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2 meter amplifier

Discussion in 'General Technical Questions and Answers' started by KD8HQY, Nov 9, 2008.

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  1. KD8HQY

    KD8HQY Ham Member QRZ Page

    looking to see if there is any information on building a home brew 2 meter linear amplifier. i know they are readily available and pretty cheap to buy already built, but i am wanting to do some experimenting and playing around to see what i am capable of building.

    more or less looking for something to "tinker" with in my spare time. plus i like the fact that i can actually say i built it, if it works.
    Last edited: Nov 9, 2008
  2. W0IS

    W0IS Ham Member QRZ Page

    I'm sure there are many designs out there. But if you're going to be using it for FM (and not AM or SSB), then it doesn't need to be "linear".

    Keep this in mind as you search for designs. You will probably find a lot more designs if you search for "two meter amplifier" than if you search for "two meter linear amplifier", because most of them are for FM and not linear.
  3. KD8HQY

    KD8HQY Ham Member QRZ Page

    thank you for the heads up. and yes it will be for FM. i have seen a few out there but figured i would ask around to see if one design is better than the next, or if someone has tried and successfully built one off of an internet design.
  4. WB3BEL

    WB3BEL Ham Member QRZ Page

    What output power level are you trying to achieve?
    What power supply voltage is available?
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    For a newer builder who hasn't homebrewed such a project before, starting out with a semi-kit that has a solid, proven design is sometimes the best thing:

    That's an example of one. It can be purchased as a full kit, a semi-kit, just parts, or just instructions: Up to you.

  6. KD8HQY

    KD8HQY Ham Member QRZ Page

    not looking for anything crazy maybe something like 200 to 300 watts. mainly just a little help while talking simplex due to the fact all the folks i talk to are around 30 miles away (as the crow fly's)
    and we don't like tieing up the repeaters for our long winded conversations.

    as far as power supply i have 2 to choose from. a 10 amp and a 35 amp, i am pretty much covered on what i would need i would think.

    thanks for the link. this is another route that i have though about taking. more or less trying to weigh my options on what would be the best for me as a beginner to making these things
  7. KA1MDA

    KA1MDA Ham Member QRZ Page

    "not looking for anything crazy maybe something like 200 to 300 watts. mainly just a little help while talking simplex"

    That's pretty funny. Would hate to see the wattage required if he needed a lot of help while talking simplex!

    Seriously- if you need 300W to talk 30 miles on simplex, I would be looking into finding a better antenna.

    Tom, KA1MDA
  8. KD8HQY

    KD8HQY Ham Member QRZ Page

    ok so you got me there, but there's no need to be an ass about it.

    and as far as the antenna goes i am using all that i can right now. space is limited and do to restrictions i am not able ot put up a out side antenna due to the apartment community that i live in.

    and as far 200 to 300 watts to me isnt much when you take into consideration that 1500 watts is legal limit.
  9. KA1MDA

    KA1MDA Ham Member QRZ Page

    "i am not able ot put up a out side antenna"

    This is just asking for trouble.

    First off, yes, the legal limit is 1500W. On VHF though, few people (outside of contest or moonbounce stations) run over 100-200W.

    Second, 300 watts to an indoor antenna is just insane. You will most likely create so much RFI to your neighbors electronic devices (not to mention your own) that you will become very unpopular. I also doubt such an installation would meet RF exposure limits.

    Although the desire to not tie up repeaters and go simplex is commendable, the end result of such an installation does not make good neighbors. You would be much better off investing in a better antenna. Even a small 3-4 element beam mounted on a camera tripod inside the apartment could offer an improvement without resorting to excessive power levels.

