# Half-Dipole Antenna Question

Discussion in 'Electronic Basics' started by Wayne, Apr 26, 2005.

1. ### WayneGuest

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?

2. ### Larry BrasfieldGuest

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
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

BTW, aiming EW instead of E or W will only drop
want to make sure you can hear the difference before
going to anything more elaborate.

3. ### Anthony FremontGuest

I think you may mean a half-wave dipole.
being a

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.
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.

4. ### WayneGuest

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.

I will try this first.

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

Thanks to you and Larry for your assistance.

5. ### Rich GriseGuest

Look up a "halo" antenna. It's omnidirectional, but flattens out the
pattern, so has some gain over a dipole, but in all (horizontal)
directions. It's also horizontally polarized.

You might also look for a "turnstile" antenna. That's typically two
folded dipoles with a 1/4 wavelength transmission line connecting them,
to give 90 degrees phase shift between the two antennas, making it
omnidirectional.

But since you have a random-wire, just put two little cuphooks in the
ceiling, and hang the loose end of your antenna from whichever one
works.

Good Luck!
Rich

6. ### Anthony FremontGuest

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.