Fixed Element Spacing and the SteppIR Yagi
First of all, there really is no "ideal" boom length for a Yagi. To get maximum gain the boom of a 3
element beam should be right around .4 wavelengths long. This would allow a free space gain of 9.7
dBi, however the front to back ratio is compromised to around 20 dB. If the boom is made shorter,
say .25 wavelengths, the front to back can be as high as 35 dB, but now the maximum gain is about 8.6
dBi. Shorter booms also limit the bandwidth, which is why right around .3 wavelengths is considered
the best compromise for gain, front to back and bandwidth. It turns out that being able to tune the
elements far outweighs being able to choose boom length. We chose 16 feet for our boom length which
equates to .23 wavelength on 20 meters and .46 wavelength on 10 meters, because very good Yagi’s
can be made in that range of boom length if you can adjust the element lengths. When bandwidth is of
no concern to you (as it is with our antenna), you can construct a Yagi that is the very best compromise
on that band and then track that performance over the entire band. It is this ability to move the
performance peak that makes the SteppIR actually outperform a mono-bander over an entire band –
even when the boom length isn’t what is classically considered "ideal". Bear in mind that a Yagi rarely
has maximum gain and maximum front to back at the same time, so it is always a compromise between
gain and front to back. With an adjustable antenna you can choose which parameter is important to you
in a given situation. For example, you might want to have a pile-up buster saved in memory, that gets
you that extra .5 – 1.0 dB of gain at the expense of front to back and SWR – when you are going after
that rare DX!
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 that results in conductor losses of -.17 dB with a Yagi
tuned to have a radiation resistance of 15 ohms, which is about as low as most practical Yagis run. The
type of balun we are using can handle tremendous amounts of power for their size because the 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 2000 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 200 watts, do not transmit while the antenna is changing
bands. A mismatch at elevated wattages may cause damage to the driven element.
SteppIR Antennas - 3 Element
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Summary of Contents for 3 Element Yagi
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