Topband: Help

[Herb Schoenbohm]

Phil,

A top wire that is long enough to produce a reasonable base impedance, such as a
3/8 wave series tuned to resonance, has a serious drawback by producing
significant high angle radiation.
That is why a balance T top top loading section is a better way to prepare a DX
antenna for 160 meters.  In cases where there may only be room in one direction
your best bet is to make a folded back top section of your inverted L.  If the
top section is an 1/8 wave folded section, a quarter wave piece of wire folded
back on itself with spacers and fed on one side with only the far end tie
together, your 60 foot vertical piece will tune nicely with a single series
tuned capacitor. ( If you don't have a nice vacuum variable, then start with a
receiving variable and  low power. After achieving your best match replace the
RX cap with a good quality fixed cap such as a G2 if you plan to run high
power.  A current of 5 amps across the cap should be expected for 1 kW assuming
a base impedance of 40 ohms and an excellent ground radial system.  Poorer
grounds of course will result in a higher base (loss resistance) impedance and
poor overall efficiency.

Although a 1/4 wave top portion and 60 foot vertical should do well in the next
county it is not the way to go if you want to maximize the radiation at lower
Dx-prefered angles.  The folded flat top radiation limiting flat top portion
works well as it insures the current top occurring near or at the top of your
vertical 60 foot wire without the drawback of an abundance of high angle
radiation.

Good Luck

Herb Schoenbohm, (ex-KV4FZ)
St. Croix, VI.






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