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From: Tim Wescott on 7 Jul 2010 17:31
On 07/07/2010 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 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.

Post a schematic?

How do you know the antenna is 50 ohms? What sort of antenna is it?

Why don't you start by matching impedances, then? Better, since the
Schottkey diode is nonlinear, why don't you try various load resistances
experimentally until you find the one that gives the most power out of
the antenna.

Because you're going to lose a bunch of voltage at the diode, you may
want to use an antenna with a higher impedance -- look up "folded
dipole". Or consider a step-up transformer into the diode.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: amdx on 7 Jul 2010 20:02
You might look into some of the high end crystal radios.
just replace the crystal with your diode and the earphones with your load.
Here's so efficiency improvements,
http://www.bentongue.com/xtalset/xtalset.html
But I think this is starting at the end rather than the beginning. Ben did
some good studies!

--
MikeK


"amdx" <amdx(a)knology.net> wrote in message
news:6ebdc$4c34dc4e$45013905$1748(a)KNOLOGY.NET...
> 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: m II on 7 Jul 2010 20:16
amdx wrote:

> You might look into some of the high end crystal radios.
> just replace the crystal with your diode and the earphones with your load.
> Here's so efficiency improvements,
> http://www.bentongue.com/xtalset/xtalset.html
> But I think this is starting at the end rather than the beginning. Ben did
> some good studies!


I was impressed with the number of crystal radio sites out there.

Just two, of the dozens....

http://www.crystalradio.net
http://www.crystalradio.us



The workmanship on a lot of these is excellent:

http://www.crystalradio.net/contest/jim.html





mike
From: Paul Keinanen on 8 Jul 2010 03:07
On Wed, 07 Jul 2010 13:26:07 -0700, Winston <Winston(a)bigbrother.net>
wrote:

>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?

A simple RF probe typically consists of an input capacitor connected
to a diode cathode (anode grounded) feeding a mA or uA meter. Some RC
filtering between the diode and the meter may help in preventing
spurious radiation from the meter leads.

From: Robert Baer on 8 Jul 2010 04:09
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 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.
Try a transformer: hi Z input (from antenna) to lo Z output (load).
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