my QDX HB build, which came out the same as yours on 20m, built as instructed BPF-wise. In my case, I can use 17m-10m on this QDX, and have a low band QDX cover 80-20m, so I didn't worry about trying to optimize the BPF on 20m, so I'm not sure how easy/difficult it may be on the high band.?
?
-Nate
N8BTR
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