High noise level on 2 meters

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K7DVS, Jun 21, 2019.

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  1. W2WDX

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    What is am using is a VHFDesign 2m QRO EME LNA preamp which has an NF of .4dB and an OIP3 @36dBm. It has built in 1200W rated T/R switching which is controlled by a sequencer in the shack, which controls timing between transceivers/amplifiers/preamplifier. I can bypass the preamp as well when I don't want to use it. I also lowered the gain to about 20dB for my noise floor and needed gain. The preamplifier is mounted in a NEMA box (which also houses the antenna polarity switching) mounted just below the rotor on my 2m cross polarity 10 el yagi. All the coaxials are Times LMR-400 and Times TC-400-NMC watertight N connectors. My runs are relatively short (under 70') so LMR-400 was sufficient. This is the preamp ...

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    I don't work EME and limit myself to phone modes, so getting signals high and noise low is crucial for communication. I live in a suburban community which has a fairly high EMI condition due to population density. The station would be prone to induced noise if I hadn't mitigated the egress of noise on cables. When I first began installing antennas I noticed my overall noise levels where quite high. So I began mitigating possible sources. From simply using the methods I discussed above I was able to bring my noise levels down by 3 S-units on 2M, before using the preamp. So I did see a change in average noise floor. Simply not using the suppressor at the entry point increased noise by 1 S-unit, so this "ground" seems to do something at VHF frequencies.

    When I mentioned 20 ohm ground impedance, that's the overall impedance of my total grounding system inside the shack. The entry point ground impedance for the VHF coaxial is clearly much lower.

    As I said earlier the dummy load test is pointless in determining induced noise on the shield. It only shows the difference between the antenna gain and no gain. The noise normally received on the antenna itself and other noise in the system, and nearly nothing. It's a myth to say you can test or evaluate a system for unwanted noise or signal ingress by disconnecting and replacing the antenna with a dummy load. Replacing an antenna with a small load significantly alters common mode impedance of the system, and removes the ingress point (the antenna's feedpoint) entirely. Dummy load substitution significantly changes system common-mode impedance. The conditions are removed that cause this type of noise.

    The only real test would come from a dummy load with the same connections and impedances (both differential and common mode) as the actual antenna. In other words the test load has to be the actual antenna to keep feed line common mode ingress the same. Obviously, casual dummy load substitution is a useless test! Real systems are vastly more complex than the simple analysis above.

    On a HF system atmospheric noise is substantially higher than on 2M. In most cases common-mode noise on the line falls below this floor. With the lower noise on 2M any other types of noise is more perceptible, particularly in a high EMI environment and in a system with both differential and common mode impedance imbalances. This condition allows egress of noise into the shield. You can reduce this by giving these currents a more direct path to ground. However, this is more difficult at VHF (as Rege has be harping at in isolation). This is why grounding at the entry, like a suppressor directly connected to good ground rod, helps to some degree since this type of ground offers the lowest ground impedance practical. To be clear this one step, not the total solution. A choke balun at the feed point helps. Ferrites in the shack can help in certain situations. Connections should be as high quality as practical. However, there is always going to be some residual noise on the line at VHF depending on the starting conditions causing it. This is the main point ... placing a preamplifier at the receiver end will increase all noise equally. It can't discriminate between atmospheric noise and other types of noise. So mitigating the line noise as much as possible and placing the preamplifier at the antenna improves the total S/N in the system. On weak signal SSB this is important due to modulation complexity. On CW less since our ears do the discriminating better even on EME, and digital no so much at all.

    Also you only use the preamplifier when needed, so overload as you describe is operator error. You don't have to use it all the time. And if you can't bypass it, that's just poor design of the system. Of course the gain of the amplifier should be chosen with care based on your typical weakest signal and normal overall noise floor.

    And Rege ... if someone is using a preamplifier on FM repeaters they have a problem with overall receive efficiency, like low antenna height. A preamplifier for that application is an incorrect approach. Additionally, discrimination and demodulation are different functions and S/N requirements. I use repeaters or FM about twice a year. I really could care less about FM. And we have repeaters on almost every available frequency in this area, most unused. My focus is reaching out via tropo from NY to Florida on SSB, for example. I have no desire to talk on a repeater in New Jersey or Pennsylvania, I have Echolink for that. :rolleyes:

    As far the original OP, the changes he was experiencing was possibly a connections issue, and most likely a noise egress due to varying ground impedance on the line due to changes in moisture. It also could be a complete failure of the ground connection, but most likely that would have to happen on both ends of the coaxial to allow noise to enter the center conductor. It also could be power pole arcing noise creating higher atmospheric noise when it's dry out, as others pointed out.
     

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