On Wed,3/22/2017 7:14 AM, K7LXC--- via TowerTalk wrote:
I need some feedback on a multiband wire antenna.
As Howie has clearly stated, any OCF antenna will put a LOT of common
mode on the transmission line. This makes it a sitting duck for noise
pickup. It is not practical to choke an open wire line -- it must be
choked after you're in coax, and the severe imbalance places heavy
stress on a choke. Which is why Howie is using two for 500W.
There's another issue that N6BV (retired ARRL Antenna Book editor and
author of HFTA) pointed out in a piece he wrote for QST several years
ago. There can be very high DIFFERENTIAL MODE loss (that is, inside the
coax) if the standing wave pattern of an antenna that's FAR from
resonance causes a current maxima at the location of a coax choke wound
around ferrite cores. The same would be true for a bifilar choke
connected as a transmission line. No question that the loss when running
an amp could fry the choke (or melt the coax).
And, as N6BV observed, the additional loss due to mismatch can be quite
high in the coax. Remember that SWR is determined by the match between
the antenna and the line, and this is what determines loss in the line.
An antenna tuner does not change that -- it only matches the line to the
transmitter so that the transmitter can put power into the line. In
Dean's extreme example of a resonant antenna loaded on its second
harmonic, much less than 100W got to the antenna with 1.5kW going into
the line.
Bottom line -- unless you're in the middle of nowhere (NO neighbors, no
cell sites, repeater systems, etc), ANY antenna feedline is going to
pick up noise unless it's choked, and it isn't practical to choke open
wire line. The antenna will also be hearing that noise, so the
contribution of the feedline is likely in the range of 2-6 dB. Not a
deal-killer, but 6 dB can cover up a lot of weak signals that you want
to work. Like the two DXpeditions in AF that I managed to put in the log
last week on 160 and 80M. For three of the four QSOs, with 2 dB more
noise, I wouldn't have heard them well enough to work them.
Another point - in Howie's example, with the choke after conversion to
coax, the part of the feedline between the antenna and the choke is
reallty part of the antenna and is picking up vertically polarized
signals and noise. That choke forces a current minima where it's placed,
but we're dealing with RF here, not DC, so voltage and current vary
along that line just like any other antenna, and the length for common
mode is for VF=1.
73, Jim K9YC
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