The AM Forum

I have heard of late of some homebrew mobile linears in both the design stage and even on the air.  Rumer has it a least one has been heard on 75 and that others are in the works. 

Well if you have something cooking, tell us what you are up to.  Which basic design?  What kind of transistors?  RF Mosfets? Switchmode mosfets?  How are you dealing with the 12 volt issue?  Is anyone switching in batteries in series for TX?  RF power levels?

I too am interested in a home brew mobile linear. I have a 75 meter linear on paper using switchmode fets.  But alas not enough time to develop it. I run 75M mobile...and piss weak at that.

Peter

The Classic Heathkit Kilowatt Mobile Amplifier  HA-14

http://www.qsl.net/w6ovp/


/Dan

Many switch mode FETs will work well in linear mode on 75m. At 12 volts a 1:4 output transformer will give you a bit over 100 watts into 50 ohms. A 1:5 you might get 200 watts PEP if the alternator is up to the task.  Metron had 4 P-P amplifiers and was rated 800 watts PEP if I remember. Now if you double the voltage 4X the power as long as you can dissipate the heat. No free lunch at 12 volts. It is all about the output turns ratio.

You need to contact K1KBW Bob. He is the one running the amps.

kc6mcw joe, Built a 16 transistor 2sc2879 amp.  It is fully bandswitched and tuned for each ham band. He can run 8 transistors or 16 of them. He runs 8 as he only has two astron 70 amp supplys at the moment.  The amp produces 300 watts AM with plenty of headroom.

C

Many new vehicles come with 100-120A alternators and larger are optional. Shut off the audio crap and run some serious power.

And Ken, KC2UDZ   He's working with Bob on that project.  I saw some nice pics of Ken's installation but somehow lost them.

Al

TNX all for the posts.

Terry and Al...TNX for the info about Bob KBW and Ken UDZ.  I had heard UDZ on the air but did not write down his call.  I sent a note to Bob asking for details of what he is up to.  Why re-invent the wheel if someone has something designed already.  It would be nice if they could post something here.

Frank....exactly yes, I was thinking of using switchmode fets in linear mode.  The current thought is to run another battery in series for 28V supply and in parallel for RX.  Can you recommend a suitable FET for 100W carrier? 

Pete

I talked to Bob on Thursday evening and among other things we talked about the amps. They are using switchmode FETs, about $1.50 each, 4 fets. Have it working good on 160 and 75, lower output on 40 as would be expected with those FETs. I think he said he was getting 100watts and a bit more carrier out of it mobile, which is a nice step up from the usual 25w carrier AM barefoot. Getting enough power out to explode a hamstick with one good "Yello!" he said. They have had to home brew mobile antennas out of #10 wire, and he said that gets warm too.

They have been building them as mono banders so far, and are still fussing with it a bit. He said he just ordered up another bunch of FETs for another round of experimenting. I think they are still working on the biasing from what I recall. All running on 12 volts, no fancy footwork with power supplies so far.

I think he said there was an older schematic on his or Ken's site, but it didn't represent the current build and had some issues. I've heard Ken on his and he puts in a respectable mobile signal up here in Ithaca.

I'd love to get something like that in the mobile for 75 meters. Might get the screwdriver a bit warm though.  ;D

Those guys are using 260n's the same FETs Steve uses in his modulators.
FQA11N90s will also work but only rated for 11 amps. It might even be easier to build an inverter to jack the voltage up. Then for AM just feed audio into the PWM chip. A 1:2 output transformer at 48 volts will make 300 watts pep.

Peter,
Ken built the first amp using IRFP260Ns using a common bipolar CB amplifier board.  It worked!  Since then he and Bob have built versions using larger ferrite of known material. But both designs still lack any real engineering to optimize the design. The goal here was to have fun and maybe make smoke.  They achieved more of the former than the latter, much to their credit.

Bob's construction seems to exhibit self destructive instability when the bias is brought into the active region. So it's pretty much class B at this point.  Several other issues of capacitor quality, tuned vs. broadband input, feeedback, etc have been discussed, but again, no numbers have been crunched.

So, after having commented extensively about their design's possible limitations I decided, until I get a chance to start building one, I'd at least get some reasonable modeling done on some designs of my own.  If your interested I'll add you to the email list as we develop the model further. 

As Frank noted, it's all about turns ratio, sort of.  I prefer to consider it all about matching drain to load. Since the goal here is to make a very cheap amp that will tolerate a 14 volt supply.   Therefore, currents are high and transformers, and their ferrite characteristics, become a big issue.  We've started using the FB-1020-43 cores used in Steve E rig designs. A ferrite for which B-H curves are lacking for this application. So, among other things I am bench testing cores to determine how many amps we can pull and how many beads we'll need.  It's all fairly preliminary at this point but I'm pretty sure they're well saturating at 5:1 and pretty much wrung out at 4:1 too.  It looks to me like something with a Uo less than type 43 will be required.  Id of 30 to 50 amps are easily pulled at 5:1.

