Is TX Shield needed?

[simonbaker]

Hi all -

I am wondering - why is it necessary to shield the TX coil?

Has anyone tried building a coil with only the RX coil shielded?

Regards,

-SB

[Max]

yes, it is.... or , well, very often it's mandatory though not always

Hi all -

I am wondering - why is it necessary to shield the TX coil?

Has anyone tried building a coil with only the RX coil shielded?

Regards,

-SB

Hi,
in VLF mds ? yes it is (but read further ahead).
I've made just Rx shielded many times... but always with bad results.

Reason is simple: depends on oscillator design if you need it or not.
In e.g. Colpitt's ones you have that raising or lowering tx coil from soil will affect your osc frequency cause of capacitive coupling with soil (and static charges around... like e.g. on grass etc).

Now... in MDs that have fixed frequency of oscillations that doesn't depend on tx coil inductance (e.g. the ones with crystal controlled trapezoidal pulse trains... like some e.g. fisher's ) it's not a trouble having tx unshielded; in tesoro's and other (simpler) mds with colpitt's or other kind of tuned oscillator it's other story.

So, depending of which md is, you may need or not having shield on tx coil too.

I say that tx shielding is "needed" cause majority of MDs and schematics here, around are of simple kind... like old tesoro's ...where that is a fact and no other issues.

If you consider , instead, some fisher design (look at quicksilver as an example) or other the things are different and you don't need any shield on tx.


If you mean , instead PI designs... things are well more different.

You may need it or not.... and must consider also that in PIs the coil is usually one, that is tx then rx during cycle of operations.

In depth PI shield is not needed cause coil is suspended well above soil (like 1m*1m frame or more). In some older Pis even 8'' coils were unshielded... that depends on timing also... they used wider first delay (35us and more) so much troubles related to capacitive effects were avoided or greatly reduced.

In modern PIs the smaller coils that are swept around at few distance from soil/sand NEED shielding: capacitive effects are well seen by faster delay machines (under 15us)... so you cannot use one really on soil without prevent capacitive effects giving you a bedlam of false signals.

Also... in more sensitive PIs the shielding is employed to avoid perturbations by e.g. near power lines.... inductive effects I mean.

Also.... shielding Pi coil is related to match RF emissions laws in commercial units ... like with other products like e.g. tv boxes.

In other kind of MDs (like 2boxes) the range of frequency employed , though tx coil is far from soil, could cause problems with law.... think e.g. at radio beacons on long waves.... you cannot sell e.g. a commercial 2boxes on long waves if you tx coil is unshielded to keep emission power under law requirements. Same apply as before for interferences due to power lines or other (e.g. radio stations) to the md unit... so usually a light shielding is performed for both reasons.

I remember that some older BFO/off-res/PLL were unshielded at tx, but I have to say that all of them I saw and used/tested had serious problems on soil, with exclusion of some fairly complicated IB designs.

Sure, the topic is about complex and would maybe require a very long thread.

Kind regards,
Max

[simonbaker]

Ok, that makes sense, good explanation.

But thinking more, if TX frequency changes, does it really change phase between TX and RX much?

I guess safest to shield TX anyway. I wonder how much shield affects Q factor of oscillator, probably not much.

Regards,

-SB