[Antennas] RE: Loading coil design software?

2002年12月12日 02:13:03 -0500

Rob Matherly, W0JRM, asked:
> Can anyone point me to a program (preferably free) that will aid me
> in designing a shortened 40 meter antenna? I'm primarily needing
> something that is focused on loading coils. What I am wanting to
> design is something similar to what was featured on page 14 of the
> October 2002 CQ.
>> For those that don't receive CQ, the antenna is a 6 foot telescoping
> whip loaded by a coil. The entire assembly then screws onto the so-
> 239 of your rig. I'm wanting to play around with different whip
> lengths to see if I could manage to squeeze a little more RF out of
> it.

I know it is a matter of personal taste, but I write software
if I have to do a large number of repetitious calculations.
Otherwise I use the handy pocket calculator, and to me, this
is a job for a calculator.
There are two simple equations that come into play here. One
determines the radiation resistance of a shortened monopole
and the other determines the capacitance. These simple
equations have been published in every edition of the ARRL
Antenna Book since before I first got my novice license (38
years ago). These equations are generally found in the
chapter on mobile antennas. A series inductor, sized to
resonate with the antenna capacitance, is used in series with
the antenna as the loading coil. Once you know the inductance
needed, another simple equation from the Handbook can help you
design the coil.
To optimize these shortened antennas, they need to be made as
long as possible. An eight foot whip will work better than
the six foot whip described. Essentially it has more
capacitance, so it takes a smaller inductance loading coil
(meaning less coil losses), and more importantly the radiation
resistance is higher. A capacity hat on the whip will help
too, as will making the whip out of a higher conductivity
material than stainless.
One problem you will likely find is that an antenna such as
this plugged directly into the rig will place the rig in the
near field of the antenna. Many modern rigs are not shielded
well enough to operate properly in such high fields. Expect
to see RF on the microphone or key cords. I have even seen
microprocessor-controlled rigs completely lock up when used
this way (and I once saw a rig receive permanent damage from
such activity). Proper grounding of the rig is vital with
such an antenna.
Also such a shortened antenna will have a high VSWR unless
attention is paid to having a matching network in addition to
the resonant antenna. Even with such, the operating bandwidth
will be small.
Having said all of this, let me add that with lots of care,
shortened antennas can be fairly efficient. It takes an
exceptionally good ground, however. If you can make the whip
at least an eighth wavelength long, most of the critical
nature of these antennas can be avoided. Of course this is
about 17 feet at 40 meters - not the sort of whip you want
plugged into the rear of the rig!
 73, Barry L. Ornitz WA4VZQ [email protected]