[Lowfer] 18.250 kHz

[Michael Sapp]

Zack: My old Heathkit HD-1420 and both of my double balanced mixer LF/MF 
converters routinely hear the French MSK station

at 18.3 kHz and the MSK bandwidth is easily detectable on the CW mode of 
ARGO as well as plainly audible in the IF receiver.  Other

MSKs in the 20 to 76 kHz range detectable as well.  The 2N3904 preamp with 
4.7uf coupling caps was hearing the 18.3 kHz French MSK in

afternoon daylight with my NE EWE into 300 kHz LPF to preamp>mixer>if 
diplexer>IF radio as analog signal.  When I decided to switch to

.22uf input and output coupling caps on the 2N3904 broadband preamp, the 
gain went down noticeably (~10dB range audible ear guess) at 18.3 kHz but 
the signal was still readily detectable.

However, my local background sinusoidal 60 and 120Hz harmonic QRM went down 
substantially at 75kHz and 137 kHz with the .22uF coupling caps in the 
preamp verssus 4.7uf

coupling caps.

If you run some Xc and Z numbers, a few things become obvious.

 4.7uf at 137 kHz Xc = .247 ohms .....(2Pifc)

.22uf at 137 kHz  Xc =  5.3 ohms.


Yet, Capacitor impedance is Z= SQT RT (R^2 +Xc^2)

If you make a ball park assumption that your input impedance is something 
close to 50 ohms resistive (Q=1 diplexer on input, etc.) , then

Cz = SQR RT (50^2 + 5.3^2) = SQR RT (2500+28.09) = 50.28 ohms.

Likewise

    4.7uF at 60 Hz = 564 ohms
   .22uF at 60 Hz = 12,057 ohms.

I decided I could live with Cz in the 55 to 60 ohm range and choose .22uf 
as a coupling cap value that would work well from 75kHz to 500kHz. But if I 
only wanted

to cover 630m wspr band, then I would go with something like .022uf whereas 
many LF/MF converters use a .1uf coupling cap value.  It would be easy 
enough to have switchable coupling caps or simply dual input and output 
connectors with different coupling caps if you wanted more frequency range 
flexibility.

So keep in mind the 60 and 120 Hz roll off when choosing  input and output 
coupling capacitors. Less "stuff"

in your rx mixer is generally a good thing.  If you look at my 630m wspr2 
reports you will not see many 60 or 120 Hz image responses reported from

the current free space QRO malware station.  Part of that solution is 
keeping 60 and 120 Hz energy out of the preamp and mixer in the first place. 
Good

grounding, eliminating audio ground loops, and other factors apply for sure, 
but it only takes one incorrect or failing coupling capacitor to adversely 
affect

receiver performance...

73 Mike wa3tts



----- Original Message ----- 
From: "Zack Widup" <w9sz.zack at gmail.com>
To: "Discussion of the Lowfer (US, European,& UK) and MedFer bands" 
<lowfer at mailman.qth.net>
Sent: Wednesday, February 14, 2018 12:18 PM
Subject: Re: [Lowfer] 18.250 kHz


> What setups are you all using for those frequencies? Antennas? Receivers?
>
> 73, Zack W9SZ
>
>