Topics

Microwave oven test


G8TZJ
 

I have heard that to test if a plastic is suitable for a microwave project, you put it in a microwave oven. I'm making a helix for 2.4 GHz and it will need some supports. So my questions are;
How big a piece do I put in the oven to test?
How long do I run the microwave oven for?
Its an 800W oven.
I know you have to put a cup of water in as well.

Any suggestions?

73 Andrew G8TZJ


Colin Ranson
 

Only a few seconds...poss 10 ?

 

Don’t forget the matching section on the first quarter turn of the helix.

 

 

I use one on the uplink to QO-100.

 

Regards

 

 

Colin. De G8LBS

 

Sent from Mail for Windows 10

 

From: G8TZJ via groups.io
Sent: 23 September 2020 12:32
To: UKMicrowaves@groups.io
Subject: [UKMicrowaves] Microwave oven test

 

I have heard that to test if a plastic is suitable for a microwave project, you put it in a microwave oven. I'm making a helix for 2.4 GHz and it will need some supports. So my questions are;
How big a piece do I put in the oven to test?
How long do I run the microwave oven for?
Its an 800W oven.
I know you have to put a cup of water in as well.

Any suggestions?

73 Andrew G8TZJ

 


Neil Smith G4DBN
 

If you do it with chocolate and don't rotate the turntable, you get a pattern of melting.  I'd suggest something over a half-wave at 2.6 GHz and support it so there is air around it, to prevent conductive heat loss. The cup of water will boil after a minute or so, and the steam will make your plastic hot, so 30-40 seconds is probably a practical limit.

If it's going to be outside, avoid nylon as it soaks up water. ABS, acrylics and polycarb are sort-of OK, PTFE/HDPE/PU/polystyrene are very good. Acetal/POM and PEEK are less than brilliant, but great mechanically. Small bore tubing is good for stiffness and light weight, or strips of Styrene modelmakers sheet (actually polystyrene) drilled and then thread two or three on to the helix and fix with polystyene cement.

If it is going inside a radome, then just use closed-cell polystyrene foam blocks as the support

Neil G4DBN

On 23/09/2020 12:32, G8TZJ via groups.io wrote:
I have heard that to test if a plastic is suitable for a microwave project, you put it in a microwave oven. I'm making a helix for 2.4 GHz and it will need some supports. So my questions are;
How big a piece do I put in the oven to test?
How long do I run the microwave oven for?
Its an 800W oven.
I know you have to put a cup of water in as well.

Any suggestions?

73 Andrew G8TZJ
-- 
Neil
http://g4dbn.uk


Clint Sharp <cjaysharp@...>
 

A glass tray or plate of thinly spread egg whites is a good way to test radiation pattern in a microwave because they turn white with a little energy


On Wed, 23 Sep 2020, 13:31 Neil Smith G4DBN, <neil@...> wrote:

If you do it with chocolate and don't rotate the turntable, you get a pattern of melting.  I'd suggest something over a half-wave at 2.6 GHz and support it so there is air around it, to prevent conductive heat loss. The cup of water will boil after a minute or so, and the steam will make your plastic hot, so 30-40 seconds is probably a practical limit.

If it's going to be outside, avoid nylon as it soaks up water. ABS, acrylics and polycarb are sort-of OK, PTFE/HDPE/PU/polystyrene are very good. Acetal/POM and PEEK are less than brilliant, but great mechanically. Small bore tubing is good for stiffness and light weight, or strips of Styrene modelmakers sheet (actually polystyrene) drilled and then thread two or three on to the helix and fix with polystyene cement.

If it is going inside a radome, then just use closed-cell polystyrene foam blocks as the support

Neil G4DBN

On 23/09/2020 12:32, G8TZJ via groups.io wrote:
I have heard that to test if a plastic is suitable for a microwave project, you put it in a microwave oven. I'm making a helix for 2.4 GHz and it will need some supports. So my questions are;
How big a piece do I put in the oven to test?
How long do I run the microwave oven for?
Its an 800W oven.
I know you have to put a cup of water in as well.

