In previous episodes of this saga -
* L401 has a habit of producing a null in the 20m band if you wind it wrong
* I wound it wrong
* For giggles, I deleted it from my QMX. Suffered no ill effects, posted about it.
* Was told I would surely awaken the wrath of the RF gods in doing so
* I pointed out that I was already operating on the air with it and encountering zero wrath
* Was told I must not be operating near any high power AM radio stations
* I decided to come back with data
Here's my data:
is the AM radio environment where I live. At night I have one 15kW station 11km away, a 25kW station 28km away, and a 50kW station 33km away. Whether you consider this a challenging RF environment is up to you, but I'm not exactly out in the sticks.
I decided to use a NanoVNA as a signal generator. Its output is approximately -22 dBm. I set it to "CW" stimulus mode at 3.501 MHz, and plugged it directly into my QMX, tuned to 3.5 MHz:
1
The frontend is complaining quite visibly! Now let's tune the NanoVNA to 1700 khz, the highest frequency in the AM radio band:
The noise floor is now jet black. One should note that the output of the NanoVNA is a square wave, meaning there is also a strong second harmonic at 3.4MHz, 100khz away from the passband.
I then unplugged the NanoVNA and hooked up a 50 ohm termination to see if there was any change in the noise floor:
To my eyes the noise floor looks identical to when I was pushing -22dBm of out-of-band noise into it.
Finally, I hooked up my antenna. It's a busy night for FT8!
In summary, one or more of the following are true:
* The Tayloe detector is very good at rejecting out of band interference.
* The low frequency rolloff provided by C401/C402/C404 (whichever is currently selected by IC402) is more than adequate to prevent AM radio stations from causing out of band interference.
Further comments:
* I still have no idea what anyone thinks L401 is supposed to accomplish in terms of filtering out signals below the passband, considering that it is a series inductor, whose effect is to produce a high frequency rolloff. If anyone can show me a SPICE simulation which demonstrates how the addition of a series inductor can produce a low frequency rolloff, I would very much appreciate it.
* I believe that in practice, my antenna tuner also produces a band pass effect which further attenuates out of band signals. This is on top of the fact that the antenna itself is only resonant at certain frequencies, and is not large enough to be resonant in the AM radio band.
