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?
  11. AC0FP

    AC0FP Ham Member QRZ Page

    Not much to go on here! The circuits your looking at should be basically a low pass/bandpass filter but that is not there only purpose! :eek:

    The filter on the input should be able to match 50 ohms to whatever the input impedance of the amplifier is. Likewise the output filter should also be able to match the collector (s) impedance to 50 ohms. This is one step beyond trivial!

    fp :)
  12. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    In my experience the filters are all 50 Ohms in and out and don't do any Z matching. The Z matching is done with a transformer at the input and another one at the output.

    The input transformer doesn't have to be so big, but the output transformer does.:p

    For a "home station" amp, I'd use 50V FETs instead of 12V bipolars...way easier and a LOT less current involved, which makes almost everything easier.
  13. W8JI

    W8JI Ham Member QRZ Page

    Solid state multiband amplifiers almost NEVER use a low pass filter on the input. I can't actually ever recall seeing one. Solid state amplifiers use a broadband transformer input system, and splitters and attenuators when required.

    The output of a multiband solid state amplifier is virtually always matched with a wideband transformer.

    The bias system for transistors has to be a stable voltage source that can supply constant voltage to the transistor base or gate. In a bipolar amp, the base current tries to push the base bias negative, moving the transistor deeper toward class C. This is why the base biasing has to be a very stiff voltage source. It normally tracks with transistor temperature, decreasing voltage a very small amount as the transistor temperature rises. This keeps quiescent current the same as the transistors change temperature.

    Class AB (there is no class AB1 or AB2 with transistors) is any bias that sets the quiescent collector (or drain) current at any current greater than zero but not so high it does not increase with drive. AB is a very wide class, that simply means collector (or drain) current conduction angle is more than 180 degrees but less than 360 degrees.

    What you really want to do is set the idle or quiescent collector current at whatever the manufacturer recommends for the device, or better yet whatever produces acceptable IM performance without excessive heat in the actual product.

    Solid state amplifiers never match impedances in the filters unless they are narrow operating frequency range designs. If they are multiple band or wideband amplifiers, the filters are the same input Z as output Z. For example I could build a 2 meter only amplifier that matches impedances in the output filter and single band input matching system. I would never match impedances in a multiple band or wide bandwidth amplifier with the low-pass output filter or match the input impedance in a selective input impedance matching system in a wideband or multiple band amp.

    73 Tom
  14. K7FE

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

    1. 800 watt low pass filters need not be massive if they have low losses. You could use 3/4" diameter toroid cores for the filter coils. If they do not saturate, then they will not contribute to the intermod, nor will they heat up and break.

    2. A base bias circuit for a pair of 2SC2879's is in the below schematic. My apologies. It is a 5 min sloppy drawing.
  15. AD6KA

    AD6KA Ham Member QRZ Page

    You're going to need to dissipate a LOT of heat.
    I would use a thick copper heat spreader bolted to a
    (large) aluminum heat sink. You can get this from CCI.
    The physical connection of the PA devices, to the spreader/heat sink especially with Bi-Polar is, critical. I would have a machinist
    mill that sucker perfectly baby bottom smooth and flat
    before bolting any PA devices to it and testing them...and of course use high temp heat sink compound and make sure they are bolted down tightly, even in testing.

    Not to sound like a wet blanket or anything, but

    If you really thought a solid state amp would need individual "band
    input filters" instead of a wideband torroidial transformer input,
    and you thought you found your needed "input filters" which
    were in fact low pass output filters, and you thought you could
    get away with putting one big commercial 30MHz low pass filter
    at the you think you're really ready for this build?

    I'm all for learning and research and experimentation.
    But maybe you should start with a more reasonable (lower power)
    project first. You speak of using an old CB heat sink, enclosure and stuff, and
    if it's big enough, great, but there are a lot of hidden
    costs in a project like this....not even counting the mods
    to your alternator and vehicle electrical system you will
    have to make to get this on the air mobile.
  16. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    This is a good thread, Keep it up. :)

    By the way;
    I don't seen why someone can't 50V or greater transistors in mobile service.
  17. K9FV

    K9FV Ham Member QRZ Page

    Building a KW size amp is a pretty big undertaking for first time..... you might consider a 500 watt size - much more manageable from a power supply standpoint.

    Terry could you post your bias circuit again? It's just a tiny image and not visible - or am I doing something wrong?

    Sue, I am really looking at those 50vdc MOSFETs for my next amp - they sure seem to have some things going for them.

    73 de Ken H>
  18. W8JI

    W8JI Ham Member QRZ Page


    My mobile amp uses 50V FET's. :)

    The inverter supply is a real problem with noise. When efficiency is high, noise is high.

    I modified a 120 V 4000 watt power inverter (I think it was 4 kW) to supply 50V dc. Cheap way to do it.

    It's nice being able to run almost a kilowatt on CW, but those headlights really blink.

    73 Tom
  19. KB3HLK

    KB3HLK Ham Member QRZ Page

    Im not to worried about about being able to supply power to the amp. I do alot of off roading and for reserve power I have 2 1100 CCA batteries and a 240 amp alt. I plan to run 00 cable and already have 2" ground strap to bond EVERYTHING!!!

    As for the HF project band pass filter, I had planed to use it for the input to keep the input as clean as possible, But what was said makes sense, that the radio is already clean with respect to the FCC guide lines. So Instead I will use a broad band 50 ohm transformer and that is it for the input.

    I think the CB amp I am using will be a sufficent heat sink for the 8 2sc2879's.
    The CB guys push the crap out of em with almost 1.5kw output and they last under those conditions.....I plan on 100 watts per device, so they wont generate the amount of heat the 11 meter crowd produces. But yes, Fans and compound will be used!!

    Im still researching the outfilters, I Think Im going to use filters in this arrangement. 10/12, 15/20, 40, 80 I figure with this arrangement it keeps down on parts and the cutoff is prior to the second harmonic.

    Now, Im just thinking about what to add for safety. I was thinking of a TEMP cutoff, SWR kickout. I was going to put both of these inline with the keyline and have them go open to stop the amplifier from keying....Still thinking about this one.

    Also what about using the internals from this in the amp?
    OH, and I am reading K0BG's website from top to bottom!!!
  20. KB3HLK

    KB3HLK Ham Member QRZ Page

    K7FE, can you repost the picture??
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