[TowerTalk] reflector for 75 meter inverted vee

K4SAV RadioIR at charter.net
Thu Oct 5 19:19:21 EDT 2006


Thanks for the post Bill.  That brings up an interesting question. I am 
aware that sometimes high angles are good on 160, such as you might get 
from a low dipole, inverted vee, or a loop. But I was referring to 
antennas that have a take-off angle of 90 degrees with very narrow 
beamwidth. A 160 meter horizontal loop at 100 feet will give a gain of 
about 7.5 dBi straight up with a beamwidth of about 90 degrees. So it 
will be useful for a lot of things.

To get more gain straight up you have to narrow the beamwidth.  It's the 
usefulness of these narrow beamwidth antennas I was questioning.  A 
simple example would be 3 dipoles at 110 feet separated by 1/4 
wavelength, feeding the outer ones 180 degrees from the center one. That 
would shoot everything straight up, producing a gain of 9.3 dBi with a 
take-off angle of 90 degrees and a beamwidth of about 40 degrees. The 
only place you get any additional gain over a simple loop is between 75 
and 90 degrees.

I wasn't thinking 160 meters before. Now the question. Do you think 
signals entering a duct on 160 would enter that duct between 75 and 90 
degrees elevation? Any speculation? What if you built an antenna with 
even more gain and even narrower beamwidth? Can you think of any other 
uses for antennas like this? Did the HAARP research ever produce any 
useful information on this kind of thing?

Jerry, K4SAV

Bill Tippett wrote:

>K4SAV:
>
> >Antennas arrays that provide large amounts of gain directed straight up
>with very narrow lobes aren't very useful for ham purposes either.
>
>         It depends on propagation.  At sunrise, sunset and during
>high auroral activity, DX wave angles are often very high.  I
>put up an inverted-V for 160 (apex 95') to complement my 3-el
>vertical array.  95' is near the optimum height to maximize
>radiation straight up.  You can see a TOA comparison of the
>inv-V (blue) versus the 3-el vertical array (black) below:
>
>http://users.vnet.net/btippett/new_page_10.htm
>
>         The TOA break even point is about 40 degrees which can
>be seen where the lines intersect.  ~95% of the time the vertical
>is +10 dB over the inv-V, but ~5% of the time it's the other
>way around.  One of those times was August 2 when I worked
>KH8SI for #321 on Topband.  The vertical array simply did not
>work (neither do Beverages for RX when angles are high) but
>after sunrise his signal peaked very nicely on the inverted-V.
>I'm very glad to have it for such times!
>
>                                         73,  Bill  W4ZV
>
>
>
>
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