HF Tx Voltage

[No1_Moose]

What sort of Voltage is produced at the Antenna output of a HF radio Transmitting at 250W?

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[bruce]

do not quite understand what you want to know, but a hf watt meter will prob give reading of over 200 watts in line with antenna or dummy load, and placing a floursent tube next to the antenna will light up when mile keyed and modulated. if not getting the watts out, then not enough 12 volt/amps going in(radio not being fully powered). if you are asking if it will burn you if you hold on, the answer is yes.

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[No1_Moose]

Ok, but is the Voltage stepped up from 12v inside the ATU?

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[tome]

More than you want to be in contact with, hundreds of nasty volts.

RF waves are AC (alternating current) - as is mains. Difference is that RF is much higher frequency and carries hugely more energy, so it's not just voltage.

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[Ships_Cat]

The voltage will vary dramatically according to the impedance of the antenna (I think you do physics, so like from P=IV and Ohm's law gives V**2=PR) and the impedance will vary according to the frequency being transmitted at. For an HF antenna on a boat where the antenna is a fixed length and the frequency can vary between MF and HF it is possible to get up to thousands of volts. These are not killing type volts, radio frequency does give severe and deep into the flesh burns though.

If you are operating into a matched 50 ohm antenna with 250 watts, then by the above the voltage will at the feedpoint be around (PR)**1/2 = 112 volts.

By matched I mean that the antenna is resonant not the fact that there is an ATU present - the ATU makes no difference to the voltage on the antenna. However, as long as the ATU has tuned, then the voltage at the coax connector out of the back of the radio itself will be around 112 volts (but probably higher as the match will not be perfect) regardless of what it is at the antenna.

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[tome]

John

Cutting through the physics, I see we're in total agreement. Nasty Volts, deep burns. Don't want to see young Moose fried by his voltmeter. Keep away from the backstay whilst transmitting.

Tom

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[tome]

No, it's already big volts before it gets to the ATU. What the ATU does is to tune the backstay to the frequency you are using. It does maximise the volts, so if you want to grab the backstay do it before the ATU does its thing. Good luck!

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[Ships_Cat]

Absolutely agreed Tom - also Moose #1 is a bit young to be hamburgered yet.

Perhaps needs to be re-emphasised that rf burns are particularly nasty. Even from 5 -10 watts if focused off a strand of wire to give a point contact on a finger or something, it will give a deep and painful burn into the flesh rather than just the skin.

John

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[Twister_Ken]

pink poo

>These are not killing type volts, radio frequency does give severe and deep into the flesh burns though. <

Sounds like it might be one cure for the starling problem!

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[ParaHandy]

Re: pink poo

lot around my way are into purple poo are your lot constipated?

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[tome]

Re: pink poo

Ours also, certainly not constipated

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[Strathglass]

As has been stated when a 250 watt transmitter is fed into a 50 ohm load then 112 volts or so will appear across that load.
This is only true if the load is purely resistive. In real life this is never the case.
This will be the aproximate voltage across the coax feeding to the ATU when it is tuned in.
It is very unlikely to be the voltage appearing between the feed wire going from the ATU to the aerial and 'ground' which invariably will be higher, possibly several hundreds of volts.

In a much simplified manner an ATU effectively adds or subtracts to the length of the aerial to make it resonate at the transmitting frequency. Just like adjusting the length of a pendulum changes the time of a clock. Thus the ATU, feed wire and the aerial combine to form the resonating element ie the whole aerial.
Along the length of any aerial the voltage will vary (in a sinusoidal manner) from zero at one or more points to several hundreds of volts.

To get back to the reason I think you were asking the original question. The voltage on the feed wire from the ATU to the backstay (or Elsewhere) can vary from quite a low voltage to a very high voltage depending on the frequency in use at any particular time.
This is why this wire should be insulated and away from hands reach. The centre of very heavy coaxial cable makes a good lead and this could be inside a plastic tube when near where people could touch it.

I have taken technical liberties to try and simplify the explanation hope this helps.

Iain

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[No1_Moose]

Thanks all

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[Ships_Cat]

Re: pink poo

Have never seen starlings around marinas here in NZ even though you lot introduced them here to poo on our boats - I would sell you the secret of getting rid of them if I knew it.

Remember when I was a little kid in 1950's we used to get enormous flocks of them which used to impress me (being a cat, of course). They declined markedly in numbers over the years and while they are still around (still get them in our garden) I think with pesticides, etc their food sources have been decimated on cropped land.

Anyway, if someone can tell me how to identify starling poo from that of other birds I will do an investigation of what colour it is out here as that will surely be a good measure of the voltage on No1 Moose's antenna.

John



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[ParaHandy]

Wheatstone bridge ...

us yins (being scientifically inclined) will devise suitable resistivity measurement of different coloured poo and thus establish (beyond doubt, i might add) the nature and source of the poo ...

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[tome]

Re: Wheatstone bridge ...

Wayne Kerr make a suitable instrument.

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[Ships_Cat]

Re: Wheatstone bridge ...

Is that the one that measures "flutter value" (re: surge/impulse testing)?

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[tome]

Re: Wheatstone bridge ...

That's the one, see <A target="_blank" HREF=http://www.waynekerrtest.com/index.htm> here</A> for details.

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