Any suggestions?

73 Andrew G8TZJ
-- 
Neil
http://g4dbn.uk


Andy G4JNT
 

And while we're on the subject of microwave ovens, I stumbled on this simple demonstration of the effectiveness of silver plating.
Put both a stainless steel and EPNS piece of cutlery into the uWave oven (along with the obligatory glass of water) and power on for 30 seconds or so.

The stainless steel cutlery will end up hot, possibly even too hot to touch, while the Electroplated Silver will remain cold

I discovered this quite by accident when making my regular deluxe Hot Choc in the evening which involves first melting chocolate buttons in a small amount of milk before topping-up.    Burnt my finger on the S-Steel teaspoon, whereas the EPNS one stays cold.
Fully recommend https://www.chococo.co.uk/  BTW :-)




On Wed, 23 Sep 2020 at 14:25, Clint Sharp <cjaysharp@...> wrote:
A glass tray or plate of thinly spread egg whites is a good way to test radiation pattern in a microwave because they turn white with a little energy

On Wed, 23 Sep 2020, 13:31 Neil Smith G4DBN, <neil@...> wrote:

If you do it with chocolate and don't rotate the turntable, you get a pattern of melting.  I'd suggest something over a half-wave at 2.6 GHz and support it so there is air around it, to prevent conductive heat loss. The cup of water will boil after a minute or so, and the steam will make your plastic hot, so 30-40 seconds is probably a practical limit.

If it's going to be outside, avoid nylon as it soaks up water. ABS, acrylics and polycarb are sort-of OK, PTFE/HDPE/PU/polystyrene are very good. Acetal/POM and PEEK are less than brilliant, but great mechanically. Small bore tubing is good for stiffness and light weight, or strips of Styrene modelmakers sheet (actually polystyrene) drilled and then thread two or three on to the helix and fix with polystyene cement.

If it is going inside a radome, then just use closed-cell polystyrene foam blocks as the support

Neil G4DBN

On 23/09/2020 12:32, G8TZJ via groups.io wrote:
I have heard that to test if a plastic is suitable for a microwave project, you put it in a microwave oven. I'm making a helix for 2.4 GHz and it will need some supports. So my questions are;
How big a piece do I put in the oven to test?
How long do I run the microwave oven for?
Its an 800W oven.
I know you have to put a cup of water in as well.

Any suggestions?

73 Andrew G8TZJ
-- 
Neil
http://g4dbn.uk


Colin Ranson
 

Out of interest – my microwave oven does not have a turntable. The antenna(s) are under the floor. Somehow they are electronically steered ?

 

I found it very weird first few times I used it.

 

Regards

 

Colin de ‘LBS.

 

Sent from Mail for Windows 10

 

From: Andy G4JNT
Sent: 23 September 2020 14:35
To: UK Microwaves groups.io
Subject: Re: [UKMicrowaves] Microwave oven test

 

And while we're on the subject of microwave ovens, I stumbled on this simple demonstration of the effectiveness of silver plating.

Put both a stainless steel and EPNS piece of cutlery into the uWave oven (along with the obligatory glass of water) and power on for 30 seconds or so.

 

The stainless steel cutlery will end up hot, possibly even too hot to touch, while the Electroplated Silver will remain cold

 

I discovered this quite by accident when making my regular deluxe Hot Choc in the evening which involves first melting chocolate buttons in a small amount of milk before topping-up.    Burnt my finger on the S-Steel teaspoon, whereas the EPNS one stays cold.

Fully recommend https://www.chococo.co.uk/  BTW :-)

 

 

Andy

 

 

 

On Wed, 23 Sep 2020 at 14:25, Clint Sharp <cjaysharp@...> wrote:

A glass tray or plate of thinly spread egg whites is a good way to test radiation pattern in a microwave because they turn white with a little energy

 

On Wed, 23 Sep 2020, 13:31 Neil Smith G4DBN, <neil@...> wrote:

If you do it with chocolate and don't rotate the turntable, you get a pattern of melting.  I'd suggest something over a half-wave at 2.6 GHz and support it so there is air around it, to prevent conductive heat loss. The cup of water will boil after a minute or so, and the steam will make your plastic hot, so 30-40 seconds is probably a practical limit.

