preamp questions

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KK4YWN, Dec 8, 2014.

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  1. KK4YWN

    KK4YWN Ham Member QRZ Page

    I'm talking as far as I can hear. I've ordered a bunch of parts and will be building a 100 watt amplifier. Well, that means I'll be talking farther than I can hear. So a preamp is on my todo list.

    My only experience with preamps was years ago on 11m and it was not a good experience. The preamp did a great job of making the s-meter go to S-8 and thats about all it did.

    I keep seeing noise figures in db and I'm wondering what that means. I presume the lower this number the better.

    How much gain is too much gain?

    Do those $19.95 kits on ebay and various web sites stand a chance of being worth the time/effort? Or should I plan on dropping some coin on a prebuilt unit?

    I'm going to be working my output power up in stages. Build and amp, run it for awhile, sell it off, roll the money into something bigger. Stop upgrading when interference becomes a problem. Knowing that I might reach the kilowatt level, what should I consider for switching?

    Are there any questions that I should be asking? I don't know what I don't know, ya know?

    Keep in mind that my station is very very very (and very) budget minded.
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    If you're operating H.F., you will probably gain nothing by a preamp. In any modern receiver, the sensitivity is far more than necessary to reach the point of diminishing returns....if you can detect thermal agitation noise, you have nothing to gain by a preamp.

  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    A preamp is nice if you can put it at the base of the antenna to make up for coax loss.

    Other than that it is a waste, unless your radio has receive problems.

    Good Luck on your project.
  4. KK4YWN

    KK4YWN Ham Member QRZ Page

    2m station. ssb work. i probably should have made these points earlier.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What exactly is your 2m SSB rig??
  6. KK4YWN

    KK4YWN Ham Member QRZ Page

    kenwood ts-711a into a 5 element "cheap yagi". 25w PEP output.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Best improvement is to install the preamp at the antenna and "switch" it. Many commercial products on the market can do that, like the MSP144VG-160, which can handle 160W transmitting power and has a 0.55 dB NF: These are all pretty pricey because they include a weatherproof housing, mast mounting brackets, and internal relays to get the job done; but it's the "right place" for a preamp.

    Something that may improve results slightly is a preamp for the receiver in the shack, which could be built right into the rig, like the little P144VDG: Much cheaper, no weatherproofing or relays involved; you wire it into your rig (inside the rig, on the RX input of the receiver, thus using the T-R relay switching already in the rig), and it might help if your RX if fairly dead.
  8. W1VT

    W1VT Ham Member QRZ Page

    I'd run a test--point your antennas to different directions and record the noise level, then switch to a dummy load and see how the noise level compares. It is possible to have a relatively high noise level and not really notice it just by listening.

    Zack W1VT
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Good suggestion.

    Also, with the beam pointed in any direction, if you can "hear" an increase in audible noise level from the receiver when you switch to the antenna (and off the dummy load), you don't need a preamp -- at least not, "in the rig," because your receiver is as sensitive as it needs to be for your location.

    It takes a pretty dead receiver to not hear an increase in noise with an antenna connected. Mine all do, even "stock" 2m SSB receivers like my 1987-vintage FT-736R.

    I notice preamps "in the shack" help most above about 2.3 GHz, and of course help more when remoted at the antenna. When you get up above 1 GHz or so, atmospheric noise levels become quite low, so receiver noise figures start to really matter. Not so much on 2m, unless you have a rather fabulous antenna with very narrow beamwidth aimed at quiet parts of the sky.
  10. W0AAT

    W0AAT XML Subscriber QRZ Page

    If you have space a better antenna like an M2 2m12 or 2m5wl will improve both TX and RX
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