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From: Wimpie on 24 Nov 2009 12:36
On 24 nov, 14:53, tone <ax...(a)btinternet.com> wrote:
> I am looking to build a simple RF detector (to prove a microwave dish is
> transmitting) working in the 18Ghz region. Any offers especially of the
> diode to use.
>
> Thanks

Hello,

With a microwave diode you should be able to detect levels of -30 dBm
(or even significantly lower with good match).

With a 3 dBi antenna this converts to a plane wave power of about 30mW/
m^2 (for example transmitter with 1.5W EIRP at 2m distance).

For this purpose you should use real schottky diodes intended for SHF.
That are the ones with reverse voltages of about 4V or less and
capacitances of 0.25pF or less. An example is BAT15 (smd, not beam
lead). If you can get special mmW diodes (capacitances around 0.1 pF
or less), and tune them, you will get more output voltage with the
same incident Power Flux Density.

To get all EMF generated by the junction, the load should be larger
then the so-called "video resistance" (factor 10 gives you 90% of
EMF). Diodes with relative high forward voltage have high video
resistance and need a buffer (for example op-amp with CMOS input
stage). By doing this, you can use the diode without bias avoiding
thermal issues.

Best regards,

Wim
PA3DJS
www.tetech.nl
remove abc in case of PM
From: tone on 24 Nov 2009 15:50
tone wrote:
> I am looking to build a simple RF detector (to prove a microwave dish is
> transmitting) working in the 18Ghz region. Any offers especially of the
> diode to use.
>
> Thanks


Thanks for all the FB. I am starting to think it's more trouble than
it's worth. I was thinking of a small unit (0.6" aerial, diode and meter
- maybe even a cap) which when in the vacinity ~2m would detect the rf
from the dish (0.3-0.6m dish) - certianly not in front.
From: Joerg on 24 Nov 2009 16:18
John Larkin wrote:
> On Tue, 24 Nov 2009 08:30:14 -0800 (PST), osr(a)uakron.edu wrote:
>
>> John, at 18 on FR4?
>>
>> You sure?
>>
>> Steve
>
> Why not? Things scale. People use dipoles+schottkies at way higher
> frequencies than this. The only reason to use FR4 is because it would
> be an easy way to solder that _really_tiny_ part down. It would
> probably be easier to use real wires for the antenna itself, because
> they'd be easy to trim to length to tune the thing. With enough
> transmit power, it wouldn't need tuning at all.
>

Really tiny? 0.060" length is a mid-size boulder :-)


> A patch antenna would be interesting. Googling that would give lots of
> hits. You could hack one on copperclad with an x-acto knife.
>
> That Skyworks diode has Xc around 50 ohms at 20 GHz, a little high but
> not lethal. Aeroflex has an 0402 schottky that's only 0.08 pF, which
> would be even better.
>
> The problem isn't quantified, so we don't know how much signal we'd
> have. Close to a transmitting dish, I'd assume a bunch.
>
> It's surprising what fast stuff you can do with tiny surface-mount
> parts on pc boards, things that used to need plumbing.
>

That is true. Except the stuff inside the sampler heads you showed us,
that probably won't fly with SMT packages.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: John Larkin on 24 Nov 2009 16:35
On Tue, 24 Nov 2009 13:18:06 -0800, Joerg <invalid(a)invalid.invalid>
wrote:

>John Larkin wrote:
>> On Tue, 24 Nov 2009 08:30:14 -0800 (PST), osr(a)uakron.edu wrote:
>>
>>> John, at 18 on FR4?
>>>
>>> You sure?
>>>
>>> Steve
>>
>> Why not? Things scale. People use dipoles+schottkies at way higher
>> frequencies than this. The only reason to use FR4 is because it would
>> be an easy way to solder that _really_tiny_ part down. It would
>> probably be easier to use real wires for the antenna itself, because
>> they'd be easy to trim to length to tune the thing. With enough
>> transmit power, it wouldn't need tuning at all.
>>
>
>Really tiny? 0.060" length is a mid-size boulder :-)

It's really a pain to solder. But I'm thankful it's not a beam lead.

>
>
>> A patch antenna would be interesting. Googling that would give lots of
>> hits. You could hack one on copperclad with an x-acto knife.
>>
>> That Skyworks diode has Xc around 50 ohms at 20 GHz, a little high but
>> not lethal. Aeroflex has an 0402 schottky that's only 0.08 pF, which
>> would be even better.
>>
>> The problem isn't quantified, so we don't know how much signal we'd
>> have. Close to a transmitting dish, I'd assume a bunch.
>>
>> It's surprising what fast stuff you can do with tiny surface-mount
>> parts on pc boards, things that used to need plumbing.
>>
>
>That is true. Except the stuff inside the sampler heads you showed us,
>that probably won't fly with SMT packages.

Agoston Agoston showed me a 20 GHz sampler he did, surface-mount parts
on FR4. A lot of stuff that used to be hybrids can be done now surface
mount, too.

Oh, here's that other detector diode:

http://www.aeroflex.com/AMS/Metelics/pdfiles/SMS201.pdf

Cute little devil.

John



From: a7yvm109gf5d1 on 24 Nov 2009 19:04
On Nov 24, 8:53 am, tone <ax...(a)btinternet.com> wrote:
> I am looking to build a simple RF detector (to prove a microwave dish is
> transmitting) working in the 18Ghz region. Any offers especially of the
> diode to use.
>
> Thanks

Why not ask the intended receiver's RX power level?
Or a bolometer?
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