[Elecraft] Helix-wound EDP

[Ron D'Eau Claire]

Thom, K3HRN wrote:

What I was hoping to do was have a helix that acted like a w3edp, ie, a 
multi-band antenna.   So, if the edp is 87 to 89 feet, would that mean that
a 
helix might be 174 to 178 feet of wire wound on the form?

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I did some experiments  with a helical antenna wound on a wooden form (2x3"
fir, painted). 130 feet (42 meters) No. 16 wire wound on a pole 12 feet long
resonates as a 1/4 wave at just 3850 kHz. On my lot where it is a challenge
to get 60 feet of wire in the air, my regular antenna is a 50 foot doublet
up 30 feet fed with true open wire line, using a link-coupled balanced
antenna tuner in the shack. 

There's nothing unusual about the W3EDP antenna. It's a simple end-fed wire
with a single-wire counterpoise. The parallel tuned network means that W3EDP
found that it presented a rather high impedance on all bands with the
counterpoise arrangement he used. Note that both the counterpoise and
radiator are tuned as a system by the network. That's why he disconnects it
on some bands to help get a match more easily.

Like *all* such antennas, efficiency is highly dependent upon the radiation
resistance of the radiator compared to that of the ground system. That means
that it is the physical size of the radiator that is important to
efficiency, not its resonant frequency. On those bands were the wire is
close to 1/2 wavelength long (40 meters) the efficiency will be high because
the impedance will be in the thousands of ohms. That will work well even
where the ground is very poor. An 85 foot wire will work fairly well even
down on 80 meters where it's only a little over a 1/4 wavelength. A 1/4 wave
wire has a radiation resistance at resonance of 30-something ohms. Even a
poor ground showing close to 300 ohms resistance will permit an efficiency
of 10%. Believe me, there's a lot of antennas, especially 80 meter antennas,
no where near that efficient. 

For example, mobile antennas. And that's where your helical design (and the
one I've played with) come in. 

Even though an antenna may be 'resonant', thanks to loading, either with a
coil or by making the antenna one huge coil such as you contemplate, it will
have a very, very low radiation resistance. In such a short antenna on 80,
expect something less than ONE ohm, even though it may be resonant. That
means a full sized W3EDP might be as much as 10% efficient with it's minimal
ground system on 80 meters but your helical antenna efficiency will be far
less than 1%. 

The full-size W3EDP is longer than 1/2 wavelength on 40 and up, so its
efficiency may well exceed 50% or 75% on those bands where the radiation
resistance is likely well above 1000 ohms.

But that won't happen with a loaded radiator of any sort. The efficiency
will stay low, although it does improve somewhat as the frequency increases.
For example, my helical antenna starts to look, in terms of efficiency, like
a decent 1/4 wave on 20 meters. That's because at 12 feet it IS a decent 1/4
wave on 20 meters, or very close to it! Of course, it's no longer necessary
or desirable to wind the helix on that band either. My matching network has
to compensate for all that inductance and the ohmic losses in the wire
remain! 

I came to the inescapable conclusion that my short doublet was a far better
80 meter antenna than the helical vertical, even with a full-size 1/4 wave
"radial" or "counterpoise". I have a purpose-built high-efficiency tuner
designed to match it without baluns. It's the classic balanced tuner design
seen in the literature everywhere*. There's a picture of it in my shack
above the K2 on QRZ.COM. It's not fast for band-changing but it's versatile
and efficient at extreme load impedances. At 100 watts, I work stations out
to about 1,000 miles on 80 with consistently excellent reports. That's just
about right for the limit on a so-called NVIS (near vertical incidence
system) antenna in which all signal goes up to be scattered off of the
ionosphere over that range in all directions. Side-by-side comparisons with
the helical/counterpoise arrangement show a 2 or more S-unit difference in
almost every test, with the doublet the best. 

Those few situations where the helical was equal to or better were at longer
distances - out near 2,000 miles or so on the eastern coast of the USA from
Oregon. Such contacts are very rare for me to make, but when I have, the
vertical sometimes showed a SLIGHT  advantage, or is, at least, considered
equal to the low doublet. That fits with the presumption that a vertical has
a lower angle of radiation. The reality is that the vertical helical is not
much different from the short doublet at long distances without an excellent
ground screen, and  far inferior at short skip ranges where high-angle
radiation is important. Without a large ground screen or radial system, the
earth losses are very high, meaning that most of the RF radiated below 20 or
30 degrees above the horizon is attenuated anyway. 

Some years ago Doug DeMaw, W1FB, published a design that might work for you
in a tight situation. I've considered it here, although the short doublet
has worked well enough that I've not tried it. He bent a doublet at 90%,
like you might for an extreme "inverted V" design, but arranged it so one
radiator was horizontal to the earth, making the other one vertical. He used
guyed aluminum pipe for the vertical radiator and a simple wire for the
horizontal radiator. The system is fed with open wire line and an efficient,
balanced ATU. I believe he settled on a compromise 25 feet for the vertical
and horizontal segments, producing a radiator with about the same electrical
characteristics as mine. The challenge there is to get the whole system up
as high as possible. That horizontal wire is an integral part of the
radiation system! But if you can put it on a roof at, say, 20 feet, he
reported quite good  results. I found that it was easier to put up my
all-wire doublet with one leg nearly horizontal and the other dropping down
to a tie point at about 12 feet above ground. 

I highly recommend Cebik's excellent web pages for more discussion about
antenna and tuner design, especially for those of us with limited space. 

Ron AC7AC

* http://www.cebik.com/link/link.html