Thanks John, I have been faced with this capacitive loss thing for a long
time. It became quite necessary to reduce it to make my all active HI-Z 8A
array work well. Especially with the early Hi-Z amps with 6dB loss.
Here is a list of elements of different lengths and their subsequent output
source capacitances and system losses. This list is for an element diameter
of 0.625 inches. And a total capacitive load of 15pf.
10 feet =34.6pf.......... .70.......... -3.01dB
11 feet =37.3pf.......... .71.......... -2.97dB
12 feet = 40.1pf.......... .73.......... -2.73dB
18 feet = 56.4pf.......... .79.......... -2.05dB
24 feet = 72.6pf.......... .83.......... -1.61dB
There is about 0.1 dB per foot increased output signal as a result of the
capacitive division and increased length. Pretty much insignificant with the
state of the art.
It becomes quite significant with highly capacitive input amplifiers, their
connecting wires, or element insulators constructed with excess capacitance
between the element and ground.
Going from 18 to 24 foot elements using both John and my numbers is about a
3.4dB increase in signal.
Here is a couple fun tools I have used over the years that apply.
"http://www.daycounter.com/Calculators/Whip-Antenna-Design-Calculator.phtml";
"
http://www.learningaboutelectronics.com/Articles/Capacitor-voltage-divider-c
alculator.php#answer"
Lee K7TJR
Lee,
The NEC-4 analysis is based strictly on gain and does not take into account
the loading of any amplifier input capacitance, which can modify the scaling
of signal level obtained at the output of the amplifier with increasing
vertical height. A very short vertical, in the range of 10 to 25 feet,
exhibits a feedpoint impedance that is almost entirely capacitive reactance.
NEC-4 shows that the capacitance is a few ten's of pF, with the capacitance
increasing with increasing length. On my own 15 foot verticals, which uses
4 foot ground rods, I measured the vertical's capacitance as about 50 pF on
160m using an accurate impedance analyzer.
This means that the voltage at the output of any feedpoint amplifier will
depend on the voltage divider relationship that occurs between the
capacitance of the vertical and the input capacitance of the amplifier.
This reduces the maximum voltage output that is available from the vertical
at the amplifier's output. The maximum output is obtained (theoretically)
when the input impedance of the amplifier presents a conjugate match to the
antenna impedance, which, in this case, would be inductive (a very large
inductance, in fact). But this is the same as saying the vertical is now
resonated to the frequency of interest by the amplifier's input inductance.
As a practical matter, the amplifier gain can easily make up any
inefficiency in coupling signal out of a short vertical, subject to the
considerations of noise added by the amplifier, as I discussed in my earlier
post.
73, John W1FV
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