[Lf] Re: LF antennas]

[Andre' Kesteloot]

john sexton wrote:

> Hi Rik,
>
> Now I have had time to study your web page on 136 antennas, I have the
> following comments, which I hope you will find useful.
>
> 1. The reason that the antenna current decreases linearly to zero at the end
> of the antenna, is because sin(x) = x approx for small values of x, and for
> most amateur antennas the lengths involved are only a fraction of a
> wavelength.
> Similarly the reason the voltage is practically constant over any continuous
> segment of wire making up the antenna is because cos(x) =1 approx.
>
> 2. There are several useful programs from G4FGQ for determining probable
> environmental loss for a given site.
>
> 3. The values of Q for loading coils, that you have used in examples are
> very conservative, it is not particularly difficult to obtain higher values
> of Q especially for small elevated coils.
>
> 4. From your explanation of how to calculate ERP, it follows that in order
> not to exceed 1 watt ERP, we must not exceed approx. 0.55 watt radiated
> power.
>
> 5. I thought at first that you had lost a factor of 4 (two squared) in
> formula 5a, but later realised that the ratio is calculated with respect to
> the average current in a monopole, which is 50%. It might be useful to make
> this clear, assuming that it isn't just me who trips over this one.
>
> 6. The results of calculations agree well with those obtained from G4FGQ's
> program TANT136.
>
> 7. The paragraph just above the graph in section 2.2, states that the gain
> that can be achieved 
is 6dB. The reason for this is that one can at most
> double the average current in the vertical, which occurs when the current is
> constant over the vertical segment.
>
> 8. Concerning Umbrella antennas, i.e. capacitive top loading with down
> sloping top loading wires, you suggest a maximum vertical descent of 50%
> should not be exceeded. In a comment regarding the Galveston NDB, someone
> (sorry I have forgotten who) said that the textbooks on VLF recommend not
> more than 30%. I decided to try to determine the optimum value and came up
> with the following formula:
> The Optimum length of a down sloper from the top is h(sqrt(1+sec(alpha))
> -1), where h is the height of the main mast and alpha is the angle of the
> sloper from the vertical. Which this length the Radiation Resistance
> compared with a monopole of the same height is increased by a factor of
> 4 * (1 + cos(alpa))^2 * (1 - sqrt(cos(alpha)/(1+cos(alpha)))^2. This
> increases monotonically with alpha. At 89 degrees, L is approx. 6.6 * h and
> the Radiation Resistance is approx. 3 * that of Monopole.
> For an angle of 45 degrees, the sloper should descend 39% of the height and
> the multiplication factor for Rr is only 1.48.
>
> 9. You give some useful results and graphs for inductive loading. I would
> only like to comment that the elevated coil need not be the full value to
> resonate the top part of the antenna. A combination of top capacitive and
> inductive loading can be beneficial, e.g a 60 metre horizontal top and a 3
> mHenry coil will practically achieve the full 6 dB increase.
>
> 10. Section 2.7 Antennas with multiple vertical elements. This looks like a
> good idea, which needs to be investigated further.
>
> 11. Section 2.10 Helical antenna. You point out that if capacitive top
> loading is added the advantage of a helical antenna will be less. This is of
> course relatively speaking. This is really a case of "less is more".
>
> Congratulations on a fine piece of work. I for one feel that at last I am
> beginning to understand LF antennas.
>
> 73 John, G4CNN
>
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