Model TRX-Kit
Determining the Right Number of Secondary Turns—The primary is provided with a
winding of three turns of #20 enameled wire. The table below shows what impedance
ratios can be achieved with various secondary windings using the insulated #26 wire provided with the kit:
Receive Antenna Transformer Kit
SECONDARY
Z RATIO
USED FOR:
3 Turns
1:1
4 Turns
1:1.8
Not commonly used
The TRX-Kit includes the materials needed to make two high performance transformers for
receive antennas. These instructions show you how to make transformers for several different impedance ratios—as used for Beverages, K9AY Loops, pennants, flags, EWEs and
other types of receive antennas. The kit includes two gas tube surge protectors with a 75 volt
breakdown rating and a high current discharge rating. The surge protector helps reduce damage due to induced static charges from storms, dry winds, and other natural phenomena.
5 Turns
1:2.8
Not commonly used
The TRX-Kit transformers have separate primary and secondary windings. This allows you
to have separate antenna and feedline grounds, which can greatly reduce unwanted commonmode currents. The windings have excellent isolation for pennant, flag and other groundindependent antennas that require minimum coupling across the transformer.
The transformers use four 1/2-inch diameter toroid cores assembled into in a “binocular”
configuration. The large amount of ferrite material assures excellent low frequency performance, including frequencies below the AM broadcast band. The winding method results
in very low loss—for example, a pennant transformer made with the TRX-Kit will have
about 1/2 dB loss, compared to 6 dB or more in the single-toroid transformer often recommended for pennants or flags!
The large size cores also enhance the ability of the transformer to withstand surges. The
transformer should be able to withstand short, accidental transmissions up to 100 watts, but
they are not guaranteed against such forms of abuse, intentional or not!
The kit includes the following parts:
(2) Transformer core sets with pre-wound three-turn primaries
(2) Surge protectors
(3 ft) #26 insulated wire
50 or 75 ohm common mode isolation
6 Turns
1:4
7 Turns
1:5.4
Low-height Beverages
Beverage-on-ground
8 Turns
1:7.1
75:532 ohms — Beverages with 75-ohm feedline
9 Turns
1:9
10 Turns
1:11.1
Pennant, EWE, flag with 75-ohm feedline
11 Turns
1:13.5
Most common winding for pennant, flag or EWE
12 Turns
1:16
50:450 or 75:675 ohms — Beverages, K9AY Loops
Pennant, EWE, Flag
The choice of cores and primary winding is optimized for the most common windings of
9 turns (1:9 transformer) and 11 turns (1:13.5). A 1:9 transformer will transform 50
ohms to 450 ohms or 75 ohms to 675 ohms, which will work well for Beverages and
K9AY Loops. A 1:13.5 transformer will transform 50 ohms to 675 ohms or 75 ohms to
1012 ohms, which is ideal for pennants, flags and EWE antennas.
These transformers provide the high common mode isolation needed for ungrounded
antennas. You may have heard comments that a “binocular” winding is not acceptable for
pennants and flags, but be assured that those comments do not apply to the TRX-Kit
transformers. The capacitive coupling between the 3-turn primary and the secondary
windings is very small. Although common mode isolation is difficult to measure accurately, we estimate that these transformers have 40 dB isolation in the 160 meter band, which
will provide full performance from pennant and flag antennas.
The surge protector provided with the kit has a capacitance of just 1 pF, and will not
affect the performance of the transformers below 30 MHz.
Winding Method—Simply thread the #26 insulated wire through the cores. “One turn”
means that the wire passes through all cores. (Note that the pre-wound primary is three
turns.) The preferred technique is to cut a sufficient length of secondary wire, fold it in half,
and—from the side with the primary leads—poke the wires through each side of the core:
TRANSFORMER CONNECTIONS
Beverage, EWE, K9AY Loop:
SECONDARY
PRIMARY
ANTENNA
FEEDLINE
Alternately wind each of the protruding wires until you have the desired number of turns:
ANTENNA &
SURGE PROTECTION
GROUND
By winding this way—one side, then the other—the windings are evenly distributed over
the primary winding. And, when you are finished, the two ends are on top, not buried under
other windings. Be sure all windings are snug, so they cover the lower windings evenly.
When you are finished, use a dab of glue to hold the secondary windings in place. Hot glue,
Duco and super glue are all OK. “Liquid electrical tape” is also a good choice. It is important to keep the windings snug—excessively loose windings add loss.
Your finished transformer should look something like this:
PRIMARY
(enameled wire)
FEEDLINE
GROUND
Pennant, Flag and other Ungrounded Antennas:
SECONDARY
PRIMARY
(UPPERMOST WIRE)
FEEDLINE
ANTENNA
SECONDARY
(insulated wire)
SURGE PROTECTOR
MUST BE GROUNDED
TO OPERATE
FEEDLINE
GROUND
The enameled primary wire has solderable insulation—a hot soldering iron will melt
through the coating and allow the solder to bond to the metal. We have found that it helps
to lightly scrape some of the insulation off the ends before soldering, as it seems to speed
up the heat transfer from the soldering iron to the wire.
Now that the windings are complete, you are ready to install the transformer in your choice
of enclosure, using the types of connectors you prefer, or attached to longer “flying leads.”
At this time, you will also connect the surge protector, which has different connections
depending on the antenna type. Pictorial diagrams showing the correct connection method
are included on the next page.
AY Technologies • www.aytechnologies.com • Antennas by K9AY