[TowerTalk] Crossed Dipoles....

[Tom Rauch]

Hi Gary

> You must be using proportional font (or someone else is and is
> complaining about it).  Non-proportional is the "rule" for news and
> E-mail.  Some programs default proportional.

That sounds to me like Maxwell's equations must sound to others. 
I know nothing about computers, and dislike hearing people talk 
about them. I'm an appliance op when it comes to computers, 
although I appreciate your trying to help (I'm beyond help on that 
topic).
 
> I assume the 90 degree phase shift can be done with a 1/4 wave line
> section at the center.  I think I've seen that in a handbook or antenna
> book over the years.

Yes. This goes back to things like the dreaded Inverted L thread.

If you bend an antenna and that's all you do, you don't wind up with 
both patterns. You wind up with ONE pattern that is the vector sum 
of the two.

So if I put up a perfect Inverted V dipole, broadside to the V I have a 
perfect horizontal pattern. Not a mixture of V&H.

An inverted Vee dipole, just like regular dipole, radiates a vertically 
polarized wave at high angles off the ends.

An inverted L half-wave fed in the center, broadside to the L, 
produces a tilted wave that is neither vertical or horizontal and still 
has a response null at 90 degrees to the main response.

The crossed dipoles, unless you feed them in phase quadrature, 
give a pattern JUST like a single dipole that is rotated 45 degrees 
to either pair. It is still (if the antenna is high enough) a dipole 
pattern with two lobes and two nulls.

So you must use a 90 degree phase delay between the two 
dipoles, or you made the same pattern change you would have 
made by simply turning a single dipole antenna to a new direction.

The phase shift isn't critical, anything between 60 and 120 degrees 
would make a pretty much omni pattern, but you must use the 
phase shift or you won't have the omni pattern.

> Thanks for the wisdom, Tom.  I had always wanted to try something like
> that but had forgotten about the phase shift.  I'm wondering how well it
> would work if the two dipoles were cut for opposite ends of the band (i.e.
> 3550/3900)?  

Since the system isn't current or phase critical, it should be OK.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm