The answer lies in a combination of things.
- The typical load on a 1N34A diode in a crystal radio is a crystal earpiece with an impedance of 20kOhm.
- The 1N34A conducts much smaller currents with a lower Vf.
The trick lies in the combination. With a lower If, Vf is also lower. Since a crystal earphone draws very little current, Vf is also lower.
As you can see from this diagram (from here,)
The 1N34A will conduct 0.5mA at about 0.3V.
0.3V divided by 20kOhm is only 0,015mA, so that's plenty of currrent to drive a crystal earphone.
Due to the I/V curve of the diode, the forward voltage of the 1N34A in a crystal radio is lower than you would expect from the Vf entry in the datasheet.
You need a low Vf because of the modulation scheme, not to avoid loading the tuning circuit.
You need the crystal earphone to avoid loading the tuning circuit and to keep the Vf low enough for the diode to demodulate the signal.
In reponse to comment:
It may well be the diode that is causing problems. The 1N5822 is made for lower frequencies and higher currents than a signal diode. Here's link to a site about crystal radios with some hints about Schottky diodes: http://www.techlib.com/electronics/crystal.html
According to that site, you will need a bias current for most Schottky diodes to get good performance. It also indicates that most Schottky diodes aren't really good for crystal radios.
From my own experience in building crystal radios, you would be well advised to build the first one using exactly the parts recommended and exactly the schematic of someone who has built one successfully. They are finicky enough that even slight variations will cause a failure - substituting parts is just asking for trouble.
Another thing to keep in mind is that they don't have fantastic range. My biggest problem when building one as a kid was that we lived so far away from any transmitter that you pretty much had to get everything perfect and have a good antenna in order to pickup anything at all. Being a kid, of course, meant that I never got things really good enough.
Please, please, please!
Use a proper crystal radio earphone!
It makes a WORLD of difference in all of your experiments with crystal radios!
A piezo buzzer is meant to BUZZ at a particular frequency. How well it will do anything else is anybody's guess.
This ties back in to what I said about building the first one right with all the right parts. Once you have a working receiver, you can try using the piezo buzzer and see what it does and how it changes things - but that is an experiment for AFTER you have a working receiver.