If it's going to be outside, avoid nylon as it soaks up water. ABS, acrylics and polycarb are sort-of OK, PTFE/HDPE/PU/polystyrene are very good. Acetal/POM and PEEK are less than brilliant, but great mechanically. Small bore tubing is good for stiffness and light weight, or strips of Styrene modelmakers sheet (actually polystyrene) drilled and then thread two or three on to the helix and fix with polystyene cement.

If it is going inside a radome, then just use closed-cell polystyrene foam blocks as the support

Neil G4DBN

On 23/09/2020 12:32, G8TZJ via groups.io wrote:

I have heard that to test if a plastic is suitable for a microwave project, you put it in a microwave oven. I'm making a helix for 2.4 GHz and it will need some supports. So my questions are;
How big a piece do I put in the oven to test?
How long do I run the microwave oven for?
Its an 800W oven.
I know you have to put a cup of water in as well.

Any suggestions?

73 Andrew G8TZJ

-- 
Neil
http://g4dbn.uk

 


G8TZJ
 

 Neil Smith G4DBN

It is going to be outside and probably subject to rain. (I'm unsure about a covering, as I want to limit the wind loading). The plastic type is unknown. Its the off cuts from soffits and bargeboards etc. from when the roof was done some time back.

73 Andrew G8TZJ


Neil Smith G4DBN
 

It will probably be uPVC then, which is not a great dielectric, but for this application, and if the sections are thin, you should get away with it.  Losses vary, but the figures I've seen are something like a tan delta of 0.02 at 1GHz for uPVC, about the same as polycarbonate, compared with 0.0003 or so for polystyrene, polypropylene, PTFE and polystyrene.  Just because the loss tangent is 70 times as bad isn't necessarily serious, seventy times a tiny amount is still a tiny amount. You wouldn't use it in a tuned cavity or as the dielectric in a capacitor or a microwave lens though.

I've seen lemonade bottles used to cover helicals, but there is always a problem with condensation if it is too enclosed, so think umbrella rather than wetsuit. PET loss tangent is something around 0.008 at 1GHz, and being thin, any loss would be very small unless you are also running 10GHz through the bottom of the bottle and it is one of those with a heavily moulded base, where you might get some weirdness.

Do the oven test and see if you detect any warming. If the material has unusual fillers or plasticisers, it could be radically different from raw uPVC

Neil G4DBN

On 23/09/2020 21:48, G8TZJ via groups.io wrote:
 Neil Smith G4DBN

It is going to be outside and probably subject to rain. (I'm unsure about a covering, as I want to limit the wind loading). The plastic type is unknown. Its the off cuts from soffits and bargeboards etc. from when the roof was done some time back.

73 Andrew G8TZJ
-- 
Neil
http://g4dbn.uk


Colin Ranson
 

Hi,

 

I had in my possession  a 16 turn RHCP helix from SAS (Satelite Aerial Systems) for 2.4GHz. (Super Cannon) It is really meant for rx’ing  video downlinks but what the hell it can handle a few watts.   The helix support appears to be made of two PVC angle sections slotted together and ‘plastic welded’ A thick slot at the bottom enables a thick threaded PVC washer and stainless steel bolt to be used to pull the support up to the groundplane (PCB).

The helix connected directly to an RG36 tail with an SMA plug.  On my LNA it was broadly  resonant at 2.4GHz showing about 140 ohm.   For simplicity I replaced the RG316 with another bit that had a factory fitted N socket attached.  I scraped the insulation off the first quarter turn and soldered an oversized ‘arrowhead’ of thin copper shim to it.  I simply trimmed it until the SWR came down to 1.3 -1.  I could not get it any better.  For want of something better I sprayed the assembly with Pu lacquer.  I may later fit an N socket to the GP if the present arrangement does not survive the winter.

