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High noise level on 2 meters

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

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Unless the receiver's NF is >3dB, "sun" and other atmospheric noise is the limiting factor in most cases so all kinds of preamps generally don't help much...if the receiver is any good to begin with.

    I was pretty active on 2m e.m.e. back in the late 70s/early 80s and never used a preamp, even for that. 7/8" Heliax for feedline, less than 75' long, contributed <<1 dB to my NF and I had no problem copying anyone who could also hear me.

    Kill the preamp.
     
  2. N4RBZ

    N4RBZ Ham Member QRZ Page

    Lots of good info for a noob like me,,, really enjoyed the read,, thanks to all
     
  3. W2WDX

    W2WDX Subscriber QRZ Page

    See ... I've had the complete different experience. I have used an FT-736r for years which has a pretty good NF on two meters, however not as good as other more modern receivers, and there is a significant difference on 2M DX reception between hearing and not hearing a station over noise. The placement of the preamp also is important since noise inherent to longer coax runs can render the advantages of a preamp useless and create more of a noise amplifier than a signal amplifier. Another consideration is the dynamic range of the front end; if it's easily overloaded than what good is it? So its gain versus its own inherent noise is also a factor (to put it simply).

    Take your 1db of Heliax noise and then stick a preamp at the end where it plugs into the transceiver. What does that 1dB of coax noise become? It certainly does not remain the same. Put the same preamp at the antenna end and what happens to the s/n ratio at the transceiver? I would bet the signal comes up substantially AND you overcome any losses regardless of how small or large, while the 1dB of coaxial noise remains close to being the same.

    Also there is great difference between using the station with grounding and un-grounded in terms of man-made or EMI type noise vs atmospheric noise (contrary to what Rege seems to think).
     
  4. AI3V

    AI3V Ham Member QRZ Page

    Coax dosn't have noise, it simply has loss.

    https://duckduckgo.com/?q=cascaded+noise+figure&t=fpas&ia=web

    Unless you live in a extraordinarily quiet location, your 2m receiver will be "noise limited" by external noise.

    The simple test is to switch between a 50 ohm dummy load and your antenna. If the noise increases when you connect the antenna no preamp in the world will make a improvement.

    A wire as short as 19 inches on 2 meters no longer acts as a "ground wire", but instead is simply a antenna.

    And no tecnique in the universe can distinguish between natural and manmade noise.

    ..........

    Imho the vast majority of stations attempting to improve vhf reception with a preamp would be better off $pending the money on a cavity or helical resonator preselector instead.

    Rege
     
  5. W2WDX

    W2WDX Subscriber QRZ Page

    So I guess the notion that common mode currents on the shield from external EMI sources do not exist and if it does it does not upset the differential on the conductors of the coax, rendering coax noiseless in all conditions. Nonsense. Boy ... I'd love to live in your universe!

    So I also guess coaxial cables also do not function if they are longer than 19" and act as antennas. Again ... nonsense. Yes ... a longer wire is less effective at VHF (higher ground loss), but who said anything about using wire or long wires for that matter. Certainly not someone who designs better. It's never perfect, but you can certainly get the impedance down to 20 ohms at VHF with some know-how. And to use nothing (since "ground does not exist at VHF" apparently) is just silly.

    Mitigating noise in a VHF system is not a single solution methodology, so I am not saying that noise on the coax is the only thing that must be considered in reducing overall receiver noise. However, specific to the OP, noise created on the coax by common mode currents from EMI sources must be reduced and certainly not amplified prior to the receivers front end. One method in reducing this type of common mode current on the coaxial shield is grounding. Filtering is another, at least in terms of broadband noise. The use of baluns is another. However, generally this type of noise on the coax is a real thing and using a preamplifier at the receiver input equally amplifies both the signal and the common mode noise.

    Cavity filters are fine (very large on 2M) but mostly are employed more for mitigating blocking issues on multi-repeater sites. I use helical filters to reduce broadband noise on 2M, which are much smaller in size and nearly as effective in this application. Additionally, some type of narrow-band filtering on the input of the preamplifier is preferred; however, is difficult to effectively implement at 2M.

    Furthermore, your comparison of noise between a dummy load and an antenna is ludicrous; and speaks nothing about the common mode noise on the transmission line, as well as the difference between that noise and the target received signal being amplified equally. This idea actually has no theoretical foundation at all. Replacing an antenna with a small load significantly alters common mode impedance of the system, and removes the ingress point entirely. Dummy load substitution significantly changes system common-mode impedance. You are basing this on your incorrect presumption that noise received by the antenna is the only noise that exists and is predominant in all conditions. The reality is, in particularly noisy EMI environments like suburban and urban locations, this common mode based transmission line noise can exceed antenna received noise levels if not mitigated. This mitigation includes, in part, not amplifying this coaxial noise at the receiver end.
     
    Last edited: Jul 16, 2019
  6. AI3V

    AI3V Ham Member QRZ Page

    Whatever dude.

    If you think "grounding" your vhf station will somehow reduce "noise" go right ahead.

    And worry all you want about "common mode", and $spend time, money and effort on "ferrites".

