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homebrewPosted - 19 June 2013 7:53  Show Profile  Email Poster  Edit Message  
So if a single diode pulls all the available power from the antenna, we'd need a grounded center tap coil hooked to two antennas at each end to effectively use a bridge rectifier? That might work on a compact loopstick set.
gzimmerPosted - 19 June 2013 11:24  Show Profile  Email Poster  Edit Message
Trouble is that if the antennas are close they will couple to each other and act as one antenna.

To get more signal you need more capture area. A phased array would work, or a parabolic reflector.

A theoretical example is the "RecTenna" as proposed for Wireless Power transmission.

http://en.wikipedia.org/wiki/Rectenna

Lots of widely spaced dipoles, each feeding a separate diode...

.............. Zim

.

_J_Posted - 19 June 2013 13:5  Show Profile  Email Poster  Edit Message
A BIG parabola!

Instead of looking for a way to use a bridge detector or multiple diodes (by going to 2 antennas), it is better to use the 2 antennas with one diode. There is no way to avoid multiple diode losses.

That said, on the kind of strong signals we want for speakers, the diode loss issues are less severe (Square Law Mode).

John Davidson

homebrewPosted - 19 June 2013 15:43  Show Profile  Email Poster  Edit Message  
Thanks, the rectenna is something new to me.
I would think a couple feet separation on wire antennas would be enough. Or in my case the attic and outside longwire which are at nearly right angles to each other.
I would think the fields on typical xistor set ferrite antennas would have fields close enough to not require a large amount of separation if mounted parallel to each other.
Changing from a bridge to a single diode for each antena (two total) would be easy enough.

Edited by - homebrew on 6/19/2013 3:57:03 PM

gzimmerPosted - 20 June 2013 1:22  Show Profile  Email Poster  Edit Message
> I would think a couple feet separation on wire antennas would be enough

A good article on stacking antennas is at: http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm

But this is for yagis with phased feed-lines, not dipoles with individual diodes.

Still thinking.


....... Zim

Edited by - gzimmer on 6/20/2013 1:46:48 AM

gzimmerPosted - 20 June 2013 1:28  Show Profile  Email Poster  Edit Message
John,

Thinking about using individual diodes v/s one common diode in the Rectenna.

One difference with individual diodes is that the phasing of the feed-lines no longer matters. It's all DC (and Audio), no matter what the spacing.

With a Phased-Array the spacing is quite critical to achieve the directivity. But with a Rectenna, perhaps the capture areas just merge together with no increase in directivity?

Do the diodes damp the dipoles sufficiently so they don't interact (eg re-radiate)?

Will have to do some reading.

Perhaps there's a special case here for Crystal sets and AM. Perhaps a Rectenna behaves quite differently to a phased array?

Perhaps homebrew is correct.


............ Zim


Edited by - gzimmer on 6/21/2013 1:57:35 AM

gzimmerPosted - 20 June 2013 3:46  Show Profile  Email Poster  Edit Message
Interesting article comparing DC v/s RF combining in an Rectenna (eg single diode v/s multiple diodes):

http://www2.ece.ohio-state.edu/~olgunu/rectenna.pdf

It supports the notion that a Rectenna has a broader pattern than a phased array (eg the combined Rectenna has the same pattern as a single dipole).

It shows that individual diodes are more efficient than a single diode (off axis), but this is with quite high field strengths.

They used 1 Watt at 2.5 Meters, so a Crystal set equivalent would be 10 KW at 25 KM ?

It does illustrate how loudspeaker crystal sets (eg with strong signals) are a special case.

.......... Zim

Edited by - gzimmer on 6/20/2013 3:55:08 AM

_J_Posted - 21 June 2013 1:41  Show Profile  Email Poster  Edit Message
I guess we could model it to be sure.

