Half-Dipole Antenna Question
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Posted by Wayne on April 26, 2005, 6:50 pm
Please log in for more thread options I made a half-dipole antenna for my home stereo from wire scraps I had. It works fine, but unfortunately, my two favorite radio stations are geographically 90 degrees apart (East, and South). With the dipole being a directional antenna, that, of course, means sacrificing one station's signal for the other. Instead of dropping money on more equipment, can I just add a second half-dipole in series with the first, making the second in the same horizontal plane, but oriented 90 degrees from the first dipole? If the two dipoles are oriented 90 degrees apart, would the interference between the two dipoles be zero, allowing me to get maximum signal from both the East and Soputh? | |||||||||||||||||||
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Posted by Larry Brasfield on April 26, 2005, 5:38 pm
Please log in for more thread options >I made a half-dipole antenna for my home stereo from wire scraps I had. It
works fine, but unfortunately, my two favorite radio
>stations are geographically 90 degrees apart (East, and South). With the
dipole being a directional antenna, that, of course,
>means sacrificing one station's signal for the other. Instead of dropping
money on more equipment, can I just add a second
>half-dipole in series with the first, making the second in the same horizontal
plane, but oriented 90 degrees from the first
>dipole? If the two dipoles are oriented 90 degrees apart, would the
interference between the two dipoles be zero, allowing me to
>get maximum signal from both the East and Soputh?
The problem with your method is that, in effect, the signal you pick up with one antenna will be partially lost (reradiated, actually) by the other. The net effect will be little different than if you had aimed your single dipole midway between the two directions. If you have only two stations that you really want to receive, you could build a pair of more directional antennae and sum their outputs in a (nearly) lossless splitter. The same effect as I describe above will occur, but you would have more signal to start with. BTW, aiming EW instead of E or W will only drop the signal about 3 dB for your antenna. You might want to make sure you can hear the difference before going to anything more elaborate. -- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me. | |||||||||||||||||||
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Posted by Anthony Fremont on April 27, 2005, 1:21 am
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> I made a half-dipole antenna for my home stereo from wire scraps I
had. It
I think you may mean a half-wave dipole. > works fine, but unfortunately, my two favorite radio stations are
being a
> geographically 90 degrees apart (East, and South). With the dipole No problem point half way in between, and of course you point a dipole by the broad side of it, not the ends. Like Larry said, you really shouldn't notice a problem unless the signal is very marginal since the 3dB beam width of the antenna is so wide. You could turn your dipole vertically and it will then be omni-directional wrt land stations. I believe that most FM broadcast stations transmit using horizontal polarization, but that doesn't mean the signal arrives at your location oriented that way. You will just have to experiment and then you will see how far the theory strays from reality. It's not that the theory is really so wrong, it's just that the nature of RF is so obtuse that you can't possibly factor in everything that is having an effect and sumarize it in a simple formula. Antenna theory is just that, theory. Reality often stands a good distance from it. I suggest you build yourself a two element cubical quad, or a two element beam. One of them will have so much more gain (hopefully without narrowing the beam width too much) than your dipole that you should have no trouble receiving both stations without re-pointing it. If you have the room for the 3-d nature of the cubical quad then I highly recommend you build one, you will be real happy with it. I once built an 8 element quad for VHF (2 mtrs) that really kicked serious butt, but it was a real bear to tune. Sadly, it was made from wood so nature eventually took it from me. > directional antenna, that, of course, means sacrificing one station's
signal
> for the other. Instead of dropping money on more equipment, can I
just add
> a second half-dipole in series with the first, making the second in
the same
> horizontal plane, but oriented 90 degrees from the first dipole? If
the two
> dipoles are oriented 90 degrees apart, would the interference between
the
> two dipoles be zero, allowing me to get maximum signal from both the
East
> and Soputh?
You are not likely to have much luck with this kind of thing without learning a ton more than you probably want or care to know, but I could be wrong. The key is to experiment with antennas, you will be amazed at what works sometimes. | |||||||||||||||||||
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Posted by Wayne on April 27, 2005, 12:01 am
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> No problem point half way in between, and of course you point a dipole
> by the broad side of it, not the ends. Like Larry said, you really > shouldn't notice a problem unless the signal is very marginal since the > 3dB beam width of the antenna is so wide. I am pointing the dipole by it's broad side, unfortunately, both stations are marginal. When I turn the antenna to get a decent signal on one, the other is too poor for me to tolerate. >You could turn your dipole
> vertically and it will then be omni-directional wrt land stations. I will try this first. > I suggest you build yourself a two element cubical quad, or a two
> element beam. One of them will have so much more gain (hopefully > without narrowing the beam width too much) than your dipole that you > should have no trouble receiving both stations without re-pointing it. > If you have the room for the 3-d nature of the cubical quad then I > highly recommend you build one, you will be real happy with it. My antenna is mounted in a large attic, so I don't have to worry about rot or space. If the vertical orientation doesn't help I will try a cubicle quad. (I may try to build one just for the heck of it.) Can you recommend a web-site that would have specs or drawings to construct one? I don't really know much about radio electronics. Thanks to you and Larry for your assistance. | |||||||||||||||||||
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Posted by Anthony Fremont on April 27, 2005, 5:20 am
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news:iqCbe.7779 > My antenna is mounted in a large attic, so I don't have to worry about
rot
> or space. If the vertical orientation doesn't help I will try a
cubicle
> quad. (I may try to build one just for the heck of it.) Can you
recommend
> a web-site
know
> that would have specs or drawings to construct one? I don't really > much about radio electronics.
http://www.softcom.net/users/kd6dks/quad.html has a nice calculator program. Your measurements will not be that critical since you are not needing to transmit. Just build your antenna with the "driven" element and the "reflector" element. You should be able to play with your spacing to adjust the front lobe so that both stations can be received nicely. By itself, a full wave loop will have about 3dB of gain vs. the dipole. In fact you may get good results by just using a full wave loop without the reflector. You should try that first and then add the reflector noting the effect it has on sensitivity (gain). Adding the reflector will give you a front and a back side. You point a loop asif you are trying to have the signal pass straight thru the open hole. The optimal shape for a loop is a circle, but squares are usually more convenient and they work fine. | |||||||||||||||||||
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