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Author Topic: Wire Dipole Direction  (Read 625 times)
KB1HJW
Member

Posts: 70




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« on: November 28, 2001, 03:41:09 PM »

I think I am about ready to start work on my first antenna. I just read much of ARRL's Wire Antenna Classics, and while my head stops spinning, I thought I'd ask a couple of questions.

The attic of my townhouse (where the antenna will go) is on the west end of a row. The feed line will either come up the southeast corner(where one open side of the house meets the next unit) or midway on the east side (abutting the next unit). I will run the legs in a "V" out to the far corners.

Three Questions:
1.Will the location of the feed point and the direction and angle that I run the wires make a difference in what direction I get the best transmission/reception?
2.If I run lines for both 10m and 20m how close can they be to each other, assuming the same feed point?
3.Would ladder line be better than #14 uninsulated for the lines, and, would it be better than coax for the feed?

You have all been great, I really appreciate it!
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KF4HEY
Member

Posts: 172




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« Reply #1 on: November 28, 2001, 04:34:21 PM »

Hello Rene,

First off, welcome to ham radio.  You will really enjoy the experience.

For attic antennas, don't worry about the direction of the dipole legs.  Just run them in any way possible to maximize the separation and to have the longest possible straight line runs.  Because the antenna will not be high enough above ground to work as a dipole in "free space", it will act as an omnidirectional radiator regardless of orientation.  

Use uninsulated wire if it will not come in contact with metal objects.  Also use stranded wire as it is easier to work with in the cramped space of an attic.

The maxim here is: "any antenna that works is a good antenna."

73 and will look for you on the bands,
Mike KF4HEY
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RobertKoernerExAE7G
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Posts: 1435




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« Reply #2 on: November 29, 2001, 04:19:12 AM »

Congrats on your ticket.

Co-axial cable will probably be a lot easier for you to run between your attic and rig; and will be easier to attach to your rig, PL259 to SO239.  You can find tons of CHEAP wire every where to make antennas, while you will probably have to order ladder line, or make it your self.

So; think that you will make your first antenna(s) fed with coax.  Get used to how it works for a few weeks, months.  Then make another kind of antenna to see if it is better than your first one.

Have FUN
Bob
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WB6BYU
Member

Posts: 13280




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« Reply #3 on: November 29, 2001, 12:56:56 PM »

The direction of the wire may make a bit of difference on the higher
bands, but there is a lot of leeway.  I have a rotatable dipole up
on a tower for 10/15/20m, and it seems to work about as well
whether I try to aim it in the desired direction or not.  Signals may
be weaker off the ends of the dipole, but the effect is not noticable
more than 30 degrees from the direction of the wire:  I can rotate
the dipole 90 degrees and not notice a difference.  So don't be
too concerned about which way it points.

I am a bit confused about your description.  If you are thinking
of putting the feedpoint on the East end of the attic, and running
the wires to the opposite corners (which would be the SW and NW
corners), then that may not work well, as the angle between the
wires would be too small.  If you put the feedpoint in the center
of the attic and run the wires to opposite corners (SW and NE, for
example), then it should work just fine.

You can space multiple wire elements quite close to each other -
some folks cut them out of a single piece of multi-conductor cable.
However, the closer the ends of the shorter wires are to the longer
wires, the more the interaction in the tuning of the different wires,
which makes it hard to adjust.  As a guess, I'd recommend at least
one foot spacing (30cm) at the ends of the shorter wires.  (To make
it easier to tune, leave the last foot or so of wire hanging down so
you can snip off pieces without having to retie the support strings.)
If your attic allows you to run one dipole NE/SW and the other
NW/SE from the same feedpoint, so much the better.  As with so
many other things in ham radio, it isn't so much a matter of "what
will work", but what makes it easier for you to adjust.
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NZ5L
Member

Posts: 220




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« Reply #4 on: November 29, 2001, 01:07:49 PM »

Any dipole erected less than 1/4 wvlength high will be essentially omni- directional - most of the radiation goes straight up anyway.  On higher frequency bands where heights of 1/2 wvlength or more are practical, there will be a pronounced null off the ends. You can get around this by erecting as a "vee". Several dipoles can be fed from one feedline, with only minor interaction if ends arew separated. Standard 300ohm twinlead will work OK for attic installation for multi-band operation. Keep power level at 50W or less. Will probably tune up all band higher than fundamental freq. but you may have to experiment with length of feeder. Don't be surprised if one or two bands out of tuner's matching range. If making your own balun, use the biggest suitable core available.
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W9GB
Member

Posts: 2623




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« Reply #5 on: November 29, 2001, 03:03:33 PM »

My only comment would be:

Get the antenna feedpoint as high as possible.

Attempt to keep the feedpoint's angle (between the 2 legs of your dipole) greater than 90 degrees.

A dipole has a 180 degree angle
An inverted V is about 120 degrees

w9gb
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NB6Z
Member

Posts: 550


WWW

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« Reply #6 on: November 30, 2001, 11:32:51 AM »

A 1/2 wave dipole is to short to be a V-beam, and so you will probably have some cancellation of signal in the direction of the V. Keep the angle greater than 90 degrees to prevent too much cancellation. You will have to cut each legg or element for a proper SWR (I assume you are going to use coax) after the antenna is mounted in the attic. Performance is not going to be predictable with your installation, but I am sure it will keep you happy until you buy that farm outside of town... ;-)
More info on multiple band dipoles here: http://home.teleport.com/~nb6z/yagi.htm
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