TopBand: L's and T's

[Dan Robbins]

When one can't have a full quarter wave vertical on 160, the inverted L is
certainly an able substitute.  Two things happen when part of the quarter
wave is bent over into the horizontal - the impedance drops and the null at
high angles of radiation begins to fill in. As the vertical portion of the L
is made shorter (and the horizontal portion made longer to keep resonance)
the worse these two parameters become.  If one desires both local and DX
QSOs a short inverted L will work fine.  If one's interests lie only towards
DX, a T antenna might be a better choice.  The T retains the overhead null
in the angle of radiation, thus minimizing close in signals.  One ham who
operated in the Pacific told me that he switched from an inverted L to a T
because when the there were thunderstorms a hundred or so miles away the T
was quieter.  He surmised this was because of the high angle null of the T.
The T will take up more real estate, of course, but for those vertically
impaired it may be a better antenna than an inverted L.  

If you can get the vertical section of an inverted L taller than, say, 70 or
80 feet, a T is probably not going to make much difference.  OTOH, if you're
vertical section is 60 ft or less, you might want to model both the T and
the inverted L on a computer to see what the differences are.  I did a quick
look on Elnec for a 60 ft high inverted L vs a 60 high T and the T had a
slightly lower angle of radiation and less vertical beamwidth.  At 75
degrees the T was about an S unit below the inverted L; at 90 degrees it was
many S units below.

For a 30 ft vertical section with an horizontal leg of 105', the inverted L
radiates a lot of high angle stuff; the peak angle of radiation was 30
degrees and at 90 degrees was only 2.5 dB down from that.  The vertical
beamwidth was 162 degrees!  A comparison T 30 ft high had a flatop of 170'
but the peak angle of radiation was 21 degrees and the vertical beamwidth
was 48 degrees.  Big difference there.

                                        Dan KL7Y


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