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Old 07-14-2011, 12:59 PM
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Default Careful with that HT

I bought a VX-7r recently and have been playing with it to learn some of its many functions.

Yesterday I found my desk computer at work losing it mouse function - yep, keying up the HT will reliably lock up the cursor. Had to reboot to clear it.

Today I stopped at the ATM to make a desposit. It took three tries to get the machine to read my card. I then went to King Soopers to pick up a few things. When the automated checkout machine wouldn't read my credit card, I put it together - don't put the radio in the same pocket as your wallet. I don't think it was keyed to transmit but don't know for sure.

This thing is way more destructive than cell phones or wifi.
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Old 07-14-2011, 01:17 PM
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Hey cool, glad you got your own VX-7r.. now you can show me how to use mine!

Good to know about the other stuff, usually when the HT comes out the wallet's long been stashed in the center console, but I will keep this more top of mind.
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Old 07-14-2011, 02:26 PM
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I could see it messing with a mouse, wired or wireless. They don't exactly use the best of shielding and suppression on a $10 throw-away mouse. Still, an HT can definitely generate a greater field strength than a cell phone, even at max power (where cell phones rarely run anyway). Your cell phone might generate a 4 V/m E-field within a meter of the phone but a 5 W HT even with a modest gain antenna can generate a 20 V/m field within a meter. This is a considerable EMI field, although not terribly harsh.

OTOH, I don't see how an ATM card would care. A mildly strong electric field of 20 V/m in free space of 377 ohms and generates a B-field of roughly 53 mA/m. This gives you an H-field around 0.7 milligauss. To erase a magnetic stripe /quickly/ would take quite a bit stronger H-field. I think those bulk card erasers produce in the range of 10,000 A/m B-fields (e.g. 125 gauss).

As an comparison, standing a few feet under typical household power lines will subject you to about 25 milligauss of magnetizing field. An MRI is around 1 tesla or about 1,000 gauss. The magnetic seal on your fridge or the magnet on car speakers are probably in the 1 to 10 gauss range. The Earth's own field is around 500 milligauss. In fact, I'd guess that residual magnetism on a pocket knife or the speaker in the radio are more likely culprits than any generated E-field.
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Old 07-14-2011, 02:35 PM
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I love it when someone can add numbers to put things in perspective. I like the speaker magnet theory since I doubt the radio transmitted while in the pocket.
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Old 07-14-2011, 02:50 PM
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I'm wondering if you can use the HT to access the grocery store PA system?

"Attention King Soopers shoppers. All Chips Ahoy and Oreo cookies are buy 1 get 5 free. Get 'em while they're fresh!"
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Old 07-14-2011, 09:34 PM
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Alright seeing as this has hit the proverbial tangent point, what do you guys think about cell phone usage & brain tumors? Seriously.. I've actually been using the headphones that came with my iphone more when I travel, and am hoping that's better for me. I worry that those bluetooth ear things aren't much better than the actual phone.

But our kids have been talking on cell phones since they were 2- it's the only phones we use. they do prefer the video/facetime a lot more, which is further from the melon. but I probably log 500 minutes/month on the thing.. probably less than half of that is with the headphones. I don't know why I started worrying about this recently.. but I do.
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Old 07-14-2011, 10:59 PM
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Bluetooth earphones are considerably safer next to your head IMO.
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Old 07-15-2011, 07:57 AM
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I put a gauss meter right on top of a 5 watt HT antenna while keyed up. 5 gauss.
HT listening to audio was about 1 gauss right on top of the speaker.
HT standing by. No audio .4 Gauss.
I put the same Gauss meter right on the knob of the running washing machine. 65 gauss.
This was the highest reading I could find in the house.

Anything with a transformer was throwing a pretty hefty magnetic field. I think the step down transformer to run the timer motor and the lack of metal shielding where the plastic knob comes out of the washing machine is why the reading is so high. I can't imagine washing machine manufacturers spend a lot of time on this aspect of design either.
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Old 07-15-2011, 09:00 AM
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Was the magnetometer a DC or AC unit, was it an analog or digital type and was it touching (or very, very close to) the antenna? A value of 5 G sounds very high to me and I suspect that your magnetometer can't handle the frequency of an RF EM field. Could also be an interference problem, since generally they're not really designed to work in the presence of high RF.

None-the-less, most are accurate up to maybe 100kHz for AC magnetic fields and are typically spec'd at 60Hz. Finding an RF magnetic field is done analytically by measuring the electric field, which is far easier to do accurately at RF. Once you know E-field strength then it's just a matter of applying Maxwell's Equations. That is how I arrived at my expected magnetic fields above, by starting with a 20V/m E-field and working forward. Lots of assumptions, materials for one make a difference on how an incident field affects you.

Now, I don't know if those are realized but I do know that an HT drives a lot less current (you'll see around 300mA into a dummy load) than a 120VAC overhead line and so I would expect as relative distances they would have similar magnetic fields. IOW, 10 feet from a 1800W household circuit strikes me as more similar to a foot from a 5W RF transmitter than not. Several orders difference seems counterintuitive.

Washing machine numbers do not surprise, motors are all about electromagnetism. It's not transformers as such, but current that generates an EM field. The physics of how transformers work necessitate they must generate large fields. Just a coincidence that things with xfrmrs also have high power. Your truck battery shorted with a huge wire will also make a electromagnet, as is the case, for example, inside your alternator and starter for the field windings.
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Old 07-15-2011, 06:51 PM
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Quote:
Originally Posted by nakman View Post
Alright seeing as this has hit the proverbial tangent point, what do you guys think about cell phone usage & brain tumors? Seriously.. I've actually been using the headphones that came with my iphone more when I travel, and am hoping that's better for me. I worry that those bluetooth ear things aren't much better than the actual phone.

But our kids have been talking on cell phones since they were 2- it's the only phones we use. they do prefer the video/facetime a lot more, which is further from the melon. but I probably log 500 minutes/month on the thing.. probably less than half of that is with the headphones. I don't know why I started worrying about this recently.. but I do.
Make sure they wear the proverbial aluminum helmet. I find the heavy duty version is more resistant to tears and damage...

But seriously... too much of anything is probably a bad idea. I guess we will find out in a decade or so.
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