For general low-band operation, I use a doublet that is 75 feet long on
each leg, and fed via ladder line to a BL-2 balun near the shack. The
fifteen feet of coax to the balun is low-loss LMR400. The low-loss coax
helps with the high SWR problem, in that the losses are minimized (for
what I can afford, that is).
This antenna loads up fine on all bands except 6m. I won't claim it's
ultra-efficient, but at least the ATU can handle it.
I chose the length of the ladder line as 61 feet, and this seems to give
good results. I'm sure other lengths could be calculated that would
work equally well.
73,
matt W6NIA
On 1/29/2017 10:17 AM, Wayne Burdick wrote:
I'd call an ad-hoc antenna that works on multiple bands with an ATU a "Kinda-Random
Antenna" (KRA). (Apologies to linguistic purists.)
A simplified definition might be:
A. long enough to work within the maximum limits of the ATU's L-network on
the lowest band used
B. presents a reasonably low impedance on all bands used (e.g., doesn't look
like an end-fed half-wave)
ATUs have limited monotonicity and granularity, as well as stray impedances, so
in practice there is a third criteria:
C. tunable on each band used despite specific L-network idiosyncrasies
This third criteria is the hardest one to predict for a given ATU design, as
the idiosyncrasies vary with PCB layout and actual component values. They may
only impact the highest bands, or for a particular antenna, the bands on which
Q is the highest. For our ATU designs, we try to minimize strays and keep the
network monotonic by using tightly toleranced capacitors and toroidal inductors.
While a wide range of wire lengths will meet the requirements of a "KRA" in the
field, we've found from experience that something in the 25'-28' range works on all bands
from 40 meters up, and roughly twice this for 80 meters up. Since it's impossible to
predict the effect of ground losses, obstructions, deployed wire angles, etc., you may
occasionally need to add or remove wire to obtain resonance on all bands used.
73,
Wayne
N6KR
On Jan 29, 2017, at 7:55 AM, Tom McCulloch <[email protected]> wrote:
I guess we need an alternate definition of "random" ;-)
Tom
wb2qdg
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Matt Zilmer, W6NIA
[Shiraz]
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