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VK3ABK > ATU 12.05.06 13:05l 89 Lines 3717 Bytes #999 (0) @ WW
BID : 29623_VK3HEG
Read: GUEST DL5GCC
Subj: Re: Roller inductor ATU etc.
Path: DB0FHN<DB0RGB<DB0MRW<DK0WUE<GB7FCR<VK7AX<VK3HEG
Sent: 060512/0407Z @:VK3HEG.#WEV.VIC.AUS.OC #:29623 [Ballarat] $:29623_VK3HEG
From: VK3ABK@VK3HEG.#WEV.VIC.AUS.OC
To : ATU@WW
Hello again.
The query from Andrew, VE1AEE, about a 3kW ATU, has raised several points of
interest.
Firstly, a reply from the one and only, 'Ampersandy' G0FTD, who is getting
himself into a spot of bother. He sent a bulletin about 'power' in a ATU
being dependant on antenna impedance, but then talks about 'voltage' doing
damage to his ATU capacitors when he tries to feed a very short antenna.
I queried this, and Amper' sent...
At around 25/30w I get flashover.
The maths..?
Well lets take a look at the following:
Volts = SQRT(watts * impedance)
= SQRT 25w * 2000 ohms
(1-j2000 ohms is just 2000 ohms reactive).
= 223 volts
Comment...
In an AC circuit, it is essential to consider the phase relationship of the
voltage and current in order to determine the power product. In the above
example, this has not been done and the result is meaningless.
This diagram is intended to show, in rectangular notation, Amper's polar
mathematics where his operator (-j) designates a capacitive reactance at his
'short' 160m antenna feed point. This, I think, makes the point, clearly.
R--->
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ B The resistive component 'R' is phase neutral
\ a ³ and the capacitive reactance, found at a
\ ³ short 160m antenna feed point, is seen as
\ ³ negative going. By convention, it is in a
\ ³ negative quadrant, giving the '-' in (-j).
\ ³ Xc
\ ³ Note. An inductive ³
\ ³ reactance would be - ³ +
\ ³ shown going up into ÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄ
\³ C the +ve quadrant + ³ -
hence a '+' (j). ³
The angle 'a' would, if to scale, give the resultant phase component as an
angle of leading phase. In other words, the difference between the resistive
and reactive components mean that we can't add or associate the two.
For this reason, I disagree with Amper's mathematics.
The problem with feeding a ridiculously short antenna on 160m is sure to test
the best of ATUs. The voltage involved is still 'voltage' no matter what the
phase, and by trying to maximise the 'power' input/output you will be sure to
exceed the manufacturer's specification and ratings.
This brings the second point... A manufacturer, unless he is a rogue, will be
trying to give the best results for the money he charges. He will employ a
good team, with better qualifications than most of us. Why do we try to be
smart, and rubbish the equipment when something goes wrong?
The next point follows this...Andrew, VE1AEE, seems to want an ATU that is
outside the 'normal' Ham requirement. No matter, whatever we need to know
outside the instruction book, or from interest, a phone call would get the
best and quickest results. Any decent company would comply, and even have a
web site or e'mail address.
Well, 'Amper' old boy, I hope you don't mind me calling you that, with such
familularity, but we mustn't be too formal on packet, must we? I'm happy to
see you now using a slightly more effective counterpoise on your shopping
trolley. :-) But have a think about my motor car analogy, and just what part
does the feedline play in antenna radiation.
You will remember...
....."A recent bulletin (Short Vertical Balun Trick) got me thinking. When do
we disperse more RF in the feed system than we do in the antenna?".....
I would be inclined to try a loading coil in a very short 160m antenna.
With a chuckle,
73. Dick. VK3ABK.
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