"overvolting" transmitters for more power

[llonen]

Re: "overvolting" transmitters for more power

If you are getting around 40 volts on the scope, assuming you have everything setup correctly and you are reading from the waveform peak to peak I would say your measurement sounds about right, I have just set this up in the workshop and I would suggest your HT is putting out around 3.2 to 4 watts in its current condition.

[eccerr0r]

Re: "overvolting" transmitters for more power

Okay, this is baffling me.

I'm using my 2440 that has measurements. Getting ~15.7 volts P-P with:

HT -> Coax -> T (on the scope) -> the stock rubber ducky antenna

This would translate to only 0.62W if the rubber ducky is at 50Ω. But anyway, 15.7V, this is a very interesting piece of data I collected as I learn more about radios...Interesting since the power source is 11.5V or so.

BUT WAIT... that was on low power!

Now...testing on high power... 40 volts P-P!!! THIS now is clearly 4W...

Interesting...very interesting. Now I need to see if a dummy load has the same behavior.

But still, this was expected - over voltage = higher power output... now the question is... this Kenwood Trio...when can I expect this radio to die?

[eccerr0r]

Re: "overvolting" transmitters for more power

And I tried to transmit at low power (0.6W) into an Ethernet "cheapernet" terminator, which is a 50Ω resistor -- The voltage is only 11.6V P-P which is now close to battery voltage. This would mean about 330mW is being dissipated in that terminator.

Weird... back to the drawing table at trying to understand this behavior despite being off-topic.

[llonen]

Re: "overvolting" transmitters for more power

There seems to be an oddity with the way I arrived at the values in my last post (it was 2am in the morning) anyways I have just setup on the bench a 2.5 watt 50mhz signal from a FT690 as I don't have anything on 2 meters that puts out less than 50 watts. This was fed into an absorption watt meter (no through line attenuator) which displayed the expected 2.5 watts (about right for the FT690) a feed was teed off and taken to a scope and the waveform produced 11.2 volts peak to peak. So if you are getting 40 volts then either your HT is putting out around 40 watts (very doubtful) or there is an oddity in your measurement, you need a non reactive 50 ohm load when making these measurements not an antenna.

[Radio Fox]

Re: "overvolting" transmitters for more power

...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...

The maths involved in explaining how you can generate a peak to peak voltage greater than the supply voltage can be quite complicated if you want to go right back to basics.

When I built my transmitters I just accepted the fact. I didn't bother to delve too deeply into the maths behind it. Very basically it's like the back emf effect of an inductor. The rapid collapse of the magnetic field when the supply is removed produces a far greater voltage than the supply voltage. Slap in a suitable capacitor & switch the supply on & off at RF frequencies & you have a sine wave of greater amplitute than the supply. This occurs in the network circuits before being fed to the antenna socket.

That's my cringeworthy basic explanation. LOL.

Someone asked a similar question here:-

https://electronics.stackexchange.co...h-a-12v-supply

btw...I believe your scope is a 300MHz bandwidth model. You will need to make a slight adjustment to the calculations as measuring a signal at around half the bandwith will decrease the amplitude by around 8%.

It would be interesting to see the same measurements made with a non reactive 50 Ohm dummy load.

[Radio Fox]

Re: "overvolting" transmitters for more power

...So if you are getting 40 volts then either your HT is putting out around 40 watts (very doubtful) or there is an oddity in your measurement, you need a non reactive 50 ohm load when making these measurements not an antenna.

I agree about the 50 Ohm load instead of the antenna, but 40v P-P is the correct voltage for 4W.

40v P-P = 20v peak = 14.14v RMS. 14.14²/50 = 4W.

[eccerr0r]

Re: "overvolting" transmitters for more power

Right, at resonance with a real antenna that has capacitive and inductive qualities, I have no problem expecting a massive voltage increase. Just that when that load is a noninductive resistor, it doesn't quite make sense. I suspect the terminator resistor must be noninductive as it was designed to not allow reflections... Perhaps the cable and the filter in the radio are the larger part of the voltage increase.

Yeah I was wondering when I'd see some measurement issues with this old scope. It indeed has a -3db point at 300MHz and yeah I might have to bump up the measured voltage slightly, alas, ignoring it is close enough for learning purposes

Interestingly enough, the internal termination of the 2440 has a VSWR rating of 1.3:1 ...
which makes sense that Tek made note of it...

[llonen]

Re: "overvolting" transmitters for more power

Well your seeing 40 volts Peak to Peak which is entirely consistent with 4 the four watts I'm suspecting your HT is developing, if you were getting a Peak value or RMS value of 40 volts then as above I would say something was wrong

[eccerr0r]

Re: "overvolting" transmitters for more power

Yeah, which is surprising because that's exactly how I calculated the output, at least assuming the transceiver behaves as it says on the specification: 2.5W RMS output using an 8.4V battery. Then with the higher battery voltage, higher wattage. I suspect the power consumption is higher but the Li-ion batteries hold more energy anyway.

Currently the imbalance between the cells measure about 20mV with pack voltage at 11.6V. I will have to monitor cycle after cycle and see if I have to artificially discharge a cell to balance the pack once in a while.

Incidentally, the incandescent backlight of the HT burned out... Gee, I wonder how it could have burned out :-P

[llonen]

Re: "overvolting" transmitters for more power

Sorry this has been bugging me for a while and I have only just had the inclination to revisit it, their schoolboy error here in the way the measurement is being made, well for one your seeing a peak to peak value on the scope and two you are seeing the result of a miss-matched transmission line and load. Were you to feed this into a a 50ohm pass-through attenuator, and properly matched cable you would see the expected voltage.

[kc8adu]

Re: "overvolting" transmitters for more power

what model is this?
many older ht's had high power options which were higher voltage packs.
you had say a 7.2v pack that ran at 2w or a 12v that ran around 5w.
i have an old htx-202 with a custom made 3s2p 18650 pack and a 2sc1972 final.
does around 7w.
i would be most concerned with heatsinking of the final in your rig.
this 202 gets hot on high power if you get longwinded!

[eccerr0r]

Re: "overvolting" transmitters for more power

Here is the final!
No there is no different voltage options, so this is it.