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In need of low-noise RF amp for VHF -UHF

Discussion in 'General Technical Questions and Answers' started by NM7G, Mar 28, 2019.

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  1. NM7G

    NM7G Ham Member QRZ Page

    Can anyone recommend an indoor 50MHz through 700 MHz RF amp with less than 1.5 dB noise figure. I need modest gain; 10-15 dB would be plenty. I have no access to the antenna, thus the indoor type will have to suffice. It can have an integrated p.s., or I'm sure I have a wall-wart suitable for powering, perhaps with some additional filtering. BNC, SMA, and type F connectors are okay.
     
  2. W1VT

    W1VT Ham Member QRZ Page

    Those are really hard to use, as they will overload on nearby transmitters.
     
  3. WA0YDE

    WA0YDE Ham Member QRZ Page

    For receive only, I have heard of folks using Radio Shack style cable tv amps. They are often found at thrift stores for a couple of bucks. Not the best solution, but for the price, it would be worth a try. No personal experience though.
     
    KC8VWM likes this.
  4. K7JEM

    K7JEM Ham Member QRZ Page

    I bought one of these, haven't tried it out yet, but supposed to be very broad band and low noise. Gain is around 20dB at the frequencies you are looking at, with .6 dB claimed NF. Has a +21dBm P1dB level, which is pretty good. That means it can put out around 100mW before getting really distorted. Draws around 100mA at 5v regulated. For less than $5, including shipping from China, hard to go wrong:


    [​IMG]


    https://www.ebay.com/itm/LNA-50-400...397985&hash=item23aa110745:g:pvQAAOSwY5FbmLKU
     
    W1TRY and K2XT like this.
  5. KM3F

    KM3F Ham Member QRZ Page

    To add some more insight, depends on the receiver and it's noise figure/sensitivity and selectivity.
    Most of the time a ….remote...….. amplifier is used to make up for transmission line losses and long runs.
    Used at the receiver might only raise the noise level along with the signal and end up not being any advantage
    plus cause intermodulation distortion from other nearby strong signals due to to much amplification. At this point the amplifier cannot separate noise from the desired signal.
    Just so you are a where of these possibilities.
    For example; if your receiver has a noise figure of several db., an amplifier may help.
    If the receiver has good sensitivity and feed line losses are a number of db. lossy; a remote low noise amp may make up for the line loss and help hear the desired signal.
    It depends on the situation.
    One way yes, another way no.
    The very best way is to increase antenna gain first, then it overcomes noise and 'most' IMD issues, then add a remote amplifier if still needed.
    Good luck.
     
    Last edited: Mar 28, 2019
  6. W1VT

    W1VT Ham Member QRZ Page

    http://www.satsig.net/noise.htm
    You may want to calculate the sensitivity of different options. The usual approach is to convert everything to noise temperatures so the contribution from each element of the system can be added up to find the total.
    A lossy piece of coax at the feedpoint will drive up the noise temperature or noise figure and make it impossible to improve the sensitivity with anything downstream, short of a better modulation system.
     
    K2XT likes this.
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    That's the problem I've found with any kind of broadband preamp here.

    But out "in the boonies" if there are no local VHF-UHF transmitters at all, would probably work better.
     
  8. KM1H

    KM1H Ham Member QRZ Page


    Those specs mean for one signal only, not a zillion from DC to Daylight
     
  9. W1VT

    W1VT Ham Member QRZ Page

    If it doesn't work a simple 1N34 diode detector can be used to verify an unusually strong unwanted signal. Alternately, you should not see a significant shift in the DC bias when you connect the antenna!
     
  10. N3DT

    N3DT Ham Member QRZ Page

    Mini Circuits PGA-103+ but you'll have to roll your own most likely. They're only a few bucks on ebay. I think distributors have a very high min order, like 20 or so. I've made a few on some old burned up Advanced Receiver Research boxes. It's not for the faint of heart making them to work with the small devices and parts needed. I cut my own PC boards, but they do work. I haven't implemented them, but you will most likely need a basic input filter to keep the thing from overloading/oscillating outside the band you want. That was what was recommended to me when I was looking for some UHF low noise amps. There was a thread on it either ere or eham a year or so ago.

    I put a Klitz amp on my TV antenna and it works fine. It may be just as good.

    For me the big issue is the RF switching. I just haven't come up with a way to do that. I had a horrible 2 BNC relay I put in a 2M amp on the roof and it worked, but then I had to build a timer circuit to switch things off the PTT. It was a bit involved, voltage comparators and the like I was switching about 3 items in order not to hot switch anything. It worked but finally got done in by the humidity in the relay box. If I did it again I'd probably make the switches integral with the rf amp with some small commercial DPDT pc board relays. But I'm getting way ahead of myself.

    The preamp at the antenna sure made for some better NF on 2M. It was sort of stunning and I was using about 65' of 1/2" heliax, so the coax loss was only in the vicinity of 3-4dB.

    There are some commercial preamps made with rf switching and all, but they're pricey.
     

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