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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.

Hi Bojan,

Thanks for the screen shot example of how elsie works. I might have to see if I can get it to work on Wine emulator.

One thing to keep in mind is that, because of the stray shunt capacitors at both sides of the inductor, the insertion loss of the BPF will be greater than predicted by your simulation. Also, for the same reason, the rolloff on the higher frequency side of the center freq will be steeper. That is both in simulation and in real life measurements.

Can you please point me to how to modify the QMX to omit L401?

BTW a clean square wave has only odd overtones, so the fist overtone for 1700 kHz would be 5100 kHz

Bojan Naglic: I agree that you gain nothing at lower frequencies but you loose about 13 dB at the frequency you want.
Please see the charts in the attachment.

Rod Smith: So tuning your NanoVNA around 1.1667 might well produce a significant audio tone (but depending, as you say, on the attenuation afforded by antenna and tuner), as might the fifth undertone around 700KHz

Bojan Naglic: I agree that you gain nothing at lower frequencies but you loose about 13 dB at the frequency you want.
Please see the charts in the attachment.

All,

Let's not politicize an engineering decision. It's free to keep L401 or delete it in your own QMX build for self-satisfaction, but I feel that this thread has become campaigns trying to persuade others that their idea is right. L401 is just an engineering decision point. I have a position on that (previously expressed enough, no desire to repeat) but not enough ego to continue threads like this.

Well, yes, but this would be because the actual signal entering the radio actually contains that harmonic. Even a sharp bandpass or high pass filter that perfectly isolated the band of interest wouldn't filter this out, because it's a signal, not an artifact. A real AM radio station would not be allowed to generate harmonics so strong that they interfere with the amateur radio band, so if we're interested in the effects of nearby AM radio stations we're really only interested in the consequence of a strong signal at the station's fundamental frequency. I'm interested in seeing if there is a realistic scenario where an out-of-band signal generates an artifact in the band of interest, either in the form of a spurious signal or increased noise floor. The primary goal of my experiment was to see if even a very strong signal in the AM radio band, stronger than anyone could realistically ever expect to see hitting the front end of their radio, resulted in any ill effects. I'm fairly satisfied that between the high pass characteristic of C401/402/404, and the selectivity of the tayloe detector, that L401 and the construction challenges it presents is unnecessary for good performance. I welcome anyone to provide evidence to the contrary.

necessary for good performance. I welcome anyone to provide evidence to the contrary.

Stephan

Whilst one should always applaude investigations, forgive me but I am failing to understand what it is you are trying to achieve here.
You have performed one set of experiments and and arrived at a conclusion which you have published on this forum.

Now you are asking us to change your mind.

This is not how science works.

Stephan, it is terrific that your are experimenting in this field and a proper study would be most interesting but I would argue that you need many more data points? (different equipment, locations, etc etc). The methods for running such a study are well established and documented.

Also I am sure it was never your intention, but they way you have titled this topic is questioning the integrity of these radios and I don't think that is very fair.?

I am in no way trying to deter you but you need far more investigation and data before you could make such a claim. I wish you good luck.

73s Ted
2E0THH.

way you have titled this topic