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Old 08-28-2018, 02:05 PM   #1
eccerr0r
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Default "overvolting" transmitters for more power

Well first off, this is a bad thing to do because this could be illegal because it changes the operation of the radio, so sorry non-hams and non-GROLs.

Barring that, it will also increase power dissipation, possibly causing silicon or other device meltdown.

But how much of it is a problem typically? I have this ham radio transceiver, normally 8.4 volts direct connect to the final transistor, but someone hacked it using LiPo packs to 11.1V (nominal) without any step down.

That's a huge increase! If all other things being equal, it's about 75% increase in power dissipation if it were a resistive load. For all you licensed hams out there who have played with this before, have people done this to increase power output, is it typical for this to be "safe" for the electronics? How long did it last, did you do anything to make it "last" longer?

Anyway, the radio seems to actually work despite this! The question is...how badly, for how long and should I revert this hack... Note to hams and GROLs: overvolting can increase spurious emissions which is illegal. Licensees should be aware of this and not transmit with this problem!

BTW, this is a nominal 2.5W transmitter, which calculates to about 4.4W with the overvoltage with all other things being equal (which it's not). The final is a tiny TO-5/TO-39 AFAIK so it might not be very healthy for this transistor, though perhaps as long as the antenna's impedance is matched, technically that wattage should be dissipated in the antenna, not the transistor. I was also concerned about the 5V series dissipative regulator the transceiver has for the digital electronics - it uses a TO-92 series pass transistor!
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Old 08-28-2018, 04:26 PM   #2
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Default Re: "overvolting" transmitters for more power

are you sure about what you have?
most of the radio's i'v seen have a voltage regulated output stage to prevent frequency drift.
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Old 08-28-2018, 04:47 PM   #3
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Default Re: "overvolting" transmitters for more power

Well you might want to connect this to a dissipative 50 ohm load and see what RF power you are actually developing, that said what make and model number is this as some handheld radios were designed to offer different power outputs depending upon battery type / voltage.

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Old 08-28-2018, 06:20 PM   #4
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Default Re: "overvolting" transmitters for more power

There are quite a few TRX's that change output power by changing the input voltage. At the end of the day, you can run more power like that for a short term (I mean short term), all depends on cooling. Look it as a turbo engine... you can run overboost for short term too.
However in the RF world, you need to tripple your output wattage to produce 1 more S-Unit on the other side. So work on the antenna and use low loss coax.
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Old 08-28-2018, 08:06 PM   #5
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Default Re: "overvolting" transmitters for more power

Well, the PLL must be regulated or at least enough so that it won't have noise with with voltage. After that, the amplifiers shouldn't care about the voltage, as long as it's got no noise on it (battery!) hence a lot of HT's decrease in power as batteries wear down. This is fairly similar to what's done with audio final amplifiers too... wow, a lot of similarities show up again and again!

Yeah I should measure the power output to see what is actually coming out of the unit. I don't have a power meter, but I guess I can always measure voltage at a 50 ohm load and hope my meter can measure at the frequency (maybe I need to pre-rectify and filter it and measure DC). I'm just guessing based on the voltage increase fed into the output stage which theoretically increases swing - and therefore theoretical output. As this is an HT the antenna is a rubber ducky.

Hmm... need to rethink this S-unit, was thinking carrier power versus signal power...

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Old 08-29-2018, 05:51 PM   #6
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Default Re: "overvolting" transmitters for more power

While it is entirely possibly to measure feed voltage and convert to watts based upon a 50 ohm impedance you will need a calibrated RF probe for this. I used this method for quite some time before acquiring an absorption RF power meter and through line 100 watt 10db attenuator, essential as absorption power metes are very easy to damage. Anyways you will need something like this formulae to convert your volts to watts, see below for details.
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Old 08-29-2018, 07:00 PM   #7
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Default Re: "overvolting" transmitters for more power

I would study the original circuit, because controlling/limiting TX power can be done a few different ways - the driver power, detuning the PA or driver etc.

Starving a stage of Vcc would just risk clipping, so I think increasing Vcc is doing something else. This is AM?
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Old 08-30-2018, 10:22 AM   #8
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Default Re: "overvolting" transmitters for more power

This is a 2m FM transceiver, so very high frequency. That is true, not sure if any stage is being over run, which indeed is part of the worry about causing spurious emissions. Unfortunately no equipment, though I should be able to hook up this transceiver output to my scope and view the output waveform and calculate it as seen...

though probably not a good idea to use the 50Ω internal termination in the scope as the dummy load... ha.
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Old 09-01-2018, 04:38 PM   #9
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Default Re: "overvolting" transmitters for more power

Most scopes that I know of are high impedance, unless you are referring to a spectrum analyser which often have input impedance's of either 50 or 75 ohm. Usually the input is limited to a few watts in which case you will still need a through line 50ohm attenuator. A good 200 meg scope will show you the output of said transmitter assuming you are using the correct termination.

