>> Can you be more specific about "deterioration"?
It needs a long, very long answer. Please feel free to edit, correct this
text, share with you club and publish it here again
Let's start with some basic definitions and assumptions.
1- Receiving system. Every component between your ear, starting from the
sky and ground wave, to your speaker, headphone or screen decoder. That's
include all your antennas, wires, cables preamp and all devices connected to
your radio, and including your house wires!
2- Dynamic, A good receiving system has over 100 db dynamic, that's why
AGC is there, any time the s meter is moving the receiver is reducing the
gain. It means you have gain enough or more then you need.
3- Weak signal. The thrill of work DX on 160m is all about weak signal. On
low bands the ionosphere always reflecks the signal back to the surface.
160 m or 1.8 MHz is always below the MUF. It means we have propagation 24
hours a day, every day every year. But to hear the DX signal, it needs to be
above noise, and the two important components are path attenuation and local
noise. Path attenuation is low during the night. During solar minimum is
common duct formation avoiding several hops, During sunset and sunrise there
are always some increase in signal with skipping some hops, so 10 to 20 db
increase in the DX signal is normal due attenuation void from missing hops.
The DX signal is always present and most of the time below the receiving
system noise floor. So "weak signal" is when the signal level is near the
noise floor of the receiver system.
Weak signals don't move the s-meter. Most radios s-meter is just the AGC
indicator and S0 should be near -120dBm for 2.4 KHz BW or SSB. S meter
scale was defined for phone, not for CW, or nowadays JT8. The DX weak signal
s meter scale goes down to -s5 or -s6 for digital modes.
4- Noise floor depends on the bandwidth, for cw weak signals 200 Hz is
common and 50Hz is very helpful, some modern SDR radios can copy well
signals with 20 Hz, BW, as example IC7610, K3S with APF, and or OrionII with
the new audio peak filter.
The noise floor for weak signal most of the time is near -145 dBm for CW
operation.
5- Local noise is a combination of 3 component's.
First one; Atmospheric noise or Skywave, local ground wave, it is 100%
vertical polarized. The horizontal signal is canceled by the ground
reflection, the factor is -1. When the antenna is above ground the reflected
wave changes phase and the total horizontal signal at the antenna increase.
Please see KE4PT articles about ground reflect signals on QEX , QST and
other publications, The maximum horizontal signal is at 1.5 wave length high
above ground.
On 1.8 MHz the only source of noise is atmospheric noise, no cosmic noise
below 10 MHz . During the winter the atmospheric noise can me ZERO, no
atmospheric activity means no atmospheric noise, so sky noise can be low as
VHF bands. The sky noise can be 70 degree Kelvin and requires a NF 2db or
less receiving system. At quiet locations the noise c from a 1/4 wave
vertical can de low as s2, and some dreaming DX locations near s0.
Here some of the excellent articles by my friend Kai Siwnik, KE4PT , Kai is
the editor of QEX magazine.
http://www.arrl.org/files/file/QEX_Next_Issue/May-Jun_2011/QEX_5_11_Siwiak.p
df
Second; Ground wave with manmade noise. Near ground we have
only vertical signals because the reflected signal factor is +1 and it
increases the RX signal at the antenna, the vertical RX antennas has more
gain near the ground, and less gain high above ground. Horizontal polarized
signals are canceled by ground reflection and cannot propagate on the
surface. Ground wave can be very high s9+ is very common on city lots, s2
to s4 on rural areas.
Third; Electronic noise and all unwanted signals that get into the
RX system.
Here where we define deterioration. The DX signal captured by the antenna
will show up at the input of the preamplifier.
The amplifier signal out of the preamplifier have two component's, DX signal
amplified and the internal device noise. The noise and signal are power
noise and does not depend on the impedance of the device or load, the way to
calculate the noise or signal to noise ratio is converting it to relative
temperature in Kelvin, you cannot add db with dBm, you only can add
equivalent temperature and then convert it back to db to express noise
figure. If don't know how to calculate it, there are several good online
calculators
http://www.rfcafe.com/references/calculators/noise-figure-temperature-calcul
ator.htm
http://www.rfcafe.com/references/electrical/noise-figure.htm
When the DX signal level is the same as the NF, the deterioration of signal
to signal + noise ratio is 2.3 db, this is the minimum acceptable to copy DX
signal at noise level. The RX system noise figure equal to the weak signal
result in 2,3 db deterioration on signal to signal + noise ratio( input &
output) When the signal is 10 db above the noise floor the RX system NF has
very low impact on the deterioration of the signal to noise ratio. This
define the maximum usable sensitivity of the RX system. If you want to deep
on the subject I recommend "The VHF/UHF DX book " by Ian White, G3SEK page
4 -8 .
The NF deterioration is very important during a winter day using a
horizontal RX antenna, but is not the only source of unwanted signal or
"noise" that deteriorate the signal to noise ratio.
Using a RX antenna, beverage or WF, the directivity is the only copoment
that INCREASE signal to noise ratio, every thing else will add noise, like
cable loss, SWR on the input of the preamplifier resulting in attenuation,
filter insertion loss before the preamp, any electronic component will
generate noise at the output, and will DECREASE the signal to noise ratio,
Even DSP and software noise reduction algorithm. The number of TAPS can eat
you weak signal, the DSP or audio peak filter can eat your DX and thought it
away with the noise.
Deterioration of the RX antenna directivity is so important as deterioration
of the NF preamplifier itself. A good RX antenna should have RDF above 10
db. The DRF has direct impact on the signal to noise ratio. The WF is a
wonderful RX antenna because you can actually measure the directivity
rotating it, and capturing the irradiation diagram with simple software like
PolarPlot. You can not turn a beverage antenna or measure the irradiation
diagram, it may be possible using GPS controlled drome's., not a easy task.
