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Building my own mobile hf Amplifier....Filter questions

Discussion in 'Amateur Radio Amplifiers' started by KB3HLK, May 8, 2010.

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  1. KB3HLK

    KB3HLK Ham Member QRZ Page

    Ok, I am planing a build for a 1kw mobile amplifier. I have looked at the Ameritron amp and SGC schematics. From what I can tell, they both have tuned inputs which are nothing more than Band pass filters for each band which in turn goes through a combiner network and to the PA's Then there seems to be nothing more than a Low pass filter on the output. ....I was under the impresion there were tuned outputs for each band...But it looks to me like a standard low pass filter......What do you guys say?????
  2. KB3HLK

    KB3HLK Ham Member QRZ Page

  3. W9GB

    W9GB Ham Member QRZ Page

    You will need metal fabrication skills as well as electronic assembly skills. Material costs will likely be > $600 plus your time.
    In addition, you will need to know what you are doing (HF amplifier theory), since what exists on the Internet (DIY builds) is incomplete and many sites are hacked Helge Granberg (sk) Application Notes (Motorola) notes without the proper information on transistor bias; IMD and output filtering.

    AVOID the mis-information from the pirate/illegal CB web sites.
    At least you care about RFI generated in a mobile.
    It is just a matter of time for the first lawsuit related to operations of a dirty HF amplifier -- that causes an automobile's electronic computers (and CAN bus) on an expressway to malfunction (e.g. Toyota acceleration) -- resulting in injuries or even loss of life. Even though very unlikely -- illegal operations give lawyers a reason to "go after you"

    READ VIRGIL's web site more carefully.
    Start with reading his construction manual (HF Projects)
    and a good Radio Handbook (Bill Orr's; ARRL or RSGB)

    This is the LOW PASS filter found on the OUTPUT of the RF amplifier (you are thinking of building).
    The capacitors and wound toroids are sized for the output power -- in this case (HF Projects amp) -- 100 watts! --
    You would burn up this unit with 1 kW RF output !!

    K6IF also makes PC boards for Low Pass Filters (OUTPUT)

    The various types of filters – Butterworth filter, Chebyshev filter, Bessel filter, etc. –
    all have different-looking "knee curves".

    I prefer 5 or 7-pole Chebyshev filter designs for soild-state HF amplifiers, which are sharper than Butterworth

    Matt, N6EAJ is building a 600 Watt HF amplifier (base not mobile) based on the CCI EB104 board,
    which was covered in one of Helge Granberg's MOSFET Application Notes for Motorola.


    Last edited: May 8, 2010
  4. AB9LZ

    AB9LZ Ham Member QRZ Page

    Yep, often they perform an impedance transformation as well, so the amp can "present" a 50 ohm load to the transceiver.

    Nope all you need is the low pass with a cuttoff at the appropriate point. Often designers will press one filter into service for two bands, like 30 and 20m, 12 and 10 etc.

    The folks at will have a lot of the stuff that you are looking for.

    Good luck on your project, building stuff is fun.

    73 m/4
  5. KV6O

    KV6O Premium Subscriber QRZ Page

    That's not an easy thing to do! Electrically the challenge is getting the 2KW or so that you'll need outta your car's electrical system (almost 150A at 14V - think big alternator, high idle switch, BIG copper wires, etc.), but probably the biggest challenge is dissipating a KW or so of heat - not an easy task! This is assuming you're planning on an AB configuration, which is what the other amps you're looking at are.

    Getting the thermal design right is going to be critical. The small surface area of solid state devices and the (relatively) small temperature delta is why tubes are still so popular in power applications where cost is an issue. I know hams who have built 1.5KW RF solid state amps and had to go with water cooling!

    Now, a class E mobile amp might be the way to go... :D

  6. KB3HLK

    KB3HLK Ham Member QRZ Page

    THANKS!!! That is some good info!!! I was planning on using the HF packer Filter for a tuned input and just using a LARGE Low pass filter in the output. I did plan on using 8 2sc2879 transistors and the heat sink from a cb amplifier (Texas Star dx1600)

    I plan on the amplifier being AB biased, I think thats .62 volts emitter to base at 13.8 volts B+
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You need to used bandswitched low pass filters on the output, not just one.

    A 30 MHz LPF passes 8 harmonics of 80m, 4 harmonics of 40m, the second harmonic of 20m, etc. That's important to remember. A single LPF is really only effective on 15, 12 and 10 meters. If won't help with the frequencies lower than that.

    BTW there was an earlier comment about the LPFs possibly performing impedance transformation; that's usually not the case. The impedance transformation is normally done using a wideband transformer and the filters are usually 50 Ohms in and out.
  8. K7FE

    K7FE QRZ Lifetime Member #1 Life Member Platinum Subscriber QRZ Page

    The tuned filter input will not be a significant benefit to you for the cost and space involved. Most modern transceivers provide a signal clean of harmonics, so there will be nothing to filter out on your amplifier's input. Since the amplifier is "broadband," it IS prone to generating/amplifying harmonics and does need an output filter.

    A low pass filter on the amplifier output will minimize any signal beyond the filter cutoff frequency from reaching your antenna. Lets say you select a single low pass filter with a 30 MHz cutoff. That means that if you are transmitting on 40M (7MHz) that you may also be passing a signal of twice that (14MHz), three times that (21MHz) and four times that (28MHz). So what good did your low pass filter do for you if it allowed you to transmit on four amateur bands at the same time?

    You will need low pass or band pass filters designed for each band of interest.

    Bias is not a specific voltage, rather one that is adjusted for the best linearity for your particular amplifier. It will be somewhere in the range that you mentioned, about +0.6 to 1.0V DC. The difference lies in the variables of the components themselves. Each has a tolerance within a specification range, thus you will need to adjust the bias voltage for your particular amplifier's collection of components. I suggest an IC voltage regulator/ driver transistor combination mounted on the same heat sink to aid in preventing thermal runaway. Each 2SC2879 pair (modules) should have their own bias supply and adjustment, so you will need four supplies built into the amplifier housing. Start off with the bias set for 200MA of collector current for each pair of 2SC2879 transistors. Each module's linearity should be tested and bias set separate from the other modules. When all modules have their bias properly set, their RF inputs and RF outputs may be combined in the final configuration.

    Eight 2SC2879's will provide about 800 watts output PEP with an intermodulation distortion of about -24dB. I would not run them any higher than that. Too much drive power will short the emitter base junction of the finals. Build a 3.5 to 4dB input attenuator in the amplifier box to prevent your 100 watt transceiver from over-driving the amplifier. 40 -45 watts of drive should do it. The input attenuator will also present a nice 50 ohm load for your transceiver.

    If the amplifier dos not have a fan cooling the internal components and external heatsink, add one......or two.

    This project will be a good use of parts from a former CB amplifier.
    Last edited: May 9, 2010
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Very well written, and I couldn't agree more.

    Keeping transistors linear over a wide temperature range is a challenge. Tubes are way easier: They're always hot.:)
  10. KB3HLK

    KB3HLK Ham Member QRZ Page

    I hope you all know, Im copy and pasting this good info to a document for later reference! This is great!!! I have four 3 inch fans for the heat sink and was also going to put two blowers side by side and blow in one end across the transistors and vent the other side, make a positive pressure ventilation system in the amp.

    My next question is, Why do the "BIG" builders use input low pass filters? I would think output filters to withstand 800w would be massive!!!

    My next question, is where can I find a schematics for active biasing for the 4 boards of 2 2879's?
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