Topband: 160m vertical with "top loading"

[k3bu at optimum.net]

Not sure what you mean by fictional parts...

I thought that I explained the "problem" with some historical development in my article.

The main difference in understanding the efficiency of "loaded" antenna is that we are not dealing with DC currents and voltages (W8JI argument) but RF currents and voltages on a STANDING WAVE antenna/element/radiator.

Difference between the standing wave and travelling wave antenna has to be remembered.

Standing wave, typical 1/4 wave radiator, has the RF going from the base to the tip of the antenna where it sees the end - high (almost infinite) impedance, it is reflected back and superimposes with forward wave. This is what we see in modeling - maximum current at the base, diminishing in cosinusoidal curve towards the tip. The voltage is just the opposite.
Voltage and current are real (standing wave), they can be measured, they burn coils, they blow the insulation. If we use say 3/8 or 1/2 wave electrical length radiator current distribution is "extended" by the rest of cosine curve going the "other way". Impedance increases, current decrease, voltage increase. Smarter engineers before us figured out that current in the radiator and it's magnitude is important for the efficiency.

The next important item to remember is that the efficiency of the antenna/radiator is roughly proportional to the area under the current curve. So we strive to have that portion of the radiator as large as possible. If you have to "load" it, it is better to do it closer to the tip, where the current is diminishing.

Then impedance is lowest at the base, highest at the tip of the 90 electrical degree - 1/4 wave radiator. Now if want shorten, "load" the radiator by say 30 degrees (down to 2/3 length) we can insert the inductance (coil, hairpin) and "eat" the part of the current curve. 

Inserted at the base, we "eat" 1/3 of the fattest part, at the center less fat, at 2/3 up is the best compromise between the amount of current "eaten" up and size of the coil. We will notice that as the impedance goes up from the base to the tip, inserted coil has to increase in impedance - number of turns required.

This is the simplification for understanding the "problem", and as W9UCW demonstrated with his RF ammeters, current at the input of the coil is larger than at the output. Just as you would have if you would stretch the coil into straight wire. The difference is that turns of coil have the RF current "going in circles" in smaller space and not participating fully in radiating. That is demonstrated in my article diagrams showing that tip of the antenna is getting less current from the top of the coil than the bottom of the coil is.

Modeling shows slight increase in the base current in loaded antennas, I think this is due to the resistance (Q) of the system (wire and inductance) causing reflected wave to have lesser amplitude when superimposing with forward wave. This would perhaps explain W9UCW finding that Q of the coils is not that significant. 

If we have travelling wave antenna, like properly terminated Beverage, that has constant impedance and current along its length, inserted coil would have the same current magnitude at both ends. Or coil inserted in the middle of the half wave radiator (dipole) would have the same current at both ends but "eating" the most of current curve area (Beta match).

Understanding current distribution in the antenna and along it's loading elements tells us about the efficiency of the system. The smaller amount of area under the current curve "eaten" by the loading element, the more efficient antenna.

W5DXP web page has the results of 80m mobile antenna shootouts - measurement tests, showing that the best is top hat loading, then loading coil 2/3 up, screwdrivers, helicals and base loaded are the worst. My best 160m mobile was Hustler with 80m coil and wire going from the top of the coil to the front bumper mast on 72 Buick LeSabre.

Hope this helps understanding wasaaap with loaded antennas, current distribution and means of loading.
Oh, and there is some radiating going on, every cycle RF goes up and down.
Mo high current curve area, mo dBs at the other end.

73 Yuri K3BU.us


> I tried to read this article, but it had equal parts of truth<BR>> and fiction. (The fictional parts were duly noted, but it is
> still confusing). It would be a lot more readable if you just stated
> what is now known to be true. The two ammeter photo is shocking.
> I never would have thought that was happening. I still don't
> understand the theory of where the current goes, but I can't
> argue with the photo. Come to think of it, in a Tesla coil,
> the current at the top is also not the same as the current at
> the bottom. The comments about loading coil Q not
> being critical are also surprising. I have always used top
> loading wires, so I guess I don't have to change anything
> based on this corrected information.
> 
> Rick N6RK
>