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LAN_403

Our Finest Hour

Sat Jan 03 2009, 09:17AM

Registered Member #1905 Joined: Sat Jan 03 2009, 08:35AM
Location:
Posts: 8

Hi,
My 1st thread on 4HV. Happy New year to everyone'

I am carefully conducting some experiments with a 1Kw magnetron (2.45Ghz)and Horn/parabolic reflectors.
What is the theoretical attenuation over distance (based on a 15dB horn or Dish)

Also if 2 wave guides (as above specs) were directed onto the same dish for transmission would there be interference waves and could these be easily overcome?

thanks

Proud Mary

Sat Jan 03 2009, 10:56AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hello and Happy New Year!

Microwave radiation is attenuated by atmospheric water vapour, and oxygen, and is scattered and absorbed by rainfall, and airborne particulates such as smog and factory smoke. .

Computer modeling with all these variables (and more!) would be needed to produce the "theoretical attenuation" you ask for.

But I will guess that you are really asking, how far will it go?

As far as the horizon viewed from your antenna, and just a little bit further. Exceptional refraction may very occasionally bend your signal round the horizon, extending the range considerably, and you might also extend your range once in a blue moon by tropospheric scatter.

The presence of buildings, trees, pylons, and so on along the signal path are likely to attenuate and reflect some or even all of the signal for practical purposes.

With microwaves of this order, increasing the power will not bring about any significant increase in range, though it will improve system reliability in adverse conditions. A competent amateur with a decent antenna would be able to get their signal to a receiver on the optical horizon with 5 or 10 milliwatts or so, so there's nothing to be gained by the use of such profligate power as you propose unless one is looking for radar returns and so on.

1kW of misdirected microwave energy can and will cause serious harm to yourself, others, and to birds and wildlife. Over short distances, sensitive electronic circuits absorbing the signal may be damaged or destroyed.

In the United Kingdom, it would be an offence to conduct such experiments without a licence.

Take care!

Dave Marshall

Sat Jan 03 2009, 08:20PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

As Harry stated, microwave propagation is a fickle creature, and can be influenced by hundreds of variables. Not only will precipitation, fog, clouds, flocks of birds, trees, and blowing grass all cause attenuation to the received signal, they can also create limited passive amplification through reflection.

A receiver in a place that probably shouldn't receive the signal (behind a big building, for instance) can sometimes hear just fine due to reflection from a passing rain storm, or a conveniently located hillside or building. Wavelengths as long as 70cm (~440MHz) are quite prone to such effects.

As a ham, I feel obligated to remind you that in virtually every ITU member country, you're engaging in an illegal activity unless you have an appropriate license, and are keeping to the appropriate frequencies (tough to do with a magnetron). Folks in your neighborhood with wifi are probably none to pleased when you power up, and any hams running microwave equipment in your area are probably similarly displeased. You should do your very best to assure your operations are limited in time frame, and potential for interference.

If you're in the US, the FCC is quick to act. They get up to $10,000 for each intentional instance of interference, after all.

-Dave

Our Finest Hour

Sat Jan 03 2009, 10:42PM

Registered Member #1905 Joined: Sat Jan 03 2009, 08:35AM
Location:
Posts: 8

Thanks Dave and Harry.
The test to date are very contained (inside meshed off faraday cage/room and aimed mainly at achieving the best antenna dimensions (likey to be a Dish with an offset waveguide feed)
The use of 2.450Ghz at 1000w is to suit the material we are trying to heat for the experiment.
Can anyone point to a simple rule of thumb for losses through air, I think it will be much less a factor than horn/dish design but is still "a box" I would like to have ticked. (same with interference ways)

Thanks

Proud Mary

Sun Jan 04 2009, 09:54AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It boggles the mind to visualize the effect of unleashing a 1kW magnetron inside a "meshed off Faraday Cage/room" - but I guess it would be somewhere between a microwave echo chamber and a microwave oven. Additive reflections in some places would multiply the absorbed power, while in other places there might be none at all. Sections of the mesh might well glow red and fuse, while coupling between the magnetron and the mesh could well destroy the magnetron by impedance mismatch. Anyone unwise enough to enter the room would risk serious harm, including blindness and sterility.

Perhaps it would be a good idea to learn more about the interesting and often curious properties of microwaves by investing in a small Gunn diode and some old pieces of waveguide, before embarking on projects with so many possibilities for grave misadventure.

Our Finest Hour

Sun Jan 04 2009, 10:51AM

Registered Member #1905 Joined: Sat Jan 03 2009, 08:35AM
Location:
Posts: 8

!amazed
Hi Harry I started to read your last and I nearly choked and wet myself/whilst laughing, all at the same time!!!
The vision you describe was something out of Frankensteins monster, huge capacitors and tessla coils arcing in between lightning flashes!!

The large bank of water barrels in the room absorbs most if not all the output that gets past the Meter. No arcing or glowing bits anywhere honest!! No one is in the room during the short 1 or 2 second bursts (they are either at the fertlity clinic or undergoing cataract surgery - just kidding)

Hope its not too cold in Brighton. Thanks for the concern:-)

Proud Mary

Sun Jan 04 2009, 12:38PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

My concern is that people who may not be experienced in microwave safety procedures may bring themselves or others to some harm by taking the magnetrons from microwave ovens and playing with them.

You appear not to need this kind of advice, so I will refer you to the very many academic papers on atmospheric microwave attenuation which you'll find on the internet, and wish you well with your experiments.

rp181

Sun Jan 04 2009, 04:09PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

You seem to take the precautions =)
Look at the bottom of the page for a graph:

Hon1nbo

Sun Jan 04 2009, 05:10PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

Harry wrote ...

My concern is that people who may not be experienced in microwave safety procedures may bring themselves or others to some harm by taking the magnetrons from microwave ovens and playing with them.

You appear not to need this kind of advice, so I will refer you to the very many academic papers on atmospheric microwave attenuation which you'll find on the internet, and wish you well with your experiments.

yeah, look at the Mythbusters when trying to beat the speed radars... idiots, she tried to test the magnetron (strait out of the microwave) in the shop, without any noticiable expience of what to do with it... then there are the idiots who do the stunt of "popping popcorn with a cell phone" and place the magnetron facing up from under the table, then they stand over it!

BTW, the ARRL makes a GREAT microwave experimentation book, I got a copy at Fry's for $4.90 (even though it normally is around $20)
it has all of the details you would need to know, and it is a nice, easy to use paper back...

Proud Mary

Sun Jan 04 2009, 06:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I cannot speak for other nations or jurisdictions, but in Britain today the government seems ever minded to 'regulate' or pass laws against any kind of perceived risk trumped up by the tabloid press.

One only has to think of the very recent regulation of green laser pointers following a tiny number of incidents of reckless misuse.

If our own hobby is to survive 'unregulated,' then it is up to us to promote professional standards of safety, and, with microwaves, to ensure that there are no emissions exceeding the legal limits that might interfere with the lives, interests, and rights of other people to enjoy their own lives in safety, peace and quiet - wifi users for example.

Microwave leakage emissions from ovens are the subject of exhaustive government studies, and emissions exceeding 5mW/cm2 are illegal in the United Kingdom. The misuse of oven magnetrons can only lead to more laws and regulation that will continue to limit and impact us all, long after the reckless misadventurers have moved on to some other interest.

Stern stuff, I know, but it must be said.

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