>> 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 _________________ Topband Reflector Archives - http://www.contesting.com/_topband
