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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 have noise in one direction at night. when the sun is up i have slightly higher noise when pointed at it.

    i dont think the receiver is weak. i'm probably asking more of it than most. the ssb action around me is picking up but the terrain is challenging here. a little more umph in the receiver would help.

    not sure about the antenna now that i've done some reading. i have giant pin-oaks that overhang the house. removing them is not an option: they shade the house in summer and that saves us megabucks. i could get away with a 10' boom but not much more. i'd planned on stacking another 6 element above the existing one, but the antenna was optimized for forward gain, so i'm wondering if one of these new LFA's might be better because they're supposed to be quieter.

    as always: the budget is the bottom line.
     
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    It's safe to say that if you're receiving solar noise, you're about as good as you're going to get. That's a pretty reliable benchmark for VHF-UHF receiver sensitivity.

    ERic
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, if you are hearing atmospheric noise now and it rises when aiming at the sun, no amount of increased "umph" is going to do anything for you except maybe start overloading your receiver.

    You need lower feedline loss (unless it's already close to zero), a bigger or higher antenna, or both.
     
  4. KK4YWN

    KK4YWN Ham Member QRZ Page

    feedline loss is kinda high. i have 50ft of lmr400 snaking up to the roof. then there is a ufh female to female connector. from there: 25 ft of mini 50-ohm barely shielded, crimped-end, radio-shack coax that was put up temporarily while i decide if i like the antenna or not.

    i like the antenna. its tuned a bit low but for what it is, it works pretty well.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The 50' of LMR400 should have very little loss, but the 25' of mini-8 might have quite a bit. I'd replace that.

    Still, "higher and bigger" is what works -- especially "higher."

    In a typical location (elevation = average terrain), raising a 2m beam by 10 feet higher above ground can greatly extend working radius; 20 more feet can often double it; 40 more feet can often triple it. Not based on math or calculation, based on real world observations over nearly 50 years.

    When working "weak signal" VHF (2m and higher frequencies) I've often found going from 30' above ground to 60' above ground using the "same antenna" without changing anything other than its height is often equivalent to increasing the size of the antenna x4 and leaving it at the same (original) height, in pure terms of "how many stations can be worked" under typical propagation.
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    I would not use a preamp unless it is close to the base of the antenna.

    Or you will be amplifying everything that the coax picks up. And suffer the coax loss.

    If the radio is working properly, a preamp at the radio location will help nothing.


    A Amp is not as good as good antenna height and good coax.
     
  7. KK4YWN

    KK4YWN Ham Member QRZ Page

    i cant go any higher and i cant get much bigger with the antenna.

    the plan was to build another antenna and stack it. i might be able to stack another pair but thats iffy. when i build antenna number two, i'll replace the 25ft of mini with something better.

    there was a net last night in ga. i could just barely hear the controller. i didnt bother to check in with my 25w of power, but checking into that net is my goal. a friend of mine was able to get checked in and hes at about 130 miles, which is just a few miles further than i.

    but he had 160w into a much bigger antenna.

    anyway, if i get a preamp i plan to put it on the roof. i'd like another filter ahead of it to help reject some of the digital noise i get from a nearby transmitter (i have no idea what it is but its random, desenses my rig, and digital in nature). actually, now that i think on it: i wonder if anyone has tried an active filter for two meters. that might be worth looking into.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Around here, many of us use these for 2m bandpass filtering: http://www.dci.ca/pdf/DCI-145-2H.pdf

    These are heavy and a bit pricey.

    Sometimes the noise and desense and "crap" is really just a local high powered pager operating in the 152 MHz range (usually on mountaintops or tall towers or tall buildings) and can be completely eliminated by a simple high-Q trap for that: http://www.parelectronics.com/amateur.php
     
  9. W0AAT

    W0AAT XML Subscriber QRZ Page

    More antenna gain would improve reception of weaker stations... and extend working range. When I had a pair of 2m5wls side by side on a ~15 foot cross boom at 35 feet with a preamp at the tower top my working range was 350 miles to other bigger stations. So yes you will receive more noise but you also increase signal strength and narrow beamwidth that can also lower the noise floor because less man made is picked up. Trade off is keeping them in the air through adverse weather conditions. I shelled out a Yeasu G800 in 2 years, went to a ham 4 and had better luck, broke a brake wedge once in 6 more years

     
  10. W0AAT

    W0AAT XML Subscriber QRZ Page

    Preamp I have going up next spring is built on a cavity, similar to this design http://pa5mscavitypreamp.blogspot.com/2009/11/building-cavity-pre-amp-for-2m.html decent out of band signal rejection, .35db NF(yes the lower this is the better to a point) and way more gain than I need at 27db but that I can deal with.

     
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