TARGET TIME CONSTANT

[baum7154]

[QUOTE=Tepco;176457]Coil is mentioned in some unrelated topic, probably surf related, never separately in coils, time ago, unable to find it now. In essence, flat spiral made of cheapest possible thin speaker wire, 2x .25mm, 12.5cm inner, 25cm outer radius, around 30 turns, both wires in parallel. Shield for this is graphite only, forget foils etc, can be connected wit that same, or any other wire, or coax.

Is the speaker wire pair wound 'flat' as ribbon is wrapped on a spool? When you say that "both wires in parallel", does this mean that at the inner and outer ends the pairs are simply soldered together and connected to the feed line with the outer pair tied to system ground? Does this configuration then represent a 'stacked spiral' coil?

Also it appears you see no benefit in better insulation dielectric properties?

[baum7154]

Tepco, I did find your prior posts on your spiral coil construction with pictures in the COILS forum with the thread titled "How to judge shield effectiveness". Thanks for the information!

Dan

[Tepco]

Sorry unable to respond earlier, one forum member actually built it for surf pi, and posted results on website, this may be useful too:



http://www.adrianandgenese.com/blogg...rom-a-diy-kit/

[baum7154]

Very good! Thanks for the link!

[baum7154]

Wow! what an improvement. I just optimized the critical damping resistor on my fast coil. The value I used is 1040 ohms and with that value the coil operates very well at a Guard Interval setting of 10. This change added some detection distance to some small gold targets. It appears the whole system operates much better! Can't wait to get it out on the gold fields.

[baum7154]

Thanks for the replies. I just got back from vacation so I'm just now catching up on the forum.

So what I get from Green's post is that doubling thickness approximates doubling the TC. This gets back to the issue of aluminum foil in .001" and .002" thicknesses. Is it correct then that the .001" thick 1"x1" square would have about 1/2 the TC of the .002" thick 1"x1" square?

"Tinkerer quote:

One square inch of household aluminum foil has a total discharge time of 50 uS and a 10 uS TC. Eric Foster mentioned, in one of his posts, that his Goldqust PI can detect this 1 sq inch foil at 12" using an 11" coil at 10 uS."

I'm a little confused by the statement above in that his 1"x1" foil has a 10 us TC. We really don't know how thick this sample was. Greens Heavy Duty Aluminum foil (.002" thick) sample 1"x1" measured a TC of 2.2 us. This would lead me to think that the 10us sample was about 4.5 times thicker.

To eliminate this ambiguity shouldn't we always specify the thickness of the sample material in addition to it's area? I now understand the relevance of area to signal strength and not to TC.

Thanks,

Dan

Sorry we got off topic on this thread. Back to my question above. One other commonly available material we could use for standardized targets is sheet metal from an aluminum can. It runs about .003" to .004" thick and is cleanly cut with plain old scissors. It's more durable too.

Just a thought. I wonder if Green could run a test on 1 square inch, 1/2 square inch, and 1/4 square inch samples? Would be interesting.

[green]

Sorry we got off topic on this thread. Back to my question above. One other commonly available material we could use for standardized targets is sheet metal from an aluminum can. It runs about .003" to .004" thick and is cleanly cut with plain old scissors. It's more durable too.

Just a thought. I wonder if Green could run a test on 1 square inch, 1/2 square inch, and 1/4 square inch samples? Would be interesting.

I would be happy to when I get back home after 6 Sept. Maybe someone else could try for comparison. Sometime I think I get a different answer maybe do to target material, equipment or procedure.

[baum7154]

Whenever you can do the test would be fine, I appreciate the effort. I would expect the can material to be 3 or 4 times the TC of .001" foil but it's nice to have empirical results.

Thanks,

Dan

[baum7154]

I would be happy to when I get back home after 6 Sept. Maybe someone else could try for comparison. Sometime I think I get a different answer maybe do to target material, equipment or procedure.

It would be interesting to see what dimensions of aluminum can material that give a 5us TC, or maybe that can be deduced from your tests.

Thanks,

Dan

[green]

Aluminum can results. I've seen T.C. for coins different than what I get. Including a plot with a nickel, penny and another run with the HD aluminum foil. The aluminum foil T.C. are a little different do to the way I drew the slope before. I think these are more correct. Don't know why my coin T.C. are different than what I've seen in other posts. Maybe I need to change my test procedure.

[baum7154]

Thanks Green, for your work here. This is very interesting to me. As expected the 1 x 1 can material is right at twice the T/C of the 1 x 1 foil I believe due to being twice as thick. It is also interesting how the shape difference of the .5 x .5 vs the 1 x .25 foil samples shows the long narrow one to be about 62% of the square shape T/C even though they are both 1/4 sq. in.

Looks like the 1 x 1 can material is a pretty good 5 us standard and the 1 x 1 HD foil is 2.5 us. Good work!

Thanks again

Dan

[green]

I just remembered when I plotted the HD foil the last time the 1inx1in plotted the same slope as the 1inx2in. I tried again to see if area made a difference with 1inx1in and 1inx2in. Don't know if I mixed recordings last time or if I'm doing something different. Should area make any T.C. difference?

[baum7154]

According to Carl's post there is a slight effect from area, but mostly T/C is affected by thickness.

Dan

[baum7154]

It is also interesting how the shape difference of the .5 x .5 vs the 1 x .25 foil samples shows the long narrow one to be about 62% of the square shape T/C even though they are both 1/4 sq. in.

Looks like the 1 x 1 can material is a pretty good 5 us standard and the 1 x 1 HD foil is 2.5 us. Good work!

Thanks again

Dan

Apparently exaggerated/increased length does have a significant impact on on T/C as evidenced by the square VS rectangular 1/4 square inch samples. The 'shape effect' in this case results in the long skinny one having a T/C 38% less than the square one of equal thickness and area, if the test is repeatable. It looks like we should stick with the square shape and stated material thickness for our T/C standards.

Dan

[green]

Apparently exaggerated/increased length does have a significant impact on on T/C as evidenced by the square VS rectangular 1/4 square inch samples. The 'shape effect' in this case results in the long skinny one having a T/C 38% less than the square one of equal thickness and area, if the test is repeatable. It looks like we should stick with the square shape and stated material thickness for our T/C standards.

Dan

I repeated the test. Cut some new targets. Different amplifier. Different coil (5 in round DD). Held target over one of the coils not in the center as before. About the same results as before.