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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