My measurements are very consistent on the last 10 years and for each 1 db
RDF increase the signal to noise ratio increases 2 db. A vertical TX antenna
has a RDF of 5 to 6 db, a good RX antenna 10 db, so you can assume that a WF
or a 800ft beverage with 10db RDF or better can increase the signal to noise
ration by 10 db.
Deterioration is not limited at the preamplifier port. Every single wire
connected to your radio can introduce unwanted signals and deteriorate the
signal to noise ratio. Again, the same calculation, if your RX signal is at
same level of your unwanted signal the deterioration is 2.3 db. If you RX
signal is 10 db above the unwanted signal the deterioration will be very
low.
Unwanted signal can come from several places. Here the most common ones.
1- AC or DC feed line on your radio system, in special the DC feeding the
preamplifier. On 18 MHz, a decoupling capacitor must have .47 uF or more to
show a low impedance, normal 1nf caps does not work. Choke is a must on
every feed line.
2- USB port or PC connection with your radio, again choke is a must.
3- All cables coming from you power amplifier, the TX antenna can be very
noise and the leaking from the relays can be bad as 30db to 60 db.
Considering 100 db dynamic range the leaking can be 40 dBm above your DX
signal.
4- Leaking from the TX antenna. Some radios are very bad, just as an example
the IC7851 isolation between the antenna port 1 and the port 4 used as RX,
has only 35 db isolation !!, so if the noise coming from you TX antenna at
Port 1 is s9, or -73 dBm, the antenna Port 4 can add -108dBm of noise on you
RX system. It is 37 db above the noise floor of -145dBm noise floor in CW
mode.
5- Reradiation of the TX antenna into the RX antenna. Here the beverage is
more robust than the WF because the gain is better on the beverage signal
level is 20 db strong than the WF. Using low gain antenna the deterioration
on the RX irradiation diagram can be very high, at a point that I always
being very honest, don't waste your time on low gain RX antennas if you
don't detune your TX antenna. Adding the TX reradiation with the RX antenna
pattern, the actual diagram will be an oval with a low front lob, no deep on
the side no front to back better than 5 or 10 db.
6- All wires, cable and every metal structure near 1/4 wave on the same
polarization of your RX antenna will reradiate noise or unwanted signal into
you RX signal path. That lovely 100 ft coaxial cable for your 144 MHz
vertical has the same signal on 160m as you TX antenna, it is common mode
signal from the outside braid of the 100ft coaxial cable, you bet 120ft
will generate more noise, he he.
7- All your "extras" antennas that you are not counting during your EZENEC
modeling, will deteriorate the directivity of you RX antenna, and impacting
the signal to noise ratio reducing the RDF. When you say your beverage does
not improve after a certain length is means you reach the level near your
common mode noise deterioration.
8- All cables connected to your RX system have a common point connected to
the ground, and here the deteriorator become more complex. The energy from
these extra antennas will flow into your connection to the ground, and it
will be amplified by your radio preamplifier. It is a complex issue to
observe if you have high common mode problems. The solution is to ground all
cable far from our receiver, like outside the wall inside a metal box, all
grounded with short straps or better an aluminum plate( 3 inches angle also
does a good job) . Let that energy flow to the ground outside, don't bring
it inside your radio. If you run your cables outside the tower, between
trees, coming from your window, guess what! on low bands, in special 160m
any RX antenna will be deteriorated and does not improve signal to noise
ratio. It is not "ok" like on high bands, 20 to 10 does not have the ground
wave energy the low band has.
9- Plastic box, open frame relay or switches, all of them, leaks signal
inside the preamplifier input. If you open the shield the noise will come
inside and deteriorate your signal to noise ratio. Shield is a "must have",
my 40 db gain preamplifier requires a steel metal shield inside an aluminum
shield and feedthrough capacitors .47 nF or larger, and I recommend all that
inside another metal box. A 20 db gain preamplifier is not that far as
shield requirements. Just because you see a lot of wrong and poor
construction using plastic boxes and open frame relays does not make them
right.
I can keep going pealing the onion of noise layers, you only see the impact
or deterioration of a noise source if it is the one on the surface of the
onion, after you peal that off you will find another one on the new
surface. Finding the main source of common mode noise is not easy. I used to
overkill it using chokes. Chokes are you best friend against noise, but does
not work if you have a poor ground system.
Common mode noise and/or Pin "one" problem requires a good understanding to
fix it, please check my friend Jim Brown K9YC http://k9yc.com RFI and
Pin 1 Problems here
http://audiosystemsgroup.com/RFI-Ham.pdf
Conclusion; EZENEC give us a good understanding of your RX antenna
performance, RFD can be easily calculated. Receiving antennas with same RDF
should have same performance on weak signals, but .. here is the BUT ..
EZENEC is a modeling program and it is based on the assumptions you input on
it.
Trash in >> trash out
If you remove the trash, the output is perfect, however if you don't tell
EZENEC all the antennas you have near your RX antenna, the TX tower, the
cable outside grounded like a vertical or L shape feeding your 2m antenna,
the capacitive leak from other sources, your actual irradiation diagram will
be very different from the one you see on your PC screen.
If you could turn your beverage like I can turn my WF you would be able to
see the same deterioration on directivity as the ones I show on my webinar
slides.
I hope this can define deterioration and help you to avoid it.
Good DX season, this will be just fantastic, be prepared top enjoy it using
a good RX antenna.
73'
JC
N4IS
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