It works fine with about 15w into QO-100.  It is still only roughly pointed so I need to twiddle it and maybe reduce power.  I have a run of about 10ft of M & P Ultraflex 10.   I monitored the FWD/REF using the facility on the SG Labs amp and with a few minutes talk time nothing altered so I guess the PVC (or whatever) helix support is not heating ?

 

It all works so I am happy.

 

Best regards

 

Colin de G8LBS Ipswich.  - now doing a shack refurb so won’t be on any bands for a while, missed the latest lifts – bah !

 

 

Sent from Mail for Windows 10

 

From: Neil Smith G4DBN
Sent: 23 September 2020 23:29
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Microwave oven test

 

It will probably be uPVC then, which is not a great dielectric, but for this application, and if the sections are thin, you should get away with it.  Losses vary, but the figures I've seen are something like a tan delta of 0.02 at 1GHz for uPVC, about the same as polycarbonate, compared with 0.0003 or so for polystyrene, polypropylene, PTFE and polystyrene.  Just because the loss tangent is 70 times as bad isn't necessarily serious, seventy times a tiny amount is still a tiny amount. You wouldn't use it in a tuned cavity or as the dielectric in a capacitor or a microwave lens though.

I've seen lemonade bottles used to cover helicals, but there is always a problem with condensation if it is too enclosed, so think umbrella rather than wetsuit. PET loss tangent is something around 0.008 at 1GHz, and being thin, any loss would be very small unless you are also running 10GHz through the bottom of the bottle and it is one of those with a heavily moulded base, where you might get some weirdness.

Do the oven test and see if you detect any warming. If the material has unusual fillers or plasticisers, it could be radically different from raw uPVC

Neil G4DBN

On 23/09/2020 21:48, G8TZJ via groups.io wrote:

 Neil Smith G4DBN

It is going to be outside and probably subject to rain. (I'm unsure about a covering, as I want to limit the wind loading). The plastic type is unknown. Its the off cuts from soffits and bargeboards etc. from when the roof was done some time back.

73 Andrew G8TZJ

-- 
Neil
http://g4dbn.uk

 


G8TZJ
 

Thanks for the advice. The "roof" plastic survived the oven test, even for 1 min. However upon closer inspection, I decided against using it. The outer layers are a hard plastic but the inside (it's about 8 mm thick) seems to be some sort of dense foam. So I guess this could soak up water unless it is adequately sealed.
Instead, I found a length of PVC L shaped plastic, 20 x 30 x 3 mm at Homebase. It seems reasonably rigid and it was ok in the oven. I just have to secure the end of the L on the aluminium reflector, probably using a bit of the "roof" plastic as an anchor point.
The helix is for QO-100 to be mounted just above the LNB to replace my patch feed.

73 Andrew G8TZJ


Clint Sharp <cjaysharp@...>
 

I think I know the type of material, the fascia boards on my house are made of it, it's a sort of foamed plastic with a polished face. 

It doesn't soak up water but the rough faces and edges will hold surface water

On Thu, 24 Sep 2020, 17:56 G8TZJ via groups.io, <g8tzj=talktalk.net@groups.io> wrote:
Thanks for the advice. The "roof" plastic survived the oven test, even for 1 min. However upon closer inspection, I decided against using it. The outer layers are a hard plastic but the inside (it's about 8 mm thick) seems to be some sort of dense foam. So I guess this could soak up water unless it is adequately sealed.
Instead, I found a length of PVC L shaped plastic, 20 x 30 x 3 mm at Homebase. It seems reasonably rigid and it was ok in the oven. I just have to secure the end of the L on the aluminium reflector, probably using a bit of the "roof" plastic as an anchor point.
The helix is for QO-100 to be mounted just above the LNB to replace my patch feed.

73 Andrew G8TZJ


G8TZJ
 

Yes Clint, that's the material. At least I know it wont soak up water.

73 Andrew