    Tom, KA1MDA
  10. KD8HQY

    KD8HQY Ham Member QRZ Page

    well i didnt give RFI a thought so i appreciate the reminder there.

    as far as the beam, i do have home brew 5 element that is not being used, i just might have to try it out and see how well it will work.

    my question with that is tho, with only being about 5 foot above my balcony seem like it would have a decent amount of reflect due to the close proximity of the buildings and railings.

    i am able to put an antenna up for a brief amount of time, usually during the dark hours due to the fact that i have no on site management to turn me in. so the beam just might work for my application.

    but altho i still would like to build an amplifier. if i use it then ok but even if i dont i at leas want ot try my hand at building more items, i have a few antennas under my belt and getting kind of bored with those so i am in wanting to move to something else now.
  11. N4CD

    N4CD Ham Member QRZ Page

    A 35w power supply will likely run about a 100-150W out amp...if you design it efficiently.

    Depending what power level you are at, you might need two stages of amplification.

    You'll need a good low pass filter with some loss it in.

    If you want a 200-300W amp, you'll need to either build a 24V or 48v power supply at lots of amps, or have a 60 plus amp 13.8vdc power supply.

    Remember, you are usually talking power out.....for 200w, you'll be running well over 350w in.......if you are starting at 10w....

    A 100w amp design and build is not a simple project. Heatsinking is extremely critical to not fry the transistors. Getting things stable is another major effort. It can be done, but it is not an easy first time project if you haven't build much before.
  12. WA9SVD

    WA9SVD Ham Member QRZ Page

    With all due respect, nobody is or is trying to be an A**. YOU are being reminded of the pitfalls of an indoor antenna with any significant amount of power, and 200-300 watts on 2 Meters with an indoor antenna IS A SIGNIFICANT amount of power.

    Aside from the interference potential (more like certainty) you will have to do a careful RF exposure analysis of your station and living quarters, as the RF field to yourself, other occupants and/or pets will be QUITE high, and almost certainly well beyond legally allowed limits. Whether or not you agree with the limits is moot; §Part 97 REQUIRES exposure levels be determined and not exceeded.

    See 47 CFR 97.13, which requires an evaluation for ANY station operating with more than 50 Watts on 2 Meters, and also covers exposure limits which may not be exceeded.

    Now if you feel pointing out problems, safety issues, and legal requirements constitutes being an A**, so be it.
  13. K9KJM

    K9KJM Ham Member QRZ Page

    I have to agree with the others.
    Especially at VHF and UHF, The ANTENNA AND the coax feed line are of the utmost importance. About 2 or so watts is all that should be needed to talk only 30 miles on two meters.
    There are lots of things that can be done even in the case of "restricted" antennas. (Disguise antenna mounted on a sewer vent pipe, etc etc)
    LOW loss coax is needed. Something like "Mini-8X" is good for about 25 feet of length only. For any longer run, Up to 70 or so feet, Get some Times LMR-400.
    ONLY after you have the best OUTDOOR antenna and feedline you can put up should an amplifier be considered.
    I stress outdoor, Because you have apparently not done the RF calculations on a nearby indoor antenna........
  14. W4INF

    W4INF Ham Member QRZ Page

    Thanks for the link to concepts page, I forgot about them. A nice 75w 2m amp kit for $120, sweet! I could use that with my IC-706 "Classic" which has only 10w max.

  15. KA9VQF

    KA9VQF Ham Member QRZ Page

    I routinely talk to people up to 30 miles away with my HTX-202 and a homebrew ¼ wave antenna. The 202 delivers about 5 watts output and the antenna is on top of my house at about 35 feet and is outside.

    I can hear the fellows with the TV tape j-pole inside the house but they usually can’t hear me very well if at all.

    There is a real simple cubical quad antenna that you can literally make out of coat hangers and sticks. If you are an ARRL member do a search on their site for the ‘Repeater Eater’ if not just type in 2 element 2 meter quad and spend the rest of the day reading on the several many sites that your favorite browser gives you.

    I built one out of regular 14 gauge copper wiring used for house electrical systems and some pine sticks I had handy. it’s a real bomb compared to a copper cactus j-pole even if you just hang it from your ceiling.

    The one I built was actually light enough to hang from a thumb tack but I used a plant hanger hook instead since it was already in the ceiling.

    They are simple enough to build that you may want to build several and point them out different windows of your apartment in different directions. They are easy to set as either vertical polarization or horizontal if you ever decide that 2M SSB is something you want to try.
  16. PA5COR

    PA5COR Ham Member QRZ Page

    The best 2 way linear is a good antenna system, it works well recieving as well transmitting.

