[Elecraft] (OT) EFHW Vertical Antenna (Mike Morrow)

[Ron D'Eau Claire]

Bill, you are absolutely right - a radiator of a given size is 1/2 wavelength long on only one frequency. 

There are two reasons for providing an RF ground for any end fed antenna:

1) To minimize the RF currents induced in the earth which cause loss.

2) To keep the RF voltage on the rig's enclosures and ancillary equipment low. 

Reason 1) is really critical when a short radiator is used - 1/4 wavelength or less - because such radiators have a low impedance, hence a relatively large RF current flows into the radiator and into the RF ground system. The RF power is divided between the antenna and ground, with the larger resistance consuming the most RF power. So a poor, high resistance (impedance) ground working with a low impedance antenna will see most of the RF power consumed in the RF ground. It's not unusual for a short antenna with a poor ground to have less than 5% or the RF radiated and 95% consumed in ground loss. 

With a given ground system, the losses can be reduced by raising the feed point impedance of the radiator because that also reduces the current flowing into the RF ground system. An easy way to do that is to make the radiator 1/2 wavelength long. Such an antenna can easily be more than 90% efficient even with a minimal ground system. Such antennas are often called 'ground independent' but, like most things, that's only partly true. There is the issue of losses near the antenna caused by RF currents induced in the ground by the RF current in the antenna. However, a 1/2 wave vertical already has the high-current area of the radiator raised 1/4 wavelength above the ground, so the coupling to the earth is much, much less than a 1/4 wavelength or shorter monopole that has its feed point (and highest current point) right at ground level. 

Reason 2) applies to any antenna that brings a voltage loop (high RF voltage point) to the rig. Usually some sort of RF ground system is needed when feeding a 1/2 wavelength radiator for this purpose. But it doesn’t have to be much. A cold water pipe connection or a single 1/4 wave "radial" will do (and the radial will not radiate significantly because very little RF current flows into it). 

The extreme high impedance found at the end of a 1/2 wavelength antenna or the extreme low impedance found at the end of a 1/4 or shorter antenna never occur at longer lengths. The impedance continues to shift up and down every 1/4 wavelength, but without the extreme values. So such true "long wire" antennas are less dependent upon a really good RF ground system and, with a suitable matching network, work extremely well over a very wide range of frequencies. 

I have a half wave end fed inverted "L" on 80 meters, and it works extremely well up through 10 meters. (It loads on 6, but I haven't tried it there yet.)

Ron AC7AC



-----Original Message-----
Stan and group, I found this post and referenced video very interesting. However I am bothered by several things.

First, a multi-band antenna cannot be a “end-fed half wave” on all bands. only on one band, as far as I know. 

I’ve read many articles about “end-fed half wave” antennas over the years, and they have always been single band antennas with special matching circuits to tune them, and most have agreed that when they are a half wave on the frequency used, a counterpoise or radial is not required. 

Which leads me to question the statement that a counterpoise or radial other than maybe a one meter or less one is required for this antenna. A 30’ antenna will only be a half wave near 20 meters, what happens on the rest of the bands? I know that sometimes using 5 watts or less, you might not notice a hot chassis or RF floating around, but, what is going to happen when you load this antenna with 100 watts?

I would appreciate hearing from others and what they think about the questions I have raised.

73, Bill, k6mgo
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