ok guys been reading up some on the dipole,and even the multiband dipole. I still have a question and I'm sure it depends on your location and where you want to talk.Does the direction of the antenna matter in any way. It looks like if you have a height of at least 50' the antenna is really omni directional anyways,then I hear people saying get a rotatable dipole. Have I skipped a paragraph or whole page somewhere in my reading.Guess it my be a stupid question to some,but I cant see purchasing a rotatable dipole if its going to be omni direction anyway. Maybe their design is for someone with limited space and height restriction,I dont know and want to know,so please explain this to me.I haven't bought the first radio yet cause I'm still trying to get my ducks in a row. With me being a South Alabama Redneck if I dont get my dipole up 50' my wife will be hanging clothes on it.Anyways if someone would explain this to me would be great,a dipole from my reading is the way to go.Thanks in advance !
In general terms, a dipole has directionality which can be quite useful by orienting it to avoid noise, or increase gain of weak signals.
-Rotatable dipoles can be handy, particularly if you don't have room;, or want a rotatable antenna, but a full size half wave wire dipole will beat small 'ham stick' types every day.
-The directional pattern of a dipole is shaped like a figure eight, and is at right angles to the antenna. In other words, the null (weakest signal) will be along the lenght of the antenna.
-Generally, higher a dipole is, the better the long distance performance is, because the Take Off Angle, decreases. Optimal height would be about 1/2 wavelenght (up to maybe 1 wavelength) above ground or better. (Someone can correct me or give more precise info, I'm too tired to drag out the antenna book at the moment).
-On the other hand as a dipole gets closer to the ground they interact, decreasing the directionality and increasing the take off angle. This can be useful for making close in contacts. In fact there is a whole interest group of users who use this effect on 80 and 40m, called NVIS, Near Vertical Incident somethingorother. Nine or ten feet is as low as you really need for maximum effect.-
-As far as washing clothes, my Southern Grandmother always recommended Mondays!
Just point the end where there is nobody to listen to! Or in other words, broadside to the people you DO want to listen to! Here when I had a dipole it was north south as I wanted to point towards Europe from here in the UK. Your height of 50 feet sounds OK, and 40/20/10 might be the way to go. Too bad you dont say if you could stick a wire in a receiver and see where "the action" is at your QTH. Advice I took to heart at the end of last year was;
1. Decide WHEN you can operate regards time of day.
2. Work out what antennas are best in relation to what band will be active when you will be on the radio.
3. Build them yourself.
My setup wont win too many contests, but its cheap and flexible and it works. If it was higher it would work even better. (W3EDP antenna fed from a IC718 with a MFJ901B manual tuner).
Start thinking in terms of wavelengths, not feet, and things will be clearer. At heights less than about 1/2 wavelength above ground a dipole starts to lose directivity. This is because the earth reflection focuses the energy at high angles, and up above the antenna we can never be in a spot directly off an end!!!
Originally Posted by MAKITAMAN
By the way, you notice pretty much everyone uses take off angle. That's as bad as thinking in feet instead of wavelengths. Take off angle probably misleads people more than any other specification.
Even my friend who wrote the common programs EZNEC lamented a long time about including take off angle. He told me he just knew it would cause problems. But because K6STI had included it in his programs, it wound up in EZNEC. What really is of interest is the absolute gain over a range of useful angles, not take off angle which (by itself) is usually meaningless.
So avoid using take off using take off angle and feet when thinking or speaking in generalizations, and you will be ahead of the game.
A dipole at low height (in fractions of a wavelength, less than 1/2 wave high) starts to have more energy at high angles and less at low angles, but the low angle gain of a dipole is often much greater than a vertically polarized antenna. The reason is complicated, but the end result is a vertical even with a much lower "take off angle" usually has less energy at low useful angles than a dipole with a very high "take off angle".
That's why we never want to use feet unless we are talking about a specific band, and we never want to use take off angle.
If you are a new Ham, just put up a wire dipole and play around. Of course everything W8JI said is true, but for us mere mortals there is theory and there is reality. The REALITY is that I have two dipoles for 40 meters, one for NW/SE stations (South America and over the North Pole) and one for NE/SW stations (Europe and Australia). They are each about 40 feet high, just a little over 1/4 wavelength. I can tell a difference when working a station in Europe when switching between the two antennas, but its not THAT dramatic.
I have another dipole that is 100' long and 60' high and on bands like 15 meters there should be lots of nulls where I should NOT hear stations well. I have not noticed them! Tom is right, the pattern might predict "a null here", but that null might only be for an angle of 11 degrees, the null might be much less prominent at 25 degrees and THAT is where the signal arrives to me.
Put up a simple antenna and THEN play around with changing things. Don't overthink it at the beginning.
[QUOTE=MAKITAMAN; With me being a South Alabama Redneck if I dont get my dipole up 50' my wife will be hanging clothes on it. [/QUOTE]
Cool I should have done that with my first wife.... with dipole I always set them up to face east and west. A rotatable one would be nice too.. I just make wire dipoles so the are fixed in place.
EZNEC sez that a 20m 1/2WL (33') dipole at a height of 1/2WL (35') has a maximum gain of 7.4 dB with 12.5 dB nulls off the ends. That is quite directional. I have a 33' rotatable dipole and it works well.
73, Cecil, www.w5dxp.com
The maximum power transfer theorem works just as well for a non-resonant antenna as it does for a resonant antenna.
Thanks guys, yeah I'm new to ham and trying to put my money in the best places. Got plenty of wire, just going to experiment a little. I'll. Print off all the suggestions here and put them to good use. Thanks again!
A well installed horizontal dipole is a good antenna to start off with. You might also consider a G5RV Jr.(51ft). This can be matched on 40m,20m &10m using a rigs internal tuner - handy! After that see what you want to do. Best of luck.
Last edited by EI4GMB; 05-02-2011 at 06:00 PM.
'Man's greatness lies in being faithful to the present moment.'
Fr. Solanus Casey (1870-1957)
A random thought on rotatable dipole. Say you have a 40m rotatable dipole. That means, you have sufficient unobstructed space for a ~210' horizontal loop antenna (40m half-wave is ~67', rotatable implies circular swing space is available, circumference of circle with 67' diameter is pi x d = 67 x 3.14 = 210 feet). A little more gain than a half-wave dipole, possibly quieter, less maintenance, easier to put up and multi-bander.