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Dual-Banding an L over FCP
(Beta Circuit and Component Values)

We are developing a way to use an already installed and working 160M Inverted L over FCP with Isolation Transformer (160 L/IsoT/FCP) as an 80 meter end-fed halfwave L (80EFHWL). We are not tacking on an 80 meter compromise antenna so we can say it covers 80 meters. The idea is expand an efficient 160 inverted L to include efficient 80M operation. Then a 40 meter solution can use shorter, smaller components without the need for lossy compromises to include 80M.

The 80 meter EFHWL is a single wire antenna without the pattern null points of a dipole or inverted vee. The EFHWL has an omnidirectional, nearly hemispherical 3D pattern that works well for both local and DX. It has a better vertical polarization component than a ground mounted quarter-wave vertical, with excellent low angle coverage for DX because the current pattern on the vertical wire is inverted versus the straight vertical. The RF current max is up at the bend, removing the main antenna fields from lossy ground effects and more quickly clearing RF absorbing trees and local clutter. It lacks the pure vertical's skip zones due to the horizontal wire, which maintains useful radiation at NVIS angles. Correctly done, the 80EFHWL certainly is a strong performer. Some will argue that the EFHWL is the best all-around single wire antenna for 80M.

The unpopular aspect of the 80EFHWL has always been the need for a remote tuning device at the base of the EFHWL. With feed Z sometimes more than 2000 ohms, the EFHWL cannot directly match 50 ohm coax. It has no commercial off-the-shelf remote tuning product designed for it.

The existing 160L/IsoT/FCP feed configuration has already established a point of opportunity for 80 meter adaptation. A relay at the center of the FCP can easily flip the FCP to an effective and efficient counterpoise for an 80EFHWL, and just as easily flip back to 160.

If you do not already have an existing 160 meter L/IsoT/FCP, you begin the 160/80 project by putting up a 160 meter L/IsoT/FCP and making all adjustments to get it working well.

Do not tune the 80 meter addition until 160 operation is satisfactory.

The 80 meter tuning circuitry is switched out of line during 160 operation. Adding/changing the 80 meter operation will not materially affect 160m. Changing the aerial wire, the FCP, or the isolation transformer for the 160 meter operation will detune 80 meters.

For 80 meters:

1) The 160 meter L aerial wire is used as is, no changes. No traps or coils or double wires are needed in the aerial wire.

2) The 160 meter isolation transformer is used as is, no changes.

3) the 160 meter FCP has one item added, otherwise physically unchanged: A knife switch or high voltage relay is added shorting between the FCP feed point and the middle of the third wire to "flip" the FCP to 80m operation. See the red connection in the following diagram:

When the red wire connection is open (not shorted), the FCP is on 160 meters. When the red wire connection is closed (shorted) the FCP is on 80 meters. The physical layout of the relay and shorting wire needs to be brief to avoid detuning the existing 160 operation and avoid increasing loss on 160 by unbalancing the net zero field sums. ** A) Keep the distance between the connection at the center of the third FCP fold and the relay points or switch as short as possible. B) After A) keep the connection from the relay to the FCP feed as direct as possible. ** The relay or switch will need to be up on the FCP center support.

At K2AV this means the relay is mounted in the usual 4x4 sealable plastic electrical box affixed to the center spreader.

At a site where the works are contained in a project box at the center of the FCP, this requirement bears on wiring layout inside the box. When picking wire and insulation, remember the 8kV p-p RF running around. Some lengths of the wire/sleeving used for the isolation transformer may be a good choice.

The shorting connection points are an RF high voltage point when open. The modeled RF voltage for 160 meter 1500 watt operation at the switching point is 8kV peak to peak, requiring attention to a shorting relay or switch able to tolerate 12 kV DC or better. Satisfying this need is discussed below.

4) For 80 meters, a DPDT knife switch or DPDT relay switches in a tapped parallel LC tuning network to tune the now high feed impedance of the unchanged aerial wire.

5) A later addition will extend this scheme to any part of 1.8-2 MHz and 3.5-4 MHz with multiple ranges. Each desired range on 160 or 80 will require a separate additional DPDT relay and pair of taps. This will require a switch box in the shack and multi-conductor cable to the switch.

Here is the diagram for a single tuned range on 160 and a single tuned range on 80. For those who only operate CW on 160 and 80, this simple circuit may be enough.

A single, separate twisted pair of wires should transport the relay energizing voltage for this single range per band setup. This wire pair will need a common mode choke at the tuning network to be prevent the wire pair becoming a source of noise to the antenna and a ground shunt to the antenna system.

