R: [amsat-bb] Alternatives to fiberglass boom

["i8cvs" <domenico.i8cvs@xxxxxx>]

----- Original Message -----
From: John Wright <g4dmf@g4dmf.co.uk>
To: Tom Clark (W3IWI) <w3iwi@toad.net>; Amsat Bulletin Board
<amsat-bb@AMSAT.Org>
Sent: Sunday, February 10, 2002 12:54 AM
Subject: RE: [amsat-bb] Alternatives to fiberglass boom


> At 12:49 07/02/2002 -0500, you wrote:
> >Wayne -- a quick way to test any material is to put a small piece in your
> >Microwave Oven (running at full power). Heat it for 10-15 seconds. If no
> >spark/flames are noted and the piece is not incredibly hot, then run the
> >microwave for 60 seconds. If the piece is still at room temperature, it
has
> >excellent microwave properties. If it is starting to get warm, it's
probably
> >OK, If you can't pick it up, or if flames have been noted, forget it !!!
> >
> >If it made it OK to 1 minute, then you might want to give it a try at 2-3
> >minutes just to make sure that the initial "good" diagnosis holds.
>
>
> As a domestic electronics engineer, I am somewhat alarmed at this reply.
If the material is good at 2Ghz, then you are running your 1Kw microwave
transmitter ( your microwave oven ) into a VERY high SWR, certainly running
it for 1 minute could irreparably damage the magnetron. It is VITAL that any
tests of this type are conducted with at least a beaker containing 250ml or
so of water accompanying the piece of dielectric to be tested.
>
> Incidentally, if you take 1 litre of water, measure its temperature, heat
at full power for 87 seconds, measure the temp again, then multiply the temp
difference by 50, this will give a close figure to the number of watts
generated by your microwave. My quick way of seeing if a microwave is
working at rated power.
>
> Running your microwave without a load is like running your HF rig into an
open circuit!, and microwaves don't have SWR protection!
>
>
> ____________________________
>
>      G4DMF  QTHR   IO93ha
>      Amsat-UK Member 5372
>          g4dmf@amsat.org
>        g4dmf@g4dmf.co.uk
>      http://www.g4dmf.co.uk
> ____________________________

Hi John,

I have in my hand the Data Book PHILIPS "C.W. Magnetrons for
Microwave Heating Applications .

The type DX206 for 2425-2475 MHz and Output Power Wo=1,2 KW
allow the following Limiting Values for  VSWR

Continuous: VSWR     max  4

Intermittent  ( Time = max 0,02 s  for max 20% of the time  ) :  VSWR max 10

The average reflected power for any one second period must not exeed
the reflected power equivalent to a VSWR of  4
When operating under these conditions,the tube should'not not permitted
to mode.

Obviously,running the MW fornace without load in it is very risky for the
magnetron because of  the very high VSWR and so i aegree with you
to fill in to the MW the mininum weigth of  water recommended by the
matufacturer togheter with the insulating material to be tested for our
purposes.

The magnetron output power can be measured considering  that the temperature
of one liter of water increases by 14,3 °C per minute if the heating output
power is 1 KW

1) Put in to the MW fornace a very thick  continer suitable for MW food
    applications about 10 cm in diameter and fill 1 liter of water in it at
    room temperature.
    ( I use an expanded polystirol continer originally used for ice cream)

2) Measure the temperature of this water and call it T1

3) Run the MW continuously at the max power for exactly one minute
    and  take the time beginning from the moment in wich the transformer
    start to buzz

4) Measure again the water temperature after one minute of  heating and
    call it T2

The output power in KW is given by the following formula:

Wo =( T2 - T1) / 14,3    [  KW]

As an example my domestic PHILIPS AVM 730 MW fornace last measurement
was:

T1 = 33°C
T2 = 40,5°C

Wo= ( 40,5 - 33 ) / 14,3  = 0,524 KW

In this conditions 1 liter of water represents a load matched to the
magnetron with a VSWR  around 2,5

For a good power determination it should be necessary to repeate the
procedure at different  T1 conditions and take the power average from it.

73" de i8CVS Domenico


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