| Topic: |
Science > Physics |
| User: |
"eKIK" |
| Date: |
08 Sep 2005 12:17:38 PM |
| Object: |
Newbie filter question |
Hello
This might get a bit confused but please bear with me.
I'm in need of a filter for radio waves (433 MHz) which only lets
"straight" (close to the normal) waves through. I know of the quarter
wave rule which says that a wave goes straight through an object if the
object is less than 1/4 of the wavelength. Is there any way to "bend" a
radiowave, just like light in optics with a filter that is thinner than
1/4 the wavelength?
| Normal
|
--------- Filter
* Receiver
In summary, I would like to have a setup like the one pictured above,
which only lets waves with an incident angle of, let's say 10 degrees,
reach the receiver. HOWTO?
Regards
Erik
.
|
|
| User: "PD" |
|
| Title: Re: Newbie filter question |
08 Sep 2005 12:54:38 PM |
|
|
eKIK wrote:
Hello
This might get a bit confused but please bear with me.
I'm in need of a filter for radio waves (433 MHz) which only lets
"straight" (close to the normal) waves through. I know of the quarter
wave rule which says that a wave goes straight through an object if the
object is less than 1/4 of the wavelength. Is there any way to "bend" a
radiowave, just like light in optics with a filter that is thinner than
1/4 the wavelength?
| Normal
|
--------- Filter
* Receiver
In summary, I would like to have a setup like the one pictured above,
which only lets waves with an incident angle of, let's say 10 degrees,
reach the receiver. HOWTO?
Regards
Erik
You want a *directional* antenna, you mean.
There are lots of ways to do it. Here is a not so straightforward way.
433 MHz corresponds to a wavelength of 69 cm. Build a diffraction
grating made of about 5 chain-link-fence posts separated by 100 cm. Put
an antenna and a tuner with a high Q tuned to 433 MHz about 10 m behind
the grating at an angle of 44 degrees from the central axis, where the
first non-central interference maximum will be.
Or look up "directional antenna design" in Google.
PD
.
|
|
|
| User: "eKIK" |
|
| Title: Re: Newbie filter question |
08 Sep 2005 01:00:24 PM |
|
|
Thank you both for your great answers!
I'm totally new to RF, so I think in very stupid ways :)
A directional antenna is of course the way to go, instead of a filter.
The bending part was because I only thought of the filter solution, and
I wanted to "bend" the waves away from the antenna, therefore avoiding
receiving the wave.
Google will probably be more helpful now that feed it with sensible
keywords :) Especially thanks to PD for the nice explanation.
.
|
|
|
| User: "Helmut Wabnig EmailAddress" |
|
| Title: Re: Newbie filter question |
08 Sep 2005 01:10:51 PM |
|
|
On 8 Sep 2005 11:00:24 -0700, "eKIK" <teh.ekik@gmail.com> wrote:
Thank you both for your great answers!
I'm totally new to RF, so I think in very stupid ways :)
A directional antenna is of course the way to go, instead of a filter.
The bending part was because I only thought of the filter solution, and
I wanted to "bend" the waves away from the antenna, therefore avoiding
receiving the wave.
Google will probably be more helpful now that feed it with sensible
keywords :) Especially thanks to PD for the nice explanation.
Newsgroup "rec.radio.amateur.antenna" will answer your questions.
w.
.
|
|
|
|
| User: "Randy Poe" |
|
| Title: Re: Newbie filter question |
08 Sep 2005 01:33:39 PM |
|
|
eKIK wrote:
Thank you both for your great answers!
I'm totally new to RF, so I think in very stupid ways :)
A directional antenna is of course the way to go, instead of a filter.
The bending part was because I only thought of the filter solution, and
I wanted to "bend" the waves away from the antenna, therefore avoiding
receiving the wave.
Google will probably be more helpful now that feed it with sensible
keywords :) Especially thanks to PD for the nice explanation.
One final note: I said you wanted a "high-gain" antenna,
but if you really want a 20-degree wide beam (+-10 degrees),
that's not really very high gain. The antenna newsgroup
sounds like a good suggestion. Make sure you know
how wide a beam you want, in both horizontal and
vertical directions.
- Randy
.
|
|
|
| User: "CWatters" |
|
| Title: Re: Newbie filter question |
09 Sep 2005 01:42:33 AM |
|
|
Perhaps I missunderstand but it sounds like you are asking for a polarized
antenna not a directional antenna. Is that correct?
.
|
|
|
| User: "eKIK" |
|
| Title: Re: Newbie filter question |
11 Sep 2005 01:23:06 PM |
|
|
I'm not sure what I want at all =)
I do know what I want to do on the other hand.
I have:
1. Omnidirectional (moving) antenna which transmits a signal.
2. The antenna discussed above, which should only receive the signal
(with enough gain) if the transmitter (1) is "inside" the 20 degree
wide beam (horizontal plane).
This is what I want....hope I made some more sense this time :)
.
|
|
|
| User: "Jan Panteltje" |
|
| Title: Re: Newbie filter question |
11 Sep 2005 02:20:01 PM |
|
|
On a sunny day (11 Sep 2005 11:23:06 -0700) it happened "eKIK"
<teh.ekik@gmail.com> wrote in
<1126462986.756073.138230@o13g2000cwo.googlegroups.com>:
I'm not sure what I want at all =)
I do know what I want to do on the other hand.
I have:
1. Omnidirectional (moving) antenna which transmits a signal.
2. The antenna discussed above, which should only receive the signal
(with enough gain) if the transmitter (1) is "inside" the 20 degree
wide beam (horizontal plane).
This is what I want....hope I made some more sense this time :)
What frequency?
Normally a 'yagi' antenne (use google) should do.
You can add more elements to make a smaller angle.
You can use the yagi vertically of horizontally.
|
source | |
* | | | | |
1 or more directors | |
| reflector
dipole(and feed)
You must have seen this on roofs (as used for radio and TV)....
There exist really big ones for lower frequencies too.
.
|
|
|
|
|
|
|
|
|
| User: "Randy Poe" |
|
| Title: Re: Newbie filter question |
08 Sep 2005 12:50:33 PM |
|
|
eKIK wrote:
Hello
This might get a bit confused but please bear with me.
I'm in need of a filter for radio waves (433 MHz) which only lets
"straight" (close to the normal) waves through. I know of the quarter
wave rule which says that a wave goes straight through an object if the
object is less than 1/4 of the wavelength. Is there any way to "bend" a
radiowave, just like light in optics with a filter that is thinner than
1/4 the wavelength?
| Normal
|
--------- Filter
* Receiver
In summary, I would like to have a setup like the one pictured above,
which only lets waves with an incident angle of, let's say 10 degrees,
reach the receiver. HOWTO?
I'm a little confused. You seem to be asking if an antenna
can be made highly directional. If so, the answer is yes.
Directionality is measured in terms of "antenna gain"
which is, roughly speaking, the sensitivity in the desired
direction over the average sensitivity in all directions.
So you seem to be asking for a high-gain antenna. A
parabolic dish would qualify but there are other
choices.
One basic rule is that the wider the aperture compared
to wavelength, the better the directionality.
On the other hand, you are asking about "bending", and
I can't figure out what you mean by that or how that
bears on directionality.
- Randy
.
|
|
|
|
|
Related Articles |
|
|
