Hi Alan,
2013/2/15 Alan Melia <[email protected]>:
> Why? I think you might need to look at the priorities first. Experience says
> you will not notice the difference unless you have tackled the problem of
> ground and environmental losses first (as Stefan has done) The improvement
> obtained by improving the Q of the loading coil may probably only increase
> the efficiency by a minute amount.
Here is my thinking. Let's assume an Rground of 18 ohm. The loss of
the 220 Q inductor is 9 ohm, making the antenna impedance about 27
ohm. The Rrad is tiny, about 0.322 ohm. The calculated efficiency is
1.17%, so from 50 W Pout, I am radiating 0.585 W.
If we somehow use an inductor with 4.5 ohm loss (Q= close to 500, so
doable), the efficiency will be 1.4%, implying about 0.7 W radiated.
That's 0.8 dB, indeed not much of an improvement.
If we push it a bit and get an inductor loss of 2 ohm (Q=1000, wow,
that's impressive), the efficiency will be 1.58%, radiated power 0.79
W. That's 1.3 dB or so, if this Q is achievable with reasonable means.
The benefit indeed increases then the ground losses are lowered.
> You say you have a Q of 200 now.... this indicates a bandwidth of about 2kHz
> meaning you will probably need to retune across the band. A Q of 400 to 500
> should be possible but unless the reduction in RF resistance is a
> substantial fraction of the Rloss it is wasted effort. It also means that
> your tuning will be very weather dependent. I feel that unles you have
> situation of Rloss <15ohms you will barely notice the difference......except
> a "flighty" system, difficult to keep peaked, and possibly a number of fried
> PA devices when it goes wrong.
Very good point Alan, I didn't think of this. I quite enjoy the
relatively broad tuning of my current system.
> Litz will improve the Q slightly, coil form factor needs to be right as
> well, and Litz is a devil to work with (note "proper" Litz has strand
> numbers are twisted in powers of 3, anything else is just bundled and will
> not achieve the theoretical advantage) If you miss one strand out of the
> soldered connection of the Litz you will lose a lot of the advantage.
The Litz wire I have is proper Litz. it has 126 strands, 0.1 mm
diameter. Interestingly, 126 = 5^3 + 1. :-) I think I have mastered
the soldering technique, but I am not really sure I connect all 126
strands!
> Top loading may well turn out to be more effective, but it all depends on
> your partcular location, and you need to make measurements of the antenna
> systtem, and possibly the field it generates, not guess (though that is very
> seductive :-)) but in my experience is usually wrong! )
If it makes you feel good, it can't be that bad! :-) In a more serious
note, I see your point about guessing versus measuring.
> You are right in that the best way is to make incremental improvements to
> the antenna, but be very critical, weighing the cost in effort and cash for
> the improvement .......what works for others may not work for you.
>
> Best of Luck
> Alan
> G3NYK
73, Dimitris VK1SV
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