Jim,
I have to disagree about balanced line. Unlike coax, both conductors are
exposed to external RF fields, so common mode noise will be rejected. If
the antenna and line are properly balanced (not always easy to do, I
admit) and if it is fed through a balanced antenna tuner, there is no
reason for it to be noisier than coax. Yes, it can't be choked, but it
doesn't need to be.
My antenna is a 10m long rotary dipole fed with open line. It has gone
through several iterations and I've been very careful to install the
line so that it is perpendicular to the antenna for as far as possible,
etc. I've tried various balun arrangements, but the system that works
best, both for reducing RF in the shack when transmitting and noise
immunity when receiving, is a Johnson kW Matchbox.
There is a building taller than mine a few hundred meters away from it,
and a very distinct noise peak when the antenna is turned toward it,
which seems to indicate that the noise is picked up by the antenna, not
the line. I notice the same noise peak with a coax-fed shielded loop
antenna, so it is definitely coming from the building, and isn't an
artifact of the alignment of the antenna to the line. I am pretty sure
I have at least a 10 dB difference in noise when a band is open (at
least, when the antenna is not aimed at that building), but I will wait
until the band is solidly dead tonight to check that out.
My pattern is a form of figure 8 on 40-10 meters, but you are right that
you can't maintain the pattern over a greater frequency range.
My main point is that there is nothing inherently noisy about a two-wire
transmission line!
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
On 26-Oct-2019 10:32, Jim Brown wrote:
On 10/25/2019 2:01 PM, Bob McGraw K4TAX wrote:
Except for my 6M yagi, all of my antennas are non-resonant antennas.
My favorite is the 256 ft center fed wire {resonant at 1.825 MHz}
with a balanced feed.
Dean Straw, N6BV, retired editor of the ARRL Antenna Book and Antenna
Compendiums, is a very smart engineer and a very good friend. But his
promotion of this sort of antenna is probably the greatest error of his
time in that job, an idea whose time is LONG past, for many reasons.
Primary -- 1) it cannot be choked to kill noise on RX, and 99.9% of hams
live surrounded by local noise and 2) it's pattern is different on every
frequency.
I am a strong believer in resonant antennas for each band, if if the
must be multi-band antennas like fan dipoles to fit in the available
space. For example, a 20/15/10 fan fits in 33 ft and works great, with a
predictable pattern on each band. An 80/40 fan works on 15M, with a
predictable pattern on both 80 and 40. Hypower Antenna company sells
loaded antennas that are resonant on 80 and 40 and fit into about 100
ft; I used one in Chicago on 30 to great effect. All of these antennas
are fed with 50 or 75 ohm coax, and CAN be choked to kill RX noise.
RX noise is a VERY big deal -- if you can't hear 'em, you can't work
'em. If you haven't worked to minimize your RX noise, you're DXing with
one hand tied behind your back! My friend AG6EE goes to remote locations
in NV, OR, and CA to light up rare grids with 1kW on 6M. Folks trying to
work him complain of one-way propagation because he hears them really
well and the don't hear him, but the REAL problem is their local RX noise.
http://k9yc.com/KillingReceiveNoise.pdf Text, NCJ article
http://k9yc.com/KillingRXNoiseVisalia.pdf Slides Visalia talk
73, Jim K9YC
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