Inductors can be made from commercial coil stock or hand-wound, or surplus
coils can be used.
Large ceramic coil forms can be found at many hamfests,
cleaned up, and used for winding coils. Military surplus coils
on ceramic forms are easy to find. Some are suitable and can be used as-is,
or new windings can be put on the forms. Note that many of the
coils offered at hamfests do not have enough turns-per-inch to obtain the
inductance needed. Since this kind of ceramic form is usually notched to set
the wire spacing, those particular forms are useless for bobtails. It is a
good idea to know what diameter, length, wire size, and turns-per-inch will
work on your band while shopping at the hamfests. This will prevent
wasting time and money. A copy of the inductance formula and a small
scientific calculator would be very useful items to bring along. I have used
the formula so much I have it memorized.
For these frequency ranges, at moderate power levels, most wire coils are going to be in the
2-4 inch diameter range, and about 3-6 inches long. As an example, one coil I
wound for 80 meters was made from 26 turns of #12 awg copper wire on a 2.5
inch diameter ceramic form. The winding was 3.7 inches long. Calculated
inductance was 22.7uH. To make the network easy to tune when a coaxial
capacitor is used,
the inductor can be wound for a little more inductance than needed and a 'top
tap' can be used to find the resonance.
You will find a direct reading LC meter such as the AADE LC Meter II (or similar) very handy
for working with these networks- or if you are only instrumented to measure L, or only C,
then in addition, a Grid Dip Meter is very helpful to determine where your
resonance frequency is.
One caution for
home builders- some materials are too lossy to be used for coil forms. Avoid
PVC (polyvinyl chloride) tubing and similar plastics used for plumbing.
Good materials for forms include
ceramic, fiberglass (like G-10 used on printed circuit boards,) and many
clear 'engineering' plastics such as Plexiglass® acrylic and Lexan®
polycarbonate.
A note to those who may be squeamish about building an LC tank circuit:
It would certainly be possible to dispense with the LC tank and direct
feed the antenna 'tail' at ground level using open wire line and a balanced
antenna tuner, if you prefer. This approach has its own problems and
challenges, but does do away with the requirement for the LC tank for
voltage feed. You would need to assure that the open wire line is spaced
up sufficiently from the ground (say 4-6 inches for 450 ohm window line),
and you should put some
twist on the line over the full run, to help maintain balance. You also
need to make sure you deal with any potential tripping hazard from the
balanced line near ground level. It would probably be best to gradually
slope it up until it's at least 8 to 10 feet high.
Coax is so much easier in this regard-
simply lay it across the ground, or perhaps bury it just slightly. See the
article by K3BC on the Bibliography page for an example of a balanced feed
arrangement.
I would be concerned about possible antenna/feedline interaction when using
balanced line. I certainly encourage the reader to go ahead and get their
feet wet with building LC networks, if they have not yet had the experience.
They are the basis for all HF tuning and matching networks a ham might
build. With coax feed on the low bands, SWR is generally low so that losses
in longer runs are not much of a concern. As a bonus, if you are able to
get hold of some cheap or free CATV hardline, as I have, you can have some
incredibly long runs (at low SWR) without incurring much of a power loss
at all. Open wire is only really needed if you are going to be running
under high SWR conditions- then it is the hands down winner.
For Bobtails, open wire feeders
would need to be run away at a right angle from the array. For the Half
Square, it could also be run away from the end being fed, probably for
about a half wave or more before changing direction, in all cases.
Regardless of whether you use coax with an LC tank or you use open wire
feeders, your 'return' should consist of an appropriately sized ground
screen- ground rods are optional and not really necessary or helpful.
Radials at this point are definitely not desirable, and might cause
significant problems if improperly applied.
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The trick is to trim back a little of the outer jacket at the high voltage
(center conductor)
end and then cut off the braid flush with the jacket. Each inch you remove
is worth 2.5pF- this is a slow tuning rate of about 54 KHz per inch at 3.5 MHz
using the inductance value from the table. The rate is about 208 KHz per inch
at 7 MHz, however, so it pays to make smaller adjustments
on 40 meters. The coax can be coiled or bent to take up less space.
Minimum coil diameter of the homemade
capacitor must be ten times the coax diameter, or about 5 inches.
A little more is better. Only polyethylene dielectric coax should be used,
never foam! Although you can make direct connections using the coax center
conductor and braid, soldering some smaller diameter wire leads to them makes
things a little easier, especially at the braid end. #14 awg is a good choice.
Remember to wire the coax braid to ground to avoid the possibility of a hot
braid arcing. Keep in mind that the center conductor at the braid end is hot
and must be clear of other objects.