Transmit experiment

[golfnut]

Always wondered if the Tx frequency alters as you approach the ground or large targets.


I made up a duplicate oscillator circuit and sniffed and buffered the output in lm393 and schmidt nand - into a counter.

When running IN AIR as normal my unit runs 11,604 Hz
Place head on ground 11,604 Hz
Offer up to large iron 12" away 11,656 Hz

This goes some way to explain why I get murdered over plough blades - often from a foot above the soil.

Id guess the Tx shift is no big deal in itself as the Rx LO tracks with it as they are one and the same. The issue is the Rx tank will be pulled off too, but by a different amount and boom theres the massive phase shift. So great that you cant Disc it out.


Non resonant designs, both for Tx and Rx would overcome these effects I reckon..

S

[Davor]

Resonance shift is near the same with Rx and Tx in a free running Tx, so there should not be a significant shift there. They follow the same path and a resulting phase difference is virtually zero.

It is a completely different story with crystal controlled Tx and resonant Rx.

[Carl-NC]

The reason Big Iron won't disc out is because it has a larger eddy current response than a magnetic response, therefore it looks non-ferrous. If you could monitor the TX frequency using, say, a PLL then that rise in frequency might be a good way to ID iron.

[golfnut]

Thanks, Im okk with the pll or even a frequency locked loop - to allow a few cycles drift but flag 10s of Hz pull as large iron.

What Im grey on is the terminology for the large eddy current response looking non ferrous - have you another analogy for this to get me over the line..

Does the energy wang about in it for that long, that it is still available later in time, so the non ferrous channel gets it?

S