Hey everyone,

Just acquired an MTR3B and been loving it for making contacts out in the backyard, but there’s one thing in the manual that has been in the back of my head ever since I got it.

It says not to run it over 12v and not to run it on any antenna with a high SWR.

The question is: what do they mean by high?

I do not run mine over 2 to 1 just to be safe

It has no over voltage protection

Remember it’s 12.0 volts not the usual 13.6 put out my the average psu.

The PA FETs are BS170s they have a max drain-source voltage of 60v. You can use that and the antenna system parameters to calculate what the max SWR can be.

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MTR is safe on a fully charged 3S LiPO, 12.6V

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VSWR < 4.9 at 3 Watts
VSWR < 3.8 at 5 Watts

It’s a badly designed PA in my opinion.

I tend not to run any radio over this level, even if the built in tuner can handle it. 1.5:1 is my threshold. Perhaps I’m being overly cautious.

I’ve done a Google search and haven’t been able to dig up the formula for that problem. I’d be forever appreciative if you’d share that with me, or even just point me in a direction.

That’s the exact battery pack that I’ve been operating from. Sawed off the extra slot of a 4S 18650 holder, then resoldered the connections. Works well.

I’m afraid that I don’t understand how you arrived at those numbers. How do you get there? I’m always eager to improve all aspects of my operating and station design.

The lack of protection for the finals does make me scared to run it into anything other than a “perfect” antenna, for sure. Are there any antenna tuners that you’d recommend, especially ones that are low loss? I enjoy homebrewing if I have a good recipe.

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In that case get another rig with SWR protection. Life’s too short to be paranoid about a few FETs. Buy something else you can use and enjoy without concern.

Link dipole then.

Considering that you can replace the 3 FETs in the PA for less than a dollar I wouldn’t be too worried. However, I’ve got an ATS3B and a pair of MTRs and the finals are all original.

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The Wikipedia page at Standing wave ratio - Wikipedia goes into some detail and includes the base formulas


The MTR3b will work down to about 5V, If you had a buck converter with you, you could drop the voltage when tuning up your antenna and protect your finals to a much greater extent. I’m thinking of adding the Tiny SWR to my MTR3b; not for tuning, but just as a warning against high SWR.
73 Matt

Have used the ATS3 and ATS4 before, and the MTR3 and 5. Mostly, I have used the MTR3 and have done about 700 activations with it. Have never fried those BS170’s. Not much worried about it any more, they are robust, and if it fried, they are easy to replace.

I have built into the radio a tinySWR indicator, by DK3IT, and I use a miniature tuner for the trapped endfed halfwave I usually use. I adjust the tuning while making a short string of dits. I have practiced tuning it just by listening to band noise and found that doing it this way always wound up better than 1.2:1.

I power the radio with a 350 mAH 3S battery from HobbyKing. I seem to average around 3 mAH per QSO, and thus can get at least two activations out of a charge. This battery weighs about 40 grams. Entire radio station including battery, radio, 6 meter pole, antenna weighs less than 1/2 KG.

Hope this is helpful to you. 73 - Fred KT5X (aka WS0TA)


I think that this is standard caveat for any radio.

As a member of the ATSprint group, I have the impression that ATS and MTR radios very rarely get their finals damaged (stories about blown finals are rare).

The manual of my MTR3b (built from kit) says:

If operating the MTR with supply voltages greater than 9 volts, please ensure there is a low SWR load connected to the rig. High SWR conditions (especially those which are reactive) when operating above 3 watts output may cause the PA stage to become unstable or cause the PA FETS to be damaged. Be very careful when using a fully charged gell cell to power the rig!
It is highly recommended that a resistive SWR bridge be used if an antenna tuner is needed to match the antenna.

I have been using a MTR3B for more than 100 activations, always with a small simple antenna tuner (a ZM4 clone - with resistive bridge in tuning mode). I simply do not trust field antennas that their swr will always the same, on every summit, after repeated dis-assemblies. Sometimes the wire will break, the coax connector will fail, the wire will touch a wet tree, whatever. So the tuner is always there, because generally I have no idea what the swr is after deploying the antenna. While tuning the rig, I put my MTR in “tune mode”, which reduces the output power if the supply voltage is above 10V.
And yes, I do use a miniature gell cell (although in series with a diode to protect from inverse polarisation) so the supply voltage is generally over 12v.

I am not proud to say that couple of times I tried to tune without antenna connected at all, luckily realized what is going on in time, before damage was done.


First off, the discussion here is very helpful!

The consensus seems to be that I’m worrying about something that I shouldn’t be overly concerned about. I’ll believe the wisdom and experience of the group.

The link dipole is next on my list to make. In fact, I’ve got the feedpoint and 20m section all done. Just need to trim it, then add 30m and 40m.

The TinySWR looks like an interesting piece of kit. I might have everything lying around for me to make it with parts on hand. Might modify an alligator clip with some inline diodes to drop the voltage instead of a buck converter though.

Out of all the simple tuning networks that exist for QRPers, is there a certain kind that has the lowest amount of loss or works better than others?

I don’t know how I missed that, but I did. The theory and math should keep me entertained for an evening, at least!

Hi Richard,

I have one, and it’s been a fantastic backpacking radio.

Can you elaborate on why you believe the PA is a poor design?

-Josh WU7H

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You don’t have to leave the buck converter in circuit if you’re not happy about rfi. Alternatively, you could add a switch to a 3 cell Li battery holder so that one cell is switched out whilst tuning, leaving you 8.2v to power the rig; much safer.
73 Matt

See previous posts!

I don’t have a MTR, but I do have another of Steve’s designs, the pfr3b. I seem to recall, though without checking the schematic it’s hard to say for certain that it had a pair of antiparallel 47volt zener diodes just after the pa stage in order to provide a rudimentary swr protection. The rig has built in tuner/swr but I suppose there is always potential for a broken connection that you are unaware of if using the coax feed rather than the balance feed.

In practice the MTR seems to be quite robust. The PA doesn’t seem to mind opens and I’ve used the wrong links on my dipole antenna by mistake on occasion and no harm was done.

I was looking at component supplies the other day and I note that I can get 10 off BS170 for 80p. That’s 24p to replace the PA transistors. I have quite a number of BS170 in stock anyway.

There is a 47v zener across the parallel PA transistors, so that should help keep the drain voltage below the 60v max.

73, Colin

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I have a MTR 2B and I have used it some times with no problems in my shack being powered with my Daiwa PS-140IIa power supply at 13.8V.


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