Hamstick or base loaded 8' whip?

Yeah, top loading is a very good way to go in terms of current distribution on the radiating element and a cap hat is very low loss compared to loading inductors. But top loading also has practical limits. It would take an awfully big cap hat to load an 8' whip on 80m. Some combo of center and top loading can work nicely on short antennas but when the antenna gets very short in wavelength terms it's typically not practical to use capacitive top loading alone.

 

Actually it does require controlled test environments and calibrated equipment, otherwise the measured test results are meaningless.

So testing in an uncontrolled environment does have it's limitations, but it can still serve useful to establish a "red light / green light " baseline for conducting antenna tests but not much else.

 

How well a base loaded whip works Depends how the base loading inductor is made. I have seen base loading takes a lot more inductance and a coil made of thin wire will have more I squared R loss. More current flows at the base and a middle or top load spreads out the current where it can get more radiation out.
My Hustler mobile antenna worked very well with the loading coils up near the tip. A base loaded ten meter antenna was not nearly as good as a Hustler, EVEN though the base loaded one was on a van roof and the hustler was side mounted just a yard above the road. Comparing reception and signal reports from all over, local and DX.

 

Yes, RF current is highest near the base of a shortened, loaded vertical. But for that reason it takes LESS loading inductance to base load rather than center load a shortened vertical of the same height. As you move the coil up from the base the inductance has to increase to achieve resonance which is why center loading is about optimum. If you keep raising the loading coil position the required inductance becomes very large and would become infinite if you tried to place the coil all the way at the top because RF current drops to zero at that point. But sure, a low Q coil, whether that's due to high conductor losses or poor geometry choices will reduce antenna efficiency whether you're talking about base or center loading.

Even though it takes less total inductance to resonate a base loaded short vertical as opposed to a center loaded vertical of the same total height (at the same frequency) the center loaded version will have higher overall efficiency given equivalent coil designs(i.e. similar conductor choices, similar coil geometry choices) due to the distribution of RF currents along the radiating element.

 

re: "How well a base loaded whip works Depends how the base loading inductor is made."

Do you think the current magnitude reaches the same value as at the base of a 1/4 WL vertical element?


@K7TRF
re: "Yes, RF current is highest near the base of a shortened, loaded vertical."

Hmmm ... which side of the loading coil?
And same question to you:
Do you think the current magnitude reaches the same value as at the base of a 1/4 WL vertical element? (on the 'antenna' side of the loading coil.)

.

 

Frankly that's a detail I've never considered nor really care about. I build antennas with various constraints (e.g. full 1/4 wavelength or shortened to fit the application) whether the current magnitude is the same in the antenna I will build vs the antenna I can't build in that position isn't really something I'm likely to worry about.

But I'm sure you can analyze or model it if this interests you.

 

Now I know why a combination of a high-Q center coil + large cap hat (at the very top) generally yields the best results for shortened verticals---thanks!

 

Top loading works very well if there is adequate spacing between the coil and capacitive top hat but it has physical disadvantages. As one moves the coil up the shaft, the coil must increase inductance which means more bulk and more weight.

Also, I once tried a coil at the top with no stinger and no top hat and it was very unstable - k7trf told us why. Since then, I always add at least a one foot stinger at the top of any loading coil.

 

The resonant feedpoint current is higher for a base loaded mobile antenna (with a high-Q coil) than it is for a center loaded mobile antenna of the same length because the feedpoint resistance is lower. The radiation resistance is also lower.

The resonant feedpoint current is higher for a short center loaded mobile antenna (with a high-Q coil) than it is for a 1/4WL vertical because the feedpoint resistance is lower. The radiation resistance is also lower.

As one shortens a vertical antenna from 1/4 wavelength, the radiation resistance decreases.

 

Crack a book:

https://archive.org/stream/TheArrlAntennaBook/Hall-TheArrlAntennaBook#page/n287/mode/1up

Six pages is all it takes.

73,


Mark.