Re: NEC-LIST: Fw: Lambda/4 monopoles lamdba/4 apart

It sounds like the problem has been resolved.

I'll add a tip for Chuck's "student". The problems encountered by
feeding an array as he describes is addressed in detail in Chapter 8 of
the _ARRL Antenna Book_. It is possible to correctly feed this array
with two coax lines -- but they end up being considerably different in
length than 90 degrees. This is described in "The Simplest Phased Array
Feed System -- That Works" in the _ARRL Antenna Compendium_, Vol. 2. I
recall that Al Christman also wrote up and published this method in the
IEEE Transactions on Broadcasting.

And as for the persistent and perhaps mysterious back lobe when you have
everything perfect -- that's due to the different current distribution
on the two elements even when the base currents are correctly phased and
perfectly matched in amplitude. You can find a longer discussion, along
with some illustrations of the current distributions, in "The Impact of
Current Distribution on Array Patterns", in Technical Correspondence,
QST, July 1990. Ironically, the pattern of an array with elements near
anti-resonance (e.g., monopoles near a half wavelength high), is better
when the elements are end fed with voltages that are equal in magnitude
and in quadrature than when end fed with quadrature currents.

Roy Lewallen, W7EL

Chuck Counselman wrote:
> At 11:00 AM -0700 9/22/03, Jim Lux wrote:
>
>> Since the monopole above a perfect ground is really like a dipole in
>> free space, have you tried a pair of dipoles spaced 90deg apart fed
>> 90deg apart (in free space) to see what you get?
>
>
>
> That won't help; in fact, the results will be essentially the same as
> for monopoles.
>
> His problem was the use of voltage sources, in combination with the
> great inequality of the feedpoint impedances (due to the strong coupling
> between the monopoles).
>
> My (corrected) suggestion to put the two voltage sources each behind
> one-quarter wavelength of TL solves the problem by transforming the
> voltage sources to current sources.
>
> In case anyone is thinking of driving the monopoles from a single
> voltage or current source via TL's whose lengths differ by 90 degrees:
> it won't work. Try it; you'll see. I leave the explanation as an
> exercise for the student. :-) Hint: The explanation is implicit in
> what I've already written.
>
> -Chuck, W1HIS
>

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