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From: MikeF on 7 Jul 2010 15:01
Hello, I am working on a senior design project that harvests energy
from a 45watt UHF Ham Radio transmitter. Currently I am getting 3.4v
peak-peak at the receiving antenna. When I attempt to rectify the
signal into a DC voltage across a Schottky diode I am losing nearly
all of my voltage (it drops to around 500mv). I believe this may be to
the fact that my impedance going from the 50ohm antenna to my circuit
are not properly matched.

My question is, does anyone have any ideas or information on how to
better match the impedance from my antennas to my rectifying circuit?
Or any other ways of getting around this issue.
From: amdx on 7 Jul 2010 15:57
Are you measuring that 3.4v across a 50 ohm resistor
that is terminating the output of your antenna?
If you are, you have harvested almost 1/4 watt.
Hmm, UHF, what are you measuring your voltage with?
Post your schematic somewhere.
Mike



"MikeF" <mike.fallat(a)gmail.com> wrote in message
news:94b27059-5882-457c-8808-00ce2ca6c455(a)d37g2000yqm.googlegroups.com...
> Hello, I am working on a senior design project that harvests energy
> from a 45watt UHF Ham Radio transmitter. Currently I am getting 3.4v
> peak-peak at the receiving antenna. When I attempt to rectify the
> signal into a DC voltage across a Schottky diode I am losing nearly
> all of my voltage (it drops to around 500mv). I believe this may be to
> the fact that my impedance going from the 50ohm antenna to my circuit
> are not properly matched.
>
> My question is, does anyone have any ideas or information on how to
> better match the impedance from my antennas to my rectifying circuit?
> Or any other ways of getting around this issue.


From: Winston on 7 Jul 2010 16:26
On 7/7/2010 12:01 PM, MikeF wrote:
> Hello, I am working on a senior design project that harvests energy
> from a 45watt UHF Ham Radio transmitter. Currently I am getting 3.4v
> peak-peak at the receiving antenna. When I attempt to rectify the
> signal into a DC voltage across a Schottky diode

The diode is in *parallel* with the antenna?

> I am losing nearly
> all of my voltage (it drops to around 500mv). I believe this may be to
> the fact that my impedance going from the 50ohm antenna to my circuit
> are not properly matched.

> My question is, does anyone have any ideas or information on how to
> better match the impedance from my antennas to my rectifying circuit?
> Or any other ways of getting around this issue.

First, delete the parallel diode and use one series diode.
Right now, you appear to be shorting out every other half cycle.

--Winston
From: Joel Koltner on 7 Jul 2010 16:24
"MikeF" <mike.fallat(a)gmail.com> wrote in message
news:94b27059-5882-457c-8808-00ce2ca6c455(a)d37g2000yqm.googlegroups.com...
> Hello, I am working on a senior design project that harvests energy
> from a 45watt UHF Ham Radio transmitter.

Hopefully some of your project conclusions will include the fact that such
energy harvesting has rather limited utility in the real world due to the
*incredibly* low efficiency with which it can be performed.

But that doesn't mean you can't have a lot of fun while you're at it. :-)

> Currently I am getting 3.4v
> peak-peak at the receiving antenna. When I attempt to rectify the
> signal into a DC voltage across a Schottky diode I am losing nearly
> all of my voltage (it drops to around 500mv). I believe this may be to
> the fact that my impedance going from the 50ohm antenna to my circuit
> are not properly matched.

Are you measuring the 3.4V with some high-impedance device? (E.g., FET probe
on a scope or spectrum analyzer?) In that case you're correct that the
problem is that 50ohms is significantly loading the antenna, and if you're
only getting ~500mV you are losing a lot of power in your Schottky diodes.

> My question is, does anyone have any ideas or information on how to
> better match the impedance from my antennas to my rectifying circuit?

As Mike suggested, please post a (link to a) schematic.

In the meanwhile, you might be encouraged to know that that are various
companies that manage to stay in business producing energy harvesting
receivers. Check out these guys: http://www.powercastco.com/ -- one of the
founders was doing similar experiments to what you are now (albeit at 915MHz)
back in 2004 and managed to make a master's level thesis out of it (which
truly amazes me, but hey, more power to him):
http://etd.library.pitt.edu/ETD/available/etd-07212004-192328/unrestricted/Harrist_Thesis_072804.pdf
..

Jim Thompson: If you're reading this, check out that thesis and tell me how
many chortles you get out of it -- especially the notes about SPICE around
pages 24/25: "This software is too limited for this project." Um hmm...!

---Joel

From: Jim Thompson on 7 Jul 2010 17:14
On Wed, 7 Jul 2010 13:24:19 -0700, "Joel Koltner"
<zapwireDASHgroups(a)yahoo.com> wrote:

>"MikeF" <mike.fallat(a)gmail.com> wrote in message
>news:94b27059-5882-457c-8808-00ce2ca6c455(a)d37g2000yqm.googlegroups.com...
>> Hello, I am working on a senior design project that harvests energy
>> from a 45watt UHF Ham Radio transmitter.
>
>Hopefully some of your project conclusions will include the fact that such
>energy harvesting has rather limited utility in the real world due to the
>*incredibly* low efficiency with which it can be performed.
>
>But that doesn't mean you can't have a lot of fun while you're at it. :-)
>
[snip]
>
>In the meanwhile, you might be encouraged to know that that are various
>companies that manage to stay in business producing energy harvesting
>receivers. Check out these guys: http://www.powercastco.com/ -- one of the
>founders was doing similar experiments to what you are now (albeit at 915MHz)
>back in 2004 and managed to make a master's level thesis out of it (which
>truly amazes me, but hey, more power to him):
>http://etd.library.pitt.edu/ETD/available/etd-07212004-192328/unrestricted/Harrist_Thesis_072804.pdf
>.
>
>Jim Thompson: If you're reading this, check out that thesis and tell me how
>many chortles you get out of it -- especially the notes about SPICE around
>pages 24/25: "This software is too limited for this project." Um hmm...!
>
>---Joel

Almost didn't. It's the sort of title that normally gets deleted
automatically. I just happened to see my name :-)

His "Master's" circuit actually bears some resemblance to circuits
I've used in RFID tag chips... Schottky's with CMOS devices used as
capacitors... but with active behavior (shunt regulation)... powered
by a STRONG RF reader maybe 30' away MAX ;-)

PSpice simulates it just fine.

I guess I know what university to NOT recommend :-(

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
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| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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Obama: A reincarnation of Nixon, narcissistically posing in
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