Aerials

by Kurt N. Sterba

(This article ran in Worldradio, May 2001)

The resonant feedline dipole

A reader sent Kurt a description of the "Resonant Feedline Dipole" and asks if, in Wise Old Kurt's opinion, it really works that way. Well, yes, it does. It was described by W2OZH in the August 1991 issue of QST.

To make one you take a length of coaxial cable and at one end attach a quarterwave length of wire to the cable's center conductor. Don't connect the cable's shield to anything.

Now go a quarter-wave down the coaxial cable and make a RF choke by winding the cable into a coil. W2OZH used 13 turns on a 6-inch diameter form for the 80-meter band. Your antenna is the quarter-wave wire and the quarter-wave coaxial cable; they form a half-wave center-fed dipole. Suspend it in the air and run the rest of the coax down to your transceiver.

The nice thing about this arrangement is that there is no feedline dropping down from the center of the dipole. The feedline comes out the end. This can be very convenient. Quite often you can use the house to support one end of the antenna and a tree in the yard for the other end. If you put up a normal dipole this way you have to run the feedline from the center of the antenna back to the house. This can be awkward and puts the feedline parallel to the antenna so it can pick up RF as it goes along and bring it into the shack.

With the Resonant Feedline Dipole the feedline comes right out the end of the antenna there at your house. In this situation most Hams put up a "random-wire" end fed. But the impedance at the end is high and you have to use a tuner. The Resonant Feedline Dipole, on the other hand, gives low SWR on its design band so you don't need a tuner at all.

How it works

The Resonant Feedline Dipole takes advantage of the fact that coaxial cable acts like a three conductor cable for RF. There is the inner conductor, the inside of the shield, and the outside of the shield. RF signals travel down the inside of the cable with equal currents on the inner conductor and the shield. The current on the shield does not penetrate the shield. Think about it, why do we call it a shield? Because it shields the RF from the outside world, that's why.

So we are left with the outside of the shield that has no RF on it that we can use as another conductor of RF. That's what this antenna does.

On transmit we send RF down the cable. The current on the inner conductor goes right on down the quarter-wave wire we've attached to it. The current on the inside of the shield comes out and goes back down the outside of the shield. It has nowhere else to go. We want it to do this but we want it to stop after going a quarter-wave down the shield. To do that we put an impedance in its path by winding the coax into a coil. This does not affect the signal coming down the inside of the cable but it stops the flow on the outside of the shield. This gives us a half-wave center fed dipole.

Now, the impedance at the end of a dipole is high; several thousand ohms. W2OZH's 13 turn coil only has about 400 ohms impedance. Not enough. But he adjusts the spacing between turns, varying the capacitance, until the inductance of the coil and the capacitance resonate on the dipole's frequency. This increases the impedance until it is several thousand ohms. So no RF flows on the transceiver side of the shield.

Kurt's end-fed dipole

W2OZH's coil requires some adjustment and it's large enough to call attention to an antenna installation that the neighbors otherwise might not notice. You know what Kurt is talking about.

So Kurt has devised an antenna using ferrite toroids to keep the coil small and to give it enough impedance so it doesn't need to be adjusted or resonated. It works very well at transceiver power levels.

The 20-meter version has a plain wire part 16 feet 4 inches long. Connect this to the center conductor of your RG-58 coax leaving the shield unconnected. Go 16 feet 4 inches down the coax and run the cable through two F240-61 toroid cores that you have either glued or taped together. Make 10 turns of the coax through the toroid. Be neat; place the turns side by side as you go and pull them tight so the coil is as small as you can make it.

Suspend this antenna up in the clear and feed the balance of the coax in to your transceiver. Kurt got an SWR of 1.5 or less over the whole 20-meter band.

Don't use this antenna with your legal limit amplifier. 100 watts continuous or a couple hundred PEP is fine. The reason for this is that the coil is high impedance, about 3,000 ohms, and ferrite cores don't like to work at high impedance. If you make them high impedance and apply lots of power they complain by absorbing some of the power and getting hot. You don't want that.

You can make this antenna for other bands by scaling the wire lengths as needed. On 10 Meters one F240-61 is enough for the coax coil. On 40 Meters you'll need four. You get the idea.

Loop around the house

A reader would like to put a full-wave horizontal loop about 20 feet above the ground and with his house located in the center of the loop. This will make a "skywarmer" for good local contacts. Will the house upset the pattern? How about RF in the house? He was told that there is no net field inside a loop.

Well, you are almost certain to lose some signal by absorption by metal structures in the house. But it may work perfectly well overall. A few dB loss is not likely to be noticeable in local contacts.

The RF field, though, may be a problem. Remember that when a wire has current in it a magnetic field forms around it. There will be places where the field from the wire on one side of the loop cancels that of the wire on the other side but that cancellation is at one location, not everywhere within the loop. Yes, there will be fields inside the loop.

Kurt wouldn't worry at all about the fields in the house but the FCC does. You have to meet their guidelines for human exposure to electromagnetic fields. Your antenna location does not fit the usual simplified tables developed to make it easy to check, but the experts at ARRL should be able to help.

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