[Antennas] Re: Termination Resistance for Pennant Antenna

[Barry L. Ornitz]

Nelson Wittstock <witstock@mail.ohio.net> asked:

> I'm thinking about putting up a Pennant antenna for low
> noise reception on 160 meters.  My concern is that I'm
> unclear as to what to use for a terminating resistor.  The
> non-standard values that are called for have me puzzled.
> How does one come up with a 945 or 903 ohm resistor?  I
> suppose a potentiometer could get this value and then be
> placed in a weatherproof container but there has to be a
> better way.  Any suggestions would be appreciated.

Trying to match this value exactly is a futile effort as the
optimum termination resistor depends highly on the exact local
environment of the antenna.  I am sure your particular antenna
will require its own value of termination resistance for the
best null.

If you will read the web page that Jesper Wolf Jespersen
<oz8ace@qsl.net> kindly referenced,

     http://members.aol.com/DXerCapeCod/pennant.htm,

you will find a method of remotely adjusting the resistance
for optimum null.  The method discussed there uses an opto-
coupler made from a cadmium sulfide photoresistor coupled to a
LED light source.  By varying the current to the LED, you
change the resistance.  The LED current can be sent remotely
as a low DC voltage.

I once used an alternative approach to terminate a Beverage
which required a lower value of termination resistance.  I
used a miniature incandescent panel lamp as the termination
resistance.  The resistance of such incandescent lamps goes up
with applied voltage as the resistance increases with
temperature.  To control the lamp resistance, just apply a DC
or low frequency AC voltage through the lamp.  This can be
done, of course, through the antenna wires.

To select the right lamp, pick a resistance slightly higher
than you expect to use.  Then pick a lamp based on its hot
resistance (normal operating voltage/normal operating
current).  As the lamp voltage is lowered from its normal
operating value, the resistance will go down.  For example, a
#47 pilot lamp is rated at 6.3 volts and 0.15 amps giving a
hot resistance of 42 ohms.  In this case, where a resistance
value between 400 and 1200 ohms is needed, I would suggest a
#1843 lamp rated at 28 volts and 0.02 amps, or two #1819 lamps
in series rated at 28 volts and 0.04 amps.  I would prefer to
use the single lamp solution to keep the DC voltage low.

Chicago Miniature Lamp presents some interesting equations
that predict the operating current of their small lamps as a
function of the applied voltage.  If you go through the math,
you can predict the resistance as a function of applied
voltage.  This function is nonlinear but can be approximated
as a simple straight line over the range of applied voltage
from 50 percent to 100 percent of the normal lamp voltage.  At
50 percent of the normal voltage, the resistance is
approximately 75 percent of the normal resistance.  At 25
percent of the normal voltage, the resistance will be
somewhere around half the normal resistance.

Lower resistances are easier to obtain, of course, with common
miniature lamps.  For high resistances, you need higher
voltage lamps rated at lower currents.  A seven watt 120 volt
nightlight lamp will give you around 2000 ohms.  However, it
takes the normal line voltage to do this, which is certainly
lethal.  You probably do not want to put 120 volts on your
antenna for safety reasons.  If you can guarantee that no one
can contact the antenna, up to 28 volts _might_ be suitable.
I personally would prefer a voltage less than 12 to 15 volts
to be safe.

If you need high resistances, the optocoupler approach
discussed in the web page might be more suitable.

   73,  Barry L. Ornitz, PhD          WA4VZQ
ornitz@tricon.net

Copyright 2003 B. L. Ornitz - Please do not repost this
article in other groups or web pages without the author's
permission.