    25 years back i build a 60 amp PSU to feed the homebrew 2 meter lineair amp, with 2 MRF 247's
    From the Motorola RF book i got the data, no computers then, just the Sinclair ZX 81 :D

    Still have it, run it once in a while, but 50 watts in a good beam on 2/70 does work fine here up to 100 miles in SSB

    Try to improve your antenna setup, coax etc, outdoor antenna.

    That will work recieve\transmit.

  17. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    As touched on earlier, a "35 Amp" power supply is not sufficient for a 200W-300W amplifier, period. Won't get you there.

    To run 200W output requires a minimum of 400W DC input power. At 13.8V, that's 29A. That doesn't include the current draw of the rig you use to drive it, which is probably another 7-9A. And the Astron "35A" power supply is actually 35A peak and 27A average current rating. It won't do it.

    At 300W output, the picture gets worse as the power supply for the amplifier alone will have to deliver 43-1/2A (just for the amp, not including the rig to drive it). So, you'd really need something like an Astron RS-70M (70A peak, 57A continuous rating) to get the job done from the power supply end.

    As others have pointed out, the antenna does most of the work in communications systems. Using a small collinear phased array here at home (only 10' tall overall, it's 1/2 wave over 1/2 wave in phase, housed inside a fibreglas tube with some rod radials at the base) only 20' above my roof strapped to the chimney, I work other home stations on 146.52 simplex who are 50-70 miles away all the time, every day. They're strong. Stations only 30 miles away frequently pin the meter on the rig. That's not far at all for two meter simplex. The work's all done by the antenna. I almost never run more than 25W output on 2m FM from home, it just isn't needed.

    If you really need the heavy horsepower (200-300W) to get the job done, you'll definitely need a different power supply and be prepared for a fairly interesting project. The transistors to run this power at 150 MHz will cost over $100 just for those parts, not including any other parts or the circuit board, heat sink, relays etc that are required.

    For "power" on 2m at home, I don't use solid state stuff at all. I use my homebrew kilowatt amplifier with a pair of 4CX250Bs in push pull. That runs 800W carrier power output with only 4W drive, class AB2. I use it on SSB-CW sometimes, and almost never on FM.

    For a "little" 300W amp, I'd just build up a single 4CX250B unit; it's actually less work to build than a solid state 300W amp and probably costs less, since I have all the high voltage parts on hand.

  18. WA7KKP

    WA7KKP Ham Member QRZ Page

    tubes or solid state?

    Solid state amplifiers use specialized capacitors, and they're kinda hard to find. And they're limited to a practical output of about 160 watts or so.

    For the power range you want, I'd look into a HB 4CX250 amplifier -- takes 10w or less to drive and they'll give you a good honest 250-300 watts output if you have the full 2kv plate voltage at 250 ma plate current.

    Gary WA7KKP
  19. WA9SVD

    WA9SVD Ham Member QRZ Page

    i'd still counsel against the amp of any kind. Running 200-300 Watts into an indoor antenna on 2 Meters (or any band , for that matter) is not the best advice.
    Last edited: Nov 10, 2008
  20. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Let me join the chorus here.

    I used to run 250 watts on FM, both at home and mobile. In view of the RF safety standards, the mobile installation was probably exceptionally dangerous. Oh, sure, I could work any repeater that I could hear, and out on the open plains, that was nice. But in both places, with an omni-directional antenna, I was generating QRM for people in all directions, not just the desired one, and this is pretty poor practice.

    As others have mentioned, a good outside antenna will do more for you than this amplifier, with a lot less attention from your neighbors.

    I've lived in apartments that didn't allow antennas, and the managers would have gone nuts if they knew I was actually transmitting from inside them. To keep peace and still enjoy some real DXing on VHF, I started hilltopping (mountain topping in California). Build yourself a nice, portable yagi and a mast you can set up and tear down quickly, then go find a decent hill. Consult a topo map and get a GPS for finding those real high spots.
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