KØFF Homebrew Tips
Balanced
Preamp and Tuner for Frame Antenna
by KØFF
The original design for the
electronics for this project are credited to Lyle Koehler, and detail can be
found on his webpage at:
http://www.computerpro.com/~lyle/bal-pre/bal-pre.htm
Simply stated, four
Varactor diodes located right at the antenna are used to resonate a frame or
other loop, controlled from an indoor box. An outside rated unit houses the
tuning diodes, as well as preamp of the familiar JFET input/ bipolar output
design theory. Lyle has gone a couple of steps further by including a balanced
design, and also stabilizing the FETs with a current source power supply, and a
transformer output.
These are the construction
details of the example that I built for use with a 10 foot frame loop with 10
turns of #16 Ga wire, owned by N1LF. Les’ main interest is LF but there is no
reason this same circuit can’t be used for BCB or 160 meters using smaller
loops. If your interest is in shielded loops, or SLOOPS, this circuit can also
be used to remote tune them as well.
Using 4 of the Varicap
diodes gives a capacitance swing of 70 to 770 pF, and more or less could be
used to tailor the capacitance swing needed.
Of course the frame loop
and the Shielded Loop or SLOOP are receive only antennas and should not be
transmitted into under any circumstances.
Refer to Lyle’s webpage for
the electronics, but I will continue to describe the mechanical details of this
example.
PHOTO
#1 shows an overview of both the control box and the outside unit. In this
case, I could not find a suitable cabinet for the indoor unit, so one was
fabricated out of heavy gauge aluminum stock.
The outdoor unit utilizes an electrical box made for wet
environments. Two H.H. Smith terminals connect to the loop wires
(This version does not use the center-tapped loop; otherwise there
would be 3 terminals). Direct mounting to a groundrod is accomplished via the
socket and screw seen on the bottom panel. If circumstances prohibit this, then
a brass screw for direct grounding is utilized
Photo
#2 shows the backside of the outdoor box, and the method of attaching the
SO-239. Here a silver plated/ Teflon SO-239 is secured with stainless steel
screws into the heavy aluminum boss at the center point, and a homemade rubber
gasket seals against moisture.
Photo #3 gives some details of the
circuit board and internal wiring of the outdoor box. The circuit card is one
of my Proto Boards and uses a grid pattern on the back, and a groundplane on
the topside. Only one jumper was needed in this layout and then only because I
wanted to keep both halves of the preamp balances in every practical way.
Ferrite beads on the control wires help to eliminate RF ingress via the leads.
Photo
#4 gives a different angle and here you can see the desiccant capsule and
neon-bulb spark gap.
Using a spark gap across a
transformer-coupled output is a very effective hedge against lightning surges.
If a surge does come in, then the transformer winding itself is a shunt to
ground. What the gas tube is for is to dampen any ringing voltage actually generated
by the inductance of the coil itself, if excited with a fast rise time current.
There is little energy ever dissipated in the gap when used in this manner.
Photo
#5displays the simplicity of the control box, and allows fabrication detail
to be examined. This cabinet is very simple and made with non-sophisticated
tools. All bending was done on a $15.00 brake that gets clamped tighter with
“C” clamps.
Control of the diode bias
and therefore the capacitance is accomplished via a simple voltage divided and
a 10 turn linear pot of 10 k Ohms.
Single point grounding
techniques were followed.
Coming off the coaxial
power socket is a leaded component that is a PicoFuse, to protect the power
source if something shorts out the control wires. Additional fusing should
always be provided when small wires are used to feed a voltage to some device,
as a fire precaution. If a short, or worse, a high resistance short occurs, the
wires can overheat and catch on fire, or cause a fire, long before the 50 amp
fuse in the shack power supply blows out. Always fuse the circuit according to
the wire size in use.
show some more details, and
the ubiquitous “Do Not Eat The Batteries” label. 73, Geo