My perspective is that we are taking power from the waves, not from the antenna. The antenna is just in the path, a medium. The waves have a fixed amount of power. Unless we have a scheme for getting more power from waves with a multi-element antenna then it will equal a single element antenna, diodes not withstanding. I don't see how putting 2 wires in the same part of the wave does that. Will a 7 strand antenna wire pick up any less power than a 21 strand one if we connect each strand to a separate diode?

John Davidson

gzimmerPosted - 21 June 2013 1:56  Show Profile  Email Poster  Edit Message
I think you are right John, two close spaced wires (with diodes) will have essentially the same capture area as one wire.

But as you spread them out, the capture area also spreads out.

Quite different to how a phased array works, but it makes sense now that I think about it.


.......... Zim


Edited by - gzimmer on 6/21/2013 1:59:03 AM

gsparksPosted - 21 June 2013 14:7  Show Profile  Email Poster  Edit Message
I played with many of the ideas and really no major difference. I have a pretty good pile of diodes I used for different rectifier configurations with no noticeable difference with baldwin headphones. What did work somewhat and may be worth trying is realizing Broadcast band was typically vertical polarized. when I was climbing ladders and Put my TV antenna up I made a loop 3' wide and 30" long, hung from TV pole, then I put 2 8" ground rods (1/2" copper tube. and #14 stranded feed wires. I recently found a telephone headst and using that with my old double tuned set with #16 wire coils, and a cheerleader megaphone was easily heard on all high powered stations in houston. My excuse for why I did this was for hurricane season. Antenna ground seemed to make most difference, although the antenna won't survive hurricane winds. and my GE superradio with fresh batteries will be preferred anyway.
Nice conversation and experments though
Sparky
_J_Posted - 22 June 2013 1:28  Show Profile  Email Poster  Edit Message
If you get the antennas far enough apart it will work. If they are real far apart you might get all 3 dB. Will that be noticeable?

John Davidson

golfguruPosted - 22 June 2013 3:50  Show Profile  Email Poster  Edit Message  
>>>>> If you get the antennas far enough apart it will work. If they are real far apart you might get all 3 dB. Will that be noticeable? >>>>

In my situation I found that about 14uW was the minimum power level where I could claim "listenable loudspeaker volume".

If I could double a received power level of 7uW to 14uW, making the transmission "listenable", then I believe it would be noticeable.

I think that the "3dB is not noticeable" is another myth. Try listening (using headphones) with one earphone, while the other ear is covered, versus listening with both earphones.
It would be just the same as when listening to transmissions on the threshold of headphone reception. Add another 3dB and the transmission might be "copyable" v. "not copyable".

.................

Garry NicholsPosted - 22 June 2013 12:56  Show Profile  Email Poster  Edit Message
golf',

I can't help but wonder whether your one ear vs two ear scenario is confounded by altering the efficiency of human neural processing of sound.

I think that it would be best to use either one ear or both ears for the A/B comparison, altering only the power to the earphone(s).

I have always felt that listening with two of my ears gave a decided increase in detectability over one ear. But I have never conducted a blind experiment to see if this was actually the case. I'm also not sure how to control the power levels for comparison.

I just use both ears, considering that both still work fairly well. But I'm starting to hear my wife say really strange and outlandish things, occasionally, especially when there is an echo in the room or background noise.

Garry

homebrewPosted - 22 June 2013 13:51  Show Profile  Email Poster  Edit Message  
If you put on a set of headphones with one bad side you will "hear" the sounds in both ears. Hooking two sets to different earpieces is a fun experiment when you tune each to a different station.
The early headphones had spacers for the diaphragms, thick for code and thin for music. A lot of hams used a thin spacer in one earphone and a thick spacer in the other one with no obvious problems with code or music.
golfguruPosted - 22 June 2013 18:52  Show Profile  Email Poster  Edit Message  
>>>> I think that it would be best to use either one ear or both ears for the A/B comparison, altering only the power to the earphone(s).

3dB is clearly noticeable when switching a 3dB pad "in", and "out", using any listening scenario.

............

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