transmitting without antenna

Lou

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Dec 2, 2001, 6:10:18 PM12/2/01

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I recall on many occassions reading/being told not to operate any sort of
transmitter without the antenna connected. I thought about a few basic
amplifier designs to see what this would do to the transistors, but I
couldn't see any harm it could do. I'm no expert on this but my thoughts
seem to boil down to if there is no antenna, there is no drain, so the
transistor can set up the desired voltage with less current, and everyone is
happy.

Am I looking at this wrong? Can someone explain an example where this isn't
the case?

Thanks,
Lou

Your Friend Bill

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Dec 2, 2001, 6:15:07 PM12/2/01

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The real question is why you would want to - apart from curiosity?
-Bill

J. Harvey

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Dec 2, 2001, 6:42:26 PM12/2/01

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"Lou" asked:

I was always confused by the shorter antennas having a lower input Z. If
the trend continues all the way down to an infinitely small antenna, then an
empty connector (open circuit) is a dead short...

:-)

What if the transistor was trying to feed a current into an open circuit ?
Then the voltage would reach infinity and damage the transistor.

J. Harvey
Ottawa
de VE1...@rac.ca /VE3

Richard Clark

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Dec 2, 2001, 6:43:13 PM12/2/01

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Hi Bill,

Not a fair response because we've all done it without the benefit of
curiosity (and for no good reason why to boot).

73's
Richard Clark, KB7Q

Drink, WP3JQ

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Dec 2, 2001, 7:37:59 PM12/2/01

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Hi Lou,
If I'm not missing your point, your question is basically, "How does
transmitting without an antenna harm your transmitter?" I'll give you a
simplified version.
The antenna or load is what resonates to allow the transmitted energy
escape your radio out to the spacemen monitoring us since the forties:)
And, a good antenna should give you a low SWR (Standing Wave Ratio: the
ratio between what goes Forward to the world and what's Reflected back to
your radio). Basically, the further from the antenna's resonant frequency
you get more reflected back into your radio than emitted to the spaceship.
If you don't have any antenna, your SWR is infinity and everything's
reflected back into the radio's circuitry. If you're running 100 watts,
it's like plugging the antenna into a light socket. "No drain" but no
escape either:)
I might've missed your point here (hope not) since you mention amplifier
design, which I understood in principle while studying but never learned
well enough to explain in detail. Anyways, I hope this helps.
73's de
Drink, WP3JQ

"Lou" <lo...@nospam.com> wrote in message
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Lou

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Dec 2, 2001, 7:39:16 PM12/2/01

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"Your Friend Bill" <"Remove The NO In The Reply Address"@coqui.net> wrote in
message news:3C0AB5FB...@coqui.net...

> The real question is why you would want to - apart from curiosity?
> -Bill

I guess that I wouldn't per se want to. But if someone tells you not to do
something it certainly makes you wonder. I guess you could say, it makes me
question how well I understand amplifier design.

CAM

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Dec 2, 2001, 8:13:22 PM12/2/01

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"J. Harvey" wrote:
> I was always confused by the shorter antennas having a lower input Z.

Shorter antennas have a lower input R but a higher input X which
equates out to a *higher* input Z. Z^2 = R^2 + X^2 where X^2
goes up faster than R^2 goes down.
--
cheers, CAM http://www.qsl.net/w5dxp

Reg Edwards

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Dec 2, 2001, 8:25:41 PM12/2/01

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You are looking at it wrong. Go back to square one and think again.

Example 1, if there's a Pi output network and the external load is
disconnected then the output transistor sees a zero-resistance load
at full current and blows up.

Example 2, if the external load is short-circuited, in order to
keep full current flowing into the very high inductive network
reactance an excessive voltage is developed across the output
collector which then breaks down.

" Troll's " are mischievous little dwarfs who live in deep dark
caves under Scandinavian mountains.

Peter J. Bertini

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Dec 2, 2001, 9:06:21 PM12/2/01

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Depends on the class of operation, the amplifier design, matching
network--if used, etc.; lots of variables, lots of possibilities. I
supose
one could use the output from a LM380 audio IC directly on LF QRP,
and see no difference if a load were connected or not.

Pete

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David G. Nagel

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Dec 2, 2001, 9:09:22 PM12/2/01

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Careful there. You don't want Frackey to get on you for using a
quasi-fractal antenna.

Dave WD9BDZ

Richard Harrison

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Dec 2, 2001, 9:48:12 PM12/2/01

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Lou wrote:
"I am no expert on this but my thoughts seem to boil down to if there is
no antenna, there is no drain, so the transmitter can set up the desired

voltage with less current, and everyone is happy."

There is a 50-ohm coax r-f output in most cases. A wave has been
generated and is traveling to the coax connector. Where it encounters an
open-circuit, current is interrupted. Energy in the H-field has no place
to go other than to the E-field, causing a voltage double. This is
called the Ferranti Effect. Some transistors can`t take it.

Best regards, Richard Harrison, KB5WZI

Your Friend Bill

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Dec 3, 2001, 7:42:12 AM12/3/01

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I was just kidding. Unfortunately I can't answer the question either.
Just a SWAG, I suspect if the transistor had zero components following
it, then theoretically your guess would be correct. But in practice you
have harmonic filters, metering and even the choked and bypassed Vcc
line and I suspect THEY would become the load in the absence of an
antenna.
-Curious Bill
-Bill