There should be a second choke on the wire pair between the tuning network and the station entry ground, either just on the antenna side of the station entry ground or approximately 65 feet (20m) from the tuning network, whichever is closest to the tuning network. Place a common mode choke on the feed coax at this same point.

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This project is still Beta, under development, so without more field experience we cannot yet provide a proven range of values for the coil and capacitor. You may have to experiment with values. Our best scientific wild guess:

We currently estimate a fixed 500 pF high voltage, high current (5 kV, 10 amps RF) transmitting doorknob or vacuum capacitor. It is easy enough to find fixed caps with the voltage rating. But the current rating of the capacitor(s) is just as important, must be known and is not always available. Other than being destroyed by heat from I squared R loss, the capacity change with heat will cause the tuning to wander. Vacuum capacitors are the most stable. Alas the physical size of a doorknob cap does not guarantee its current carrying ability.

500 pF, voltage, amperage, can be provided in a number of ways. A vacuum variable, or vacuum fixed cap can be used, if one owns such or is willing to procure them. Russian surplus transmitting doorknob caps are available that show a kVA reactive rating, Russian lettering looks like kBAp or KBAP. 15 kVA should handle QRO. Availability and prices below are subject to change, and intended to be representative of satisfactory solutions.

Surplus Sales of Nebraska for one, carries High Energy Corp (HEC) format transmitting doorknob caps. Three HEC-style 7.5 kV 170 pF doorknob caps in parallel for 510 pF will carry 7.5 kV at 12 amps. Part # (CFC)HT580170-75, 17 USD each, 51 USD for three plus S&H. MFJ sells these as part # 290-0170-7 for $22.95 each.

For those who would rather shoot the fly with a shotgun and never worry, Max Gain sells a fixed vacuum cap JCSL-500-5S, 5 kV @ 41 amps for 95 USD plus S&H.

We currently estimate a one-size-fits-all coil at 10 to 12 bare turns, 4 turns per inch, of solid bare AWG 12 (IEC 4 sqmm) or larger copper wound on a 3 inch (75 mm) diameter form. This can be any sufficiently rigid high current coil format providing about 3 uH inductance that will support moving taps. To reduce loss the form should be removed from the coil for actual operation. Support the coil at each end. However, do not remove the four polystyrene bars that support manufactured coil stock. 3/16 or 1/4 inch copper tubing hand wound on a 3 inch form and mounted without the form to insulators at the ends may be the least expensive and most efficient winding.

3 inch diameter 4 TPI bare #10 is available as "coil stock" from MFJ (404-0024), used in the output networks of their AL-82,-1200,-1500 amplifiers. A good choice for this application with enough space between wires to support taps, it is supplied in 11 inch lengths, at far less than Barker and Williamson prices. Please advise us of changes in supply and/or new sources of supply of identical or equivalent material.

For relays, we are evaluating a pair of 12 volt Deltrol 20852-81, possibly available from large suppliers. Array Solutions keeps on-hand stock of these as their RF-10 DPDT relays, 20 USD each, total 40 USD plus S&H. One DPDT relay is used as is to switch in the tuning network, the other modified to short across the FCP to change band.

A simple modification to the shorting DPDT relay produces a doubly isolated SPST shorting relay for the FCP with .160 inch (4 mm) of total contact gap space. Using the 30 kV/cm dry air gap breakdown rule this gap provides the needed 12 kV isolation. These Deltrol relays need to be protected from dampness in a weatherproof enclosure. See the first four bullet points at Balun Design's Installation Notes PDF for useful time-tested advice on weather-proofing an enclosure.

An installer may have a vacuum relay on hand or reasonably available. This may be substituted for the modified Deltrol relay providing it has contacts rated at least 12 kV DC, or has contacts which can be series connected, adding individual contact ratings. Verify the model number with manufacturer specs. The most common vacuum relays, RJ-1A, HC-1, etc are not rated to 12 kV. Single throw vacuum relays may require making modification to the circuit above to accommodate NC or NO contacts. Many available HV vacuum relays have 24-26 VDC windings and so will require a power supply other than the common shack 12 VDC to power the switching.

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Do not tune the 80 meter addition until 160 operation is satisfactory.

80 meter tuning is done by alternately moving the 50 ohm tap, and a resonating tap which sets the electrical base of the coil. You will probably need to set the base and 50 ohm taps by moving clip leads while watching an RF analyzer. The tuning points can be marked and then soldered to for connection to the relay(s).

Switching from the shack is accomplished by supplying relay voltage for the band assigned the normally open contacts on the relays. Using the Deltrol relays, coil voltage is supplied for 80 meters.