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1-10 of 25 messages
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Surface waves in the desert: the X experiment
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by EXPERIMENTX on November 5, 2009
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Long ago physicists and engineers diverged on "surface waves".
Sommerfeld, Tesla, Heaviside, Norton contended and disagreed.
Can you assist a new experiment? Seeking advice for best practical
way to measure surface wave by interference effect in isolated dry desert.
Goal: Measure field strength as a function of distance from
a CW transmitter. Dependence on 1/r and 1/sqrt{r} + interference will all occur. Plan to use several frequencies and vary distance from fixed transmitter(s). Work in far field up to miles until signal faces. Wavelength 1m -30 m will keep measurements on human scale. No absolute calibration needed, overall scale can be arbitrary. Yet relative errors of order 50% or less are needed to see definitive effects. Overdetermination by measuring each thing many ways is golden; internal cancellation of errors a sign of genius.
Resources exist but are limited at this stage. PhD physicist with 20 years experience has found nobody knows radio like hams. Skip textbooks, skip engineers, consult hams. Infinite possibilities exist from $3k amps and scope with batteries on donkey to step
attenuators with $30 VOM. All are wide open for your advice. It's not clear higher tech is better. Planning to read meters with binoculars. Have you experience and advice to share?
thanks, ExperimentX
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RE: Surface waves in the desert: the X experiment
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by WA3SKN on November 5, 2009
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OK, so you want to build/use an "antenna range". Low power and a calibrated receiver are all that you need.
As for "surface waves", there is the ground wave, the direct wave, and then the waves reflected off things off the ionosphere and any large metal objects in the area. I assume the desert will be used to eliminate the metal objects. And you plan to measure direct waves in the local area.
You will want an area that is flat and has at least 100 wavelength radius. No metal in the area.
Do you have that kind of area in mind?
-Mike.
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RE: Surface waves in the desert: the X experiment
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by EXPERIMENTX on November 5, 2009
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Responding to MIke-WA3SKN: That's right, we'll use a very large isolated flat area without reflectors.
The exact nature of the "ground wave" on dielectrics turns out to have
been evaluated incorrectly by electrical engineers. Their
literature generally says it does not exist. What hams recognize as ground wave may be a different surface effect not generally recognized.
Receiver need not be calibrated - just very consistent.
thanks ! JPX
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RE: Surface waves in the desert: the X experiment
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by WA3SKN on November 5, 2009
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You just want a low power transmitter with a stable output level, a couple of antennas, and a receiver that you can turn off the AGC and use a VU or dBm meter on the audio, then you will have an accurate 20-30 dB scale. That will be plenty for relative signal strengh measurements. Add a step attenuator if you need more range.
73s.
-Mike.
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RE: Surface waves in the desert: the X experiment
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by N3OX on November 5, 2009
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Take an untuned square loop:
http://vk1od.net/antenna/SmallUntunedSquareLoop/index.htm
run it into a wide dynamic range log detector, possibly with a switched attenuator stage like Mike said to increase dynamic range further:
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1011,C1743,P12432
Digitize the output with a high bit depth analog to digital converter controlled by a microcontroller.
Interface the same microcontroller with an inexpensive GPS unit and a big flash memory stick.
Now you've got a datalogging field strength meter that records coordinates and field strength. For your application, you will have to really think about GPS resolution. Alternatively you could lay out a detailed grid by hand and use the GPS as a rough tag that you're at your well marked location. Or you could dispense with the GPS, mark a grid, and have a button to press on the unit to log a data point. I would make a LCD display to verify input just so you make sure you get every point in the right order and don't scramble data.
Sparkfun electronics (www.sparkfun.com) has a lot of the stuff you'd need to do any of this, aside from the log detector.
I think you could toss together several units like this for a few hundred dollars.
73
Dan
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RE: Surface waves in the desert: the X experiment
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by N3OX on November 5, 2009
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AD8310 might be a good log detector choice:
http://www.analog.com/en/amplifiers-and-comparators/log-ampsdetectors/ad8310/products/product.html
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RE: Surface waves in the desert: the X experiment
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by EXPERIMENTX on November 5, 2009
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Responding to Mike WA3SKN: Thanks ! Assuming AGC="automatic gain control", your expertise is helpful. I guessed such a device must exist, without guessing the name. I'm a physicist, not a ham. Being ignorant of your devices, I thought I'd have to rig an amp from scratch to avoid the built-in technology of the 21st century. Given this new info, and a minute online, it appears many receivers have an AGC on/off switch. That is so cool !
Do AGC-off switches exist on CB stuff at 11 meter? On any walkie- talkies? I'd assume impossible circuitry nightmare turning AGC off if there's no switch on the box. Right ?
Even with AGC off, why should I assume a commercial amp is linear?
Regardless, I can expect to calibrate using precision signal generator
and attenuators in the lab. Right ?
THANKS JPX
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RE: Surface waves in the desert: the X experiment
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by N3OX on November 5, 2009
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I personally wouldn't go the route of using a receiver with AGC off for your application.
I do it all the time at home for quick FS measurements because I already own a receiver and a sound card.
"Even with AGC off, why should I assume a commercial amp is linear? "
HF receivers will be quite linear up until some point, because if they weren't they wouldn't make good receivers in the face of a band full of strong signals. You'd end up with extra unwanted mixing products all over the place. (this does eventually happen, sooner with cheap or poorly designed receivers)
Still, you're not going to want to assume anything. A bench calibration over many decades of dB would be an important part.
Of course if you have a very log-linear measurement system to start with, calibration is essentially just verification and establishing your error bars.
I wouldn't even look at CB or walkie talkie type systems. Too much work to make them do what you want.
I think the log amps are probably the way to go. If you want to stay simple, a log amp and a voltmeter would be simple.
If you're who I think you are, your library has QST. The June 2001 issue has an article called "Simple RF Power Measurement" by W7ZOI and W7PUA that has a very quick and easy AD8307 log amp circuit with analog meter.
If you don't want to get fancy that might be the ticket.
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RE: Surface waves in the desert: the X experiment
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by EXPERIMENTX on November 5, 2009
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Reply to Dan N3OX: Thanks! Square loop is a great idea. Calculation by VK1OD looks ok. Not that it matters. I'm reading about the log amp. 100 dB looks like overkill. Device seems to be superb. Project described in 2001 QST also looks very good. Option of receiver/AGC-off might be a good unfussy consistency check. You're an expert -THANKS JPX
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RE: Surface waves in the desert: the X experiment
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by N4OGW on November 5, 2009
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>Long ago physicists and engineers diverged on "surface waves". Sommerfeld, Tesla, Heaviside, Norton contended and disagreed.
Please cite published source documenting there is such a "disagreement" within electromagnetic theory.
Tor
N4OGW
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