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preamp questions

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

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  1. W1VT

    W1VT Ham Member QRZ Page

    Even today, the MGF1801 is still a very good device for 2M preamps.

    My 2M preamp for NF contests uses an MGF1801 and a piece of tapped LDF4-50--a piece of copper hardline is likely to have less issues with the weather than a copper pipe cavity.

    Zack W1VT
     
  2. KK4YWN

    KK4YWN Ham Member QRZ Page

    cavity preamps are new to me. thanks for the tip-off guys.

    i need some test equipment. or rich ham buddys :)
     
  3. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    YWN:

    Test equipment doesn't have to require a large outlay of cash. One definitely does not have to have the latest, and greatest, test equipment. There are older pieces of test equipment that have excellent capabilities but, because they are "old", the prices are very reasonable.

    Glen, K9STH
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Absolutely.

    My bench...

    [​IMG]
    [​IMG]

    [​IMG]

    ...and that's the newer stuff.:eek:
     
  5. KK4YWN

    KK4YWN Ham Member QRZ Page

    I can almost smelll that old stuff.

    I used to have a lot of old lab equipment and very well stocked junk boxes. Life took me on all sorts if adventures and none of them are regrettable. But I sure wish I had some of that junk back.

    Anyway my shop always had that smell. I thought it was the Bakelite. If anyone ever figures out where old electronics get thier smell they should make an air freshened with it :)
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Bakelite doesn't actually have a smell. It's the other stuff inside the gear.

    Bakelite was the forerunner to "plastic," and has many of the qualities of plastic (but not quite).

    I grew up in northern NJ where Bakelite was pretty famous, and several electronics companies spun off from the Bakel factories. Even "Boontonware," made in Boonton, NJ, was actually based on formulations from Bakel. It takes incredible heat, is very difficult to burn, and their specific formulation made it nearly unbreakable -- you could slam it onto a cement floor and it would bounce, unlike "China" dishware, which unfortunately wouldn't bounce.

    Every kitchen in NJ back in the 1950s used "Boontonware," which could not have existed if not for its forerunner, Bakelite.

    P.S. I also knew Peggy Fahnestock...yes, the granddaughter of the guy who invented the "Fahnestock clip," which you still see used to this day. She went to my high school.
     
  7. KK4YWN

    KK4YWN Ham Member QRZ Page

    this is one of those cases where i'm going to ignore logic and continue to attribute the pleasing odor of old electronics to bakelite. its rare that i ignore logic.

    on cavities...

    i'm just guessing here, but it appears to be a coaxial transmission line tuned to about a quarter wavelength. the tap handles the impedance transform (?).

    from the amp-side, its a quarter wave shorted stub. from the antenna side its a less-than one quarter wave open stub. it is also a quarter wave shorted stub on some really really high freq.

    am i mistaken?
     
  8. W1GCI

    W1GCI Ham Member QRZ Page

    I wouldn't want to be to one to introduce technical theory here, but it's worth mentioning that Dr. Ulrich Rohde has written a number of papers on the amplification and mixer chain in receivers. There is a very close relationship where too much front end gain - even at a low noise figure - is harmful to the overall receiver performance. This is especially true where you have other (big gun) stations in your local area. With many of the mid range transceivers built for VHF/UHF, unless you are alone on the band, any additional pre-amplification will kill the mixer and defeat the whole purpose. If you can bypass the existing preamp (stage or stages before the first mixer) and substitute a preamp like the gasfet previously described, then you can achieve a lower noise figure without impacting the 3rd order distortion characteristics of the overall receiver. Putting an attenuator between the low noise preamp and the receiver front end can provide additional advantage with the neighbor's KW and a few Khz away from the DX station you're trying to hear. Putting additional amplification infront of a receiver that's already too front end gain heavy will only hurt your overall performance - regardless of how low noise it happens to be.
     
  9. W6RZ

    W6RZ Premium Subscriber QRZ Page

  10. W1VT

    W1VT Ham Member QRZ Page

    I found that on 1296, you can do even better with a half wave cavities instead of the traditional quarter wave cavities.

    Zack W1VT
     
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