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John S · October 25th 14, 03:47 PM posted to rec.radio.amateur.antenna

On 10/24/2014 12:56 AM, wrote:
On Thursday, October 23, 2014 6:25:12 AM UTC-5, John S wrote:
On 10/22/2014 1:57 PM, Helmut Wabnig wrote:

Somebody wrote:

If 10 watts is delivered to a short antenna, where does it go if it is not
radiated just as well as 10 watts delivered to a long antenna?

Make it 100 Watts, make it 1000 Watts,

Transmit for 5 minutes, then switch off and measure temperature.

Is the short antenna hotter?

w.

I'd have to ask how short is short... ??

Theoretically, yes. For #18ga Cu wi

Lambda Efficiency
0.5 0.9690
0.25 0.9402
0.1 0.8622
0.05 0.7571

Hence, about 24% of the power in the shortest antenna is lost due to
I^2*R. In addition, the shorter antenna has less surface area to rid
itself of the heat.

However, as a practical matter, it would be exceedingly difficult to get
much power into the antenna as its terminal impedance is 0.69-j5323.51 ohms.

John KD5YI

Yep, you want a fat radiator and good connections with a real small
antenna. But most whips and such as themselves will still run fairly
cool unless you were really cranking the power. I've never run more
than 100w mobile.. But I've never had a whip get warm with that power.
I don't think my loading coil gets hot either. Or at least I've never
noticed it when taking it off to change the tap on the coil.
A KW is a lot of juice, so if it got a coil warm, I wouldn't be too
surprised.. The mast and whip might get a little bit warm, but I don't
think it would be too much compared to the coil.

It's the small HF loops and such where the fat tubing, and solid low
loss connections really pay off. May apply some also to the mobiles,
but fat tubing and such is kind of impractical on a car..

I knew you knew that, Mark. For a bit of information for others, if I
change the .05 lambda radiator (wire) to #8 Cu (about .125"+ diameter),
I then get an efficiency of 91%. The terminal impedance is now
0.57-j4397. Still not an easy feed, but you know that.

I guess the lesson is that you can heat almost anything if you try hard
enough.

Cheers,
John KD5YI

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