Effect of 2 Dipole Antennas Close to each other

[wiregain]

Hi,

I have a couple of question about on inverted V dipole setup.

I have an inverted V up in a tall tree at about 1 wavelength high, at that height it has some directional properties, which is fine, but I wanted to put up a second dipole perpendilcaur to the first one and then hook both dipoles up to an antenna switch so I can switch between them for different directions.

My questions are,

a) Will the dipoles react or interfere with each other (ie affect the radition pattern of each other) ? I plan to have each on a seperate branch a few meters apart and at slightly different hieghts, plus at 90deg to each other ?

b) Will the anetnna switch effect the radition patter, since hooking both antennas to the switch will result in a common earth so that antenna are essentially partially connected through the elements connected to the outer shield of the coax ?

thanks.
Steve

 

[zz0468]

Actually, I don't think you need to do a thing. The pattern of an inverted vee is close to omni-directional. I don't think putting a switch in there would be worth the time or trouble. As for any interaction, it's hard to say. There are too many unknown variables.

 

[wiregain]

An inverted V in free space or close to the ground is almost omnidirectional, but get a wavelength of height above the ground and you get 7 dbi broadside and around -5dbi off the sides.

You also move the angle of radation from around 30 deg down to 15 deg and get another wavelength of height into it and you see above 5 deg angle of raditation .... very good DX antenna.

Height is the key with an inverted V ( or dipole !)

regards.

 

[k9rzz]

Certainly there may be some interaction between the two, however if one has a lobe in a direction that the other has a null, then being able to switch beween the two would be of great benefit!

Of course, give them some time to play out, put in time to see which is better in what conditions.

Sounds like a plan to me!

 

[Don_Burke]

They will interact, but putting them perpendicular will minimize the effect.

It sounds like you are on the right track.

 

[zz0468]

HUH!!?? 7dbi gain off an inverted vee? I have never, ever seen that in any reference books, ever, anywhere. What is your source?

 

[ka3jjz]

I'm going to take a WAG that he is thinking of a V-beam, which is a totally different animal. Most folks don't have the room to put up such an antenna...73s Mike

 

[wiregain]

I modelled it in Enzec, anything above 1 wavelength in height will give 7+ dbi gain (for a dipole or inverted V)

http://www.mansfieldweather.com.au/inv.jpg

I've done some more modelling with both antennas in place and they can be stacked ontop of each other with 1 metre seperate and at 90 deg to each other and there is almost no effect.

 

[zz0468]

Something doesn't seem right, there. 7 DBi gain for either a dipole OR an inverted vee? A dipole is, by definition, ZERO dbi gain in free space. An inverted vee is going to be even less. That kind of gain would require several elements.

 

[wiregain]

Yes, but this isn't modelled in free space, it is modelled in real space at 1.2 wavelength above a real ground. There are 2 wires in the model setup as an inverted V. A dipole becomes more directional, gets more gain and has a lower angle of radition as height increases (to a point).

Try it in Eznec for youself if you wish .. seeing is beleiving

Regards.

 

[kb2vxa]

What makes me suspicious is the one wavelength above ground thing. Unless it's cut for 10M or a higher frequency band and 36' or more above ground that statement is false. In any case stop trying to out tech each other and get down to brass tacks. For all practical purposes an inverted V like any dipole has unity gain and unless mounted unusually high it's an omnidirectional NVIS antenna. That rather makes an X configuration a moot point BUT it has been used successfully with horizontal dipoles. In such a configuration there is minimal interaction because the fields cross at right angles, parasitic elements must be in the same parallel plane.

Don't ask, don't argue, get a copy of the ARRL Antenna Handbook and read it.

Wire, if EzNEC told you a dipole has gain under any conditions there's a glitch in the program. Where do you think the term dBd came from? I frankly hate dBi (isotropic) because it's a theoretical model of a spherical radiation pattern that cannot exist in nature, so reference to a dipole (dBd) is what you use in the real world.

The azmuth pattern of a dipole does not change with it's height above ground (it changes with frequency multiples) while the elevation pattern does, so in that respect you interpreted your results correctly.

I don't have the inclination to explain further but I will tell you that if you study antenna theory basics you'll better interpret the results you get from any modeling program. You'll know WHY you get those results, near field, far field, ground reflection, recombinant wave front and all that has a lot to do with it.

Oh I don't claim to be some sort of genius, only pointing out you can't build a house without a good foundation. A computer is only a tool that does the math and draw the graphics for you. It won't teach you a thing all by itself and you'll never know why an antenna behaves the way it does unless you go beyond the program or "think outside the box" if you prefer yuppie jargon.

 

[wiregain]

kb2vxa,

With respect I think you are still thinking of a free space antenna without ground reflections, not a real wolrd situation. Perhaps I'm wrong, if I am please let me know how.

Have a look at the following page related to dipole height and gain, how come they report gain on a dipole and why does EZNec ?

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=12798 Click on the Diple Height Article.



Cheers,

 

[wiregain]

Here's the EZNEC file if anyone wants to have a closer look,

http://www.mansfieldweather.com/invertedv_1wavehigh.EZ