At 04:18 PM 2007-12-12, Dick-K9OM RLVZ@aol.com wrote:
>I've been doing research on 160-meter Inverted L's and would appreciate your
>thoughts on the following:
>
>1) Some people report that the 160-m. Inverted L can be fed directly with 50
>ohm coax with good results. Is that true?
>
>2) Some people feed their Inverted L's with fixed 500 or 1,000 pF
>capacitors. What advantages does this offer, such as: lower SWR
>at resonance, and/or
>increased bandwidth? (again these are "fixed" capacitors not variable)
>
>3) Can you offer a good resource for the 500 or 1,000pF capacitors that will
>handle a Kw?
>
>I realize a variable capacitor is best as you can move around the band
>better... but for this temporary 160-m. Inverted L... I'd prefer
>Options 1 or 2
>above.
Another approach, that gives a 50 ohm match without capacitors or
transformers, is to use an inverted-L that is about 3.5 feet shorter
than resonance and place a shunt coil (about 4.7 uH) across the
feedpoint. This forms a virtual L-network to step up the naturally
low impedance of an inverted-L. I use this approach with a four-leg
inverted-L with a very short vertical section. The loss in the
matching coil is well under 0.1 dB.
Using the shunt coil match (also known as beta or hairpin match)
actually gives a very slightly wider SWR bandwidth than the
series-cap matching approach. In addition, the shorter horizontal
wire of the shunt-matched inverted-L decreases the high angle
radiation. Of course, if you're mostly interested in short-haul
contacts, the longer wire needed with the series-C match would be an advantage.
The gain at low angles is very similar regardless of the matching
approach used. The ground losses are indeed lower with the series-C
approach, due to the higher feed Z, but all of the extra signal takes
off at high angles. I'll forward to Dick another message containing
patterns and SWR numbers comparing series matched, resonant, and
shunt matched inverted-L's that I ran recently.
73, Terry N6RY
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