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Old 2012-05-06, 05:32 AM   #1
GHZ24
 
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Question Wave Guide to propagate a 213 MHz signal?

Judging by online waveguide calculators much smaller and channel 13 frequencies won't propagate in a circular waveguide much smaller than 33 inches.

I tried to model a waveguide enclosed yagi for uhf (like a yagi in a garbage can.) and it proved to me how hard it is to "shield" your way into a high front to back ratio.

My frustrated attempts to get these models to report a F/B or F/R at least as good as a stagger stacked yagi made me try fully enclosing the yagi just to see if the mesh was bleeding signal (holes to large maybe?).

That is not allowed by 4nec2 (it freaked) but it proved the holes in the mesh were small enough.

It was like the garbage can (waveguide) was conducting the signal around to the front. If I remember right there was not only a poor F/B but an actual rear lobe! I abandoned the concept, though I may revisit it sometime.

Heed holl_ands warning to model it first!
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Old 2012-05-06, 12:10 PM   #2
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Quote:
and it proved to me how hard it is to "shield" your way into a high front to back ratio.
This guy makes it look easy, heh.
http://www.prism.gatech.edu/~wn17/

Also note the 3 to 33 mv increase on the scope before and after. Im not sure exactly how to read his scope, but it does seem to imply an increase in gain from a higher F/B F/R.

On the next page, he puts a two bay bowtie inside of a garbage can with even better results.
http://www.prism.gatech.edu/~wn17/We...20page%202.htm
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Old 2012-05-06, 01:36 PM   #3
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I think he would have to build a cage, a 55 gal drum with a diameter of 24 inches is too small for channel 13, half wavelength of around 27 inches.
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Old 2012-05-06, 03:01 PM   #4
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I thought those steel drums were 30 inches ID. They seemed larger when I was a young kid

The cage is a good idea, with climbing plants on three sides; or even for growing cucumbers and tomatoes.
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Old 2012-05-06, 03:04 PM   #5
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NOTE: Resizing from UHF to Hi-VHF Band is THREE TIMES BIGGER....
Which means it needs to be big enough to WALK into....
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Old 2012-05-07, 12:30 AM   #6
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Default big enough to walk in?

The 33 inches I quoted was a minimum size for a wave guide to propagate a 213 MHz signal.

http://www.turnpoint.net/wireless/cantennahowto.html

plug in 33 inches in the can diameter and see. then try 45" (to big to propagate correctly)

BTW the scale factor from channel 13 to 14 is 2.25

Quote:
This guy makes it look easy, heh.
http://www.prism.gatech.edu/~wn17/
I didn't see any F/B. F/R or any plots.
Glad the spectrum analyzer shows it worked for him to block weaker reflected signals at some unknown angle.
But his results prove only that the cage didn't destroy the reception of the antenna inside it.
He could have a F/B ratio of 1 and not know it cause no interfering signal happens to come from that direction in his environment.
Show me a model of a cage that works to get better F/B ratio.
I'm not saying it cant work just that I didn't get as good of results as I thought it should have (not good at all)
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Old 2012-05-07, 04:04 PM   #7
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I didn't see any F/B. F/R or any plots.
True, he does'nt have any. But since he's got the antenna in an open ended (back end closed I assume) screen mesh Faraday cage, the F/B and F/R ratios have to be very large.
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