Also, current derived feedback will be a key feature in my design for stability and gain control.  Bias circuits are also a major talking point for both gain control and IMD reduction. 

In the meantime, Ken and Bob have stuff on the air using two modules and simple combiners using bifilar wound toroids.  I'll keep you in the loop, or torus as the case may be :)
 

Mark

This one sure seems popular.  ITs a simple 4 transistor CB amplifier design with band pass filters.


http://www.xs4all.nl/~pa0fri/Lineairs/HLA300/hla300eng.htm

C

I have simulated an amp with 22 11N90s runing 80 volts that will do over 2 kW out. Also tried one at 130 volts whick will do over 5KW. 11N90s work fine up to 80 meters but the efficiency goes bad at 40 M. A pair of amps with 1:4 transformers will do about 200 watts pep and a pair with 1:5 transformers will do around 400 watts pep out into a 50 ohm load. you can play tricks with the transformer ratio after the combiner to reflect a lower Z at the amps and get more power. There is no free lunch when you talk linears. Driving above that power will not be linear.

Ken, KC2UDZ is currently running one he designed in his truck... he is pretty loud a lot of the time from southern NJ up here to north of Catskill NY...

                 _-_-bear

I recall an design in a Handbook from the mid sixties that added a second alternator and brought out the three phase AC. Reverse connected filament transformers and three phase rectifiers made HV for the toobs.

I remember that too, wasn't it in the Bill Orr handbook? If I'm not mistaken it was a full KW amp, and the tubes were water cooled.  ;D

14th Ed  The Radio Handbook.

Two water cooled Eimac X531 tetrodes in parallel. 3 phase alternator feeding 3 plate transformers. 120V pri, 2200V center tap.......   used a fuel pump to cool the tets.



klc

Mark,
I have worked with those cores in HF BB amplifiers. I used a pair in an MRF429 amp at around 40 volts with DC going through the transformer. 160 meters a pair ran hot and 75 meters 2 were acceptable. This is at 40 volts and less current. Current will reduce the permability so maybe 4 cores would be better. I prefer type 61 material with transmission line transformers. All the work I did with the Erbtech amps convinced me trye 61 is better for broadband operation and will work 160 through 6 meters. Only ui=125 so turns need to be adjusted to get minimum reactance. Layout is very critical in a low Z linear amp. Don't worry about matching the drain to the load consider the load transformed on the drain. RDS on just needs to be low enough withoug the gate getting too hard to drive. Also consider shunt feeding the drains to keep the DC off the transformer. This will make life easier for permability.
I bet 2 cores would work with shunt fed DC on 75M

Frank,
  We're on the same page. I agree that the lower u core would be preferable at the higher currents expected at 13 volts.  The 43 cores are being used for the current prototypes because they were readily available from all the E rig work being done.  We all know higher voltages will work better but that's not part of the design criteria here, which are, 13.8 volts and cheap semis. Not necessarily in that order. :)   

And shunt feed is also my preferred method of providing DC but not just for getting DC off the xfmr but deriving a current feedback source instead of a voltage source off the drain.  I'm also considering applying that feedback in the source lead, not unlike the Norton Noiseless Feedback technique. Doing this may ameliorate some of the layout issues as well since ground fluctuations will be more readily canceled, or at least swamped by the feedback voltage.  Modifying Vgs from the source may have other benefits in terms of gate Z. We'll see... I will eventually get some bench time on this. For now it's just model-play.  In the meantime, Bob and Ken are smokin' from the mobile and nothing's blowing up. :)
Thanks for the input.

Mark
   

Just wondering if a simple switch mode DC-DC booster circuit would be useful here. Would taking the MFJ 4416B, and modding for 15v output give a useful boost when using 13.8V transistors? In stock form they can transform 9-12V battery to a fixed 13.8V at 20+ amps (load) at ~ 90% efficiency. Might be easy to modify.

http://www.mfjenterprises.com/Product.php?productid=MFJ-4416B

Jim
WD5JKO

Mark,
Bipolar RF transistors have emitter resistors built into the die to provide negative feedback and balance the current in each segment of the die. I read FETs have better high order distortion numbers without these resistors. My simulated linear with source resistors to balance the current between packages gave good distortion numbers when the value was .33 to .5 ohms.
This value would need to be much lower at 12 volts to keep efficiency up.
Now noiseless Norton feedback with a transformer and FETs running grounded gate would be impressive.
Since most mobile antennas are low impedance why work at 50 ohms? Say you move the amplifier out to the base of the antenna...HMMMM

Hmmm indeed. Since the amp is being designed specifically for the mobile environment, why not toss aside the 50 ohm requirement. It's probably only going to be for the lower bands, 160-40, and the typical center loaded whip is going to be in the 5-20 ohm range down there from what I've read and experienced. I don't know about putting the amp out on the mount, but maybe that could be worked out.

Now a 1:4 transformer will make some real power into 10 ohms

Hmmmmmmm mm   Yes indeed.  If I remember corectly,  Walts book made a mention of doing this on some satalight he was werking on........


klc