    I'm not saying "common mode" never is a issue, but ive fixed thousands of radio setups and never once did a hunk of ferrite make a bit of difference.

    A cavity or helical resonator on the other hand....

    Even on hf, for field day at the radio club, the item that made multi band operation possible was preselectior filters.

    It has become quite profitable for some sellers to sell "common mode" cures.

    Right up there with cures for baldness....

    :)

    Rege
     
  7. W2WDX

    W2WDX Subscriber QRZ Page

    You clearly have no idea what I am talking about. Who said anything about ferrites? I was talking about broadband noise induced in the coaxial shields and suggesting to the OP that this type of noise will be amplified, by tens of dB, using a preamplifier at the receiver end of the coaxial. I don't think you have a clear understanding of the variety of noise sources that exist on receive; be they atmospheric (the only type you seem to think is relevant), thermal, common mode, corrosion rectified, or the plethora of other sources. Nor how to mitigate each mode.

    And by the way, controlling common mode impedance via ferrites is a real thing too. You know? ... Choke baluns and such. But I guess they don't exist since for you "never once did a hunk of ferrite make a difference". And I guess they can't possibly work at VHF, type 66 mixtures and it's measurable choking impedance at VHF notwithstanding. Use of ferrites, while related, is a subject outside the scope of what I am discussing in this thread.

    I've noticed you have a tendency to talk about things in isolation. No working radio system is limited to thinking of components in isolation ... it is a system with a cumulative interaction of the whole. So the best approach is to use preventative measures in initial system design and installation. Quite often the cost of being safe is less than a few percent of the initial system expense.

    I didn't want to get into a treatise on reducing noise, especially the source of the type of noise I initially mentioned. Obviously it takes a combination of reducing ground resistance and/or adding series impedance on the cable shield to significantly isolate any low-noise receiving antenna from conducted ground noise over the feed line's shield. Also, I do agree that ferrites can be ineffective, with caveats. Decreasing ground impedance at the antenna reduces common-mode excitation of the antenna feedpoint and reduces noise ingress, although adding a feed line choke would sometimes help. There is no reason to go to extremes in choke value, because a simple ground or two, or even a buried cable, can multiply effects of any series impedance. Also, once the suppression system takes common mode significantly below antenna signal levels, any additional choke impedance is immeasurable and totally unobservable. Probably what your "hunks" experience comes from.

    One general possible isolation scheme would be to use an isolated primary and secondary in the matching transformer, and ground the feed line shield some distance away from the antenna's signal ground. This will introduce several hundreds of ohms (or more) of reactance in the common-mode signal path, as well as provide another path to earth for common-mode noise, such as what occurs using suppressors at the entry point at ground level. However, there are many methods and all are dependent on the need and vary based on frequency. Since we can't easily measure noise contribution, we shouldn't take chances. It makes no sense to gamble that unwanted signals or noise are so low that they will never contribute to noise in a special antenna installed to reduce noise and interference, such as a 2M yagi. But again ... all of this, and much more, is beyond the scope of my original suggestions on preamplifier use.

    Come on Rege ... This is basic stuff.
     
    Last edited: Jul 16, 2019
  8. W2WDX

    W2WDX Subscriber QRZ Page

    Sorry meant to say Type 46 mixtures, not 66.
     
  9. AI3V

    AI3V Ham Member QRZ Page

    Sure its basic, and thinking you can get a vhf "ground" under 20 ohms shows you havn't a clue.

    I'll type it one more time.

    The vast majority of issues with vhf and above radios, when sammy hammy adds a "preamp" are simple overload issues.

    Under the very, very best circumstances, like a eme sized array pointed into space a preamp makes a small difference.

    For a fellow running a yacomwood fm rig, and a omni antena it's a waste of money, and a very good chance of making things worse.

    I'm outta this thread.

    Rege
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That loss contributes to the usable noise figure of the station receiver; it's just an expression.
    Well, I agree and perform exactly that test. I had a (good at the time, c. 1980) ARR MFG1402 GaAsFET switched preamp at the antenna system for my 2m e.m.e. system, and it had a measured NF of 0.6 dB, but with too much gain so local stuff if aimed at could overload it and actually degrade S/N for weak signals. ("Local stuff" included out-of-band but very powerful signals from hilltop pagers @ 153 MHz and such.] I had to mistune the preamp a bit to drag its gain down but improve its intercept point, following instructions provided at the time. So, I had an ~1 dB NF preamp right at the antenna combiner. Since transmitting would switch that out of line (using very good SHF-rated coaxial relays), I could just press my footswitch with the transmitter turned off to simulate transmitting, but it only actually switched the preamp in and out of the line.

    Difference on very, very weak CW e.m.e. signals? Basically undetectable. I could hear the very weak (S0 indicated) signals with or without the preamp. So, I removed $450 worth of hardware (preamp, relays) on the combiner and just ran without it. The feedline loss was <1 dB, but the preamp should have made "some" difference...but the difference was so small I just couldn't really tell.

    The noise level does rise when I switch from a shielded dummy load to an antenna, so as you indicate, when that happens a preamp won't be doing anything useful.
     

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