Rick, there are compromises with all antennas, in the SteppIR's case the
element spacing is not adjustable plus it takes time to QSY. FWIW, I just
checked the model's gain of my homebrew 20m OWA Yagi which has no moving
elements or relays that you referred to. The gain from 14.000 to 14.350 MHz
varies by only 0.1dB and the SWR is less than 1.2:1 at the band edges.
John KK9A
Richard (Rick) N6RK wrote:
Unfortunately, the pattern of a Yagi can go all to hell as you move away
from the design center frequency, no matter what your ability of match
it. The "2:1 VSWR bandwidth" doesn't tell you much about how the
pattern holds up across frequency.
I can easily illustrate this with my MonstIR. If I set it up for the
bottom end of 40, then drive it at the top end of the band, I can
use the driven element tuning (independent of the parasitic elements)
to somewhat mitigate the VSWR. But the pattern degrades considerably
and I lose several dB or more of gain.
> Like everything, there is no free lunch in engineering trade-offs.
At least in terms of performance ONLY, the SteppIR is a free lunch.
AFAIK, it beats anything else in terms of gain across the entire
band, given a boom length and number of elements. Yes, there are
various non-performance tradeoffs, but a properly working one is
tough to beat. YMMV, but my MonstIR has worked perfectly for over
10 years. BTW, the SteppIR driven element has an impedance of
22.2 ohms, and the loss in the transformer/balun is extremely low. I
measured in on a VNA before I put up the antenna.
There are alternate designs that emulate the SteppIR in a monobander
by individually tuning the elements using various modalities, such
as motor tuned or relay switched capacitors or inductors. They
produce similar results if properly implemented.
Rick N6RK
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