SteppIR 4 Element Yagi, Instruction Manual

Page: 29 / 36

SteppIR Antennas - 4 Element 29
RF Power Transmission with the SteppIR Yagi
The RF power is transferred by brushes that have 4 contact points on each element that results in a very
low impedance connection that is kept clean by the inherent wiping action. The brush contact is .08 in
thick and has proven to last over 2 million band changes. The copper beryllium tape is .545 inches wide
and presents a very low RF impedance. The type of balun we are using can handle tremendous amounts
of power for their size because there is almost no flux in the core and they are 99% efficient. That
coupled with the fact that our antenna is always at a very low VSWR means the balun will handle much
more than the 3
000 watt rating, how much more we don't know. Jerry Sevicks book "Transmission
Transformers" (available from ARRL) has a chapter (Chap. 11) that discusses the power handling ability
of ferrite core transformers.
WARNING: WHEN OPERATING WITH MORE THAN 2 00 WATTS, DO NOT TRANSMIT
WHILE THE ANTENNA IS CHANGING BANDS. A MISMATCH AT
ELEVATED WATTAGES MAY CAUSE DAMAGE TO THE DRIVEN
ELEMENT.
Balun / Matching System
The SteppIR has a matching system that is included in all Yagi antennas (a balun is available as an op-
tion on the dipole). Our antenna designs are all close to 22 ohms at all frequencies, so we needed a
broadband matching system that would transform 22 ohm to 50 ohm. We found an excellent one de-
signed by Jerry Sevick, that is described in his book “Building and Using Baluns and Ununs”.
Our matching network is a transmission line transformer that is wound on a 2.25 inch OD ferrite core
that operates with very little internal flux (
Figure 40 ), thus allowing it to function at very high power
levels. The transformer includes a 22 ohm to 50 ohm unun and a balun wound with custom made, high
power, 25 ohm coax for superior balun operation. Jerry has espoused these transformers for years as an
overlooked but excellent way to match a Yagi, he would probably be proud to know they are being used
in a commercial Yagi. This matching network does not require compressing or stretching a coil, or sepa-
rating wires to get a good match – something that can easily be bumped out of adjustment by birds or in-
stallation crews.
Balun
Figure 40