I am guessing this is not a yaesu, trio or icom set then.
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Old 09-01-2018, 04:48 PM   #10
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Default Re: "overvolting" transmitters for more power

it's a boat anchor!!
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Old 09-01-2018, 05:03 PM   #11
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Default Re: "overvolting" transmitters for more power

i understand am but i get stuck here with fm ....
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Old 09-01-2018, 05:20 PM   #12
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Default Re: "overvolting" transmitters for more power

Yes it's a boat anchor Kenwood HT, and my scopes have a built in 50Ω termination as they are Tektronix scopes...
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Old 09-01-2018, 09:47 PM   #13
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Default Re: "overvolting" transmitters for more power

Well you should be good to go then, check the power handling of the 50 ohm termination but I suspect you will still need something like a 10:1 ratio through line attenuator, if this is a hand held then you should be good with a 25watt attenuator. Most el-chepo swr power meters will give you an approximation of power delivered which used with a 50ohm load, so long as you are not too concerned with accuracy.

Furthermore there are a number of fairly simple designs out there for making more accurate absorption rf power meters which I did consider at the time I was looking at rf power meters for the workshop, these being prohibitively expensive as I recall as the rf power detector were many times the price of the meter itself. Again they would still need a through line attenuator as they natively don't handle much current.

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Old 09-01-2018, 10:37 PM   #14
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Default Re: "overvolting" transmitters for more power

Was reading the specs of my scope...looks like the internal terminator can handle only 0.5W, so feeding in the signal directly would fry the termination.

No matter, I have a dummy load I had been using prior to acquiring my license. Probably best to just hook up the dummy load and use high impedance mode and probe off the load.
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Old 09-02-2018, 05:38 AM   #15
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Default Re: "overvolting" transmitters for more power

Quote:
Originally Posted by eccerr0r View Post
Was reading the specs of my scope...looks like the internal terminator can handle only 0.5W, so feeding in the signal directly would fry the termination.

No matter, I have a dummy load I had been using prior to acquiring my license. Probably best to just hook up the dummy load and use high impedance mode and probe off the load.

In my Medium Wave pirate days, I used the same method to check the output of my transmitters.

Using a 50W dummy load, I would tap the voltage across it using my Tektronix 2445B scope & use P=VČ/R, where V is the RMS voltage.

For 4W you should see around 40V P-P.
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Old 09-02-2018, 02:23 PM   #16
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Default Re: "overvolting" transmitters for more power

I still find that amazing that I should get 40V P-P on a 50Ω impedance load from a 4W transmitter with the final transistor powered off of ~12V (say, from a CB) or 11.6V (from this weirdo overvolted 2m radio). I can get why I could possibly get it from an actual antenna but not from a dummy load.

This mystery gets solved real soon now.
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Old 09-02-2018, 04:30 PM   #17
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Default Re: "overvolting" transmitters for more power

Regarding DBm volts and watts

Using a 50 ohm system you could expect the following

14.1 volts 4 watts
22.4 volts 10 watts
44.6 volts 40 watts etc

These were un-ashamedly lifted from this table

https://ww2.minicircuits.com/app/AN40-012.pdf

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Old 09-02-2018, 05:40 PM   #18
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Default Re: "overvolting" transmitters for more power

Yes, yes, you calculate it with P=V^2/R and that's what you'd expect, e.g.:

4W RMS = V^2 / 50Ω
4W*50Ω = V^2
V=Sqrt(4W*50Ω) = 14.142... volts RMS.

...so no table needed but... explain how does a 12V driven final amplifier transistor gets you 44V across the 50Ω load!

Again, I can understand resonance that would boost voltage, but to a dummy load resistor, not so much. However maybe there is still resonance at play...
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Old 09-02-2018, 06:13 PM   #19
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Default Re: "overvolting" transmitters for more power

Unless I am missing something obvious, you've done the maths and that agrees with the table, are you getting the 40 odd volts from the scoped waveform ?
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Old 09-02-2018, 06:23 PM   #20
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Default Re: "overvolting" transmitters for more power

Right the calculation agrees with the table, but this doesn't mesh: One of my VHF 2m transceivers is rated for 15W but the final is hooked up to 12V line. How does it get 15W to the antenna?
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