Measuring a loop antenna

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I have been taxed with verifying that an underground loop antenna for a parking lot exit gate falls within the 75-100 micro henry (uH) range...

EDIT: thanks everyone for the correction!

Any suggestions for taking an accurate reading of the existing underground loop would be appreciated - that's such a small range!!??? How can I be sure that lead capacitance etc is not skewing my readings??

Thanks
John

Just some guy

Last Edited: Sat. Aug 31, 2013 - 02:20 PM
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What did you say to your boss ?

Have you been sent to the stores for a "long weight" yet ?
Or perhaps to get some "greased nuts" ?
Or "Sky hooks" ?
Or ...

David.

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Quote:

75-100 micro hertz range...

"Loop antenna"? You would want to be measuring henries then, not hertz. Right?

We've done some of that work here from time to time, including making "prefab" loops encased in a rectangle of PVC pipe. Our shop inductance meter has the correct range(s) so we just hook up the leads to the meter where they would be connected to the control system.

Note that the leads, once they leave the loop, need to be well twisted.

Also note that you can make your own inductance meter with an AVR. ;) There should be circuits and firmware floating around here and elsewhere on the Web. This article gives hints about the frequencies used in traffic loops:
http://www.fhwa.dot.gov/publicat...

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Absolutely correct - sorry about the hertz thing...thanks for the help!

Just some guy

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Quote:

Also note that you can make your own inductance meter with an AVR.

http://www.mcselec.com/index.php...
http://www.kerrywong.com/2010/10...
http://cappels.org/dproj/EvenBet...
http://www.dansworkshop.com/2010...
...
(top Google results for "inductance meter atmel avr")

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Of course if this is just a one-time thing to check a single loop, then just borrow/beg/rent/steal an appropriate meter and take the reading. Probably "readings" as the excitation frequency matters. Thus, you'd probably want to use the excitation frequency/frequencies that the loop controller uses.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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I was wondering about microhertz. Wow, that's LOW frequency.

Since a henry is sort of a reversed farad, you can measure it with an oscillator, resistor and scope.

When you step the voltage 1 volt, a 1 henry inductor will take 1 second for the current to reach 1 amp. If you're looking for 75-100uH, then when you step the voltage 1 volt, it will take 75 to 100 microseconds for the current to reach 1 amp.

(If you can beg, borrow or finagle an inductance meter, use it.)

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

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theusch wrote:
Quote:

75-100 micro hertz range...

"Loop antenna"? You would want to be measuring henries then, not hertz. Right?

theusch... are you sure its a loop antenna?

surely it would perform the inductive part of a tank circuit. the car or such performs the task of a capacitor? antenna or tank circuit ... whats your opinion.

N

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micro Hertz? Never heard of that in all my years.
The loop would be characterized by either its inductance (units = Henrys) or its resonant frequency when installed at a site (units = Hertz). I think those buried loop sensors are part of an oscillator whose frequency changes by a large % when ferrous material is nearby.

Maybe the units are micro-Henrys? (uH).

http://www.wikihow.com/Measure-I...

Or look at vendor specs, like Swarco.com's spec sheets. Controllers can accommodate varying loop types, say, in the range of 80 to 2000 uH.

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Yes Steve, it is uH - I agreed with the correction by Theusch, I guess I should have edited the OP!

Thanks!

Just some guy

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You still have time to correct it!

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RickB wrote:
You still have time to correct it!

No he doesn't. We're already making fun of him

The largest known prime number: 282589933-1

In my humble opinion, I'm always right. 

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Thanks everyone,

Quote:
No he doesn't. We're already making fun of him

Well, I often, "lead with my chin," so I am used to jibbing!

Begin excuse 1:
I posted the question from my iPhone while sitting in class and at the time there was a FLUKE 87 picture in front of me so I was actually looking at the HZ button on the fluke while thumb typing:)

Begin excuse 2:
I graduated in '87 and do not use/work with electronics everyday and usually do a refresher before/during/after my "once in a blue moon" projects or tasks!! HOWEVER - I love it and miss it so I hang out here whenever I can and think most 'Freaks here are pretty amazing folks!!

So, I am back in town now and have had some time to ponder this task while consider the help above and I have a few thoughts to share.

First -

Quote:
Thus, you'd probably want to use the excitation frequency/frequencies that the loop controller uses.

I agree Theusch - in my mind there is no way to directly measure this value from an underground loop that has been in service for years. Corrosion has surely led to some moisture and conductivity issues that would make any direct reading faulty.

I propose the only way to accurate measure the Henry value of the loop is to calculate it mathematically after taking several other value readings from the controller - especially during gate activation.

I will let you know how I fare - thanks for the tips!!

(BTW: I Googled for help with this before posting, but found NOTHING!! I did find a great deal of info from HAM radio operators, but nothing directly related to these exit gate systems!)

Just some guy

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If you know the dimensions of the loop and the number of turns, you can calculate the inductance.

I presume that you have a resonant circuit. The frequency will alter as objects approach. This will be partly due to the capacitance and partly due to the conductance / permittivity of the object.

There must be all sorts of app notes on this subject. Google is your friend.

David.

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I guess I was Google'ing the wrong terms:

Quote:
The lead-in or "home run" from the corner of the loop to the detector must be twisted a minimum of five times per foot. The detector powers the loop causing a magnetic field around the wire. The loop then tunes to a resonate frequency and the detector constantly monitors the frequency for changes. When a vehicle enters the loop, the frequency increases causing the detector to send an output to the gate operator.

And the answer is...

http://www.gateinfo.org/knowledg...

:D

Just some guy