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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. Rather we enhance an existing efficient 160 inverted L to include efficient 80M operation. Then a 40 meter solution can use shorter, smaller components without the need for long and 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. It has 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, which removes the main antenna fields from lossy ground effects and more quickly clears RF absorbing trees and local clutter at takeoff. 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 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:

With the red wire connection open (not shorted), the FCP is on 160 meters. Closed (shorted), it's on 80 meters. The relay and shorting wire's physical layout needs to be brief. Otherwise they will detune the FCP's 160 operation and increase loss by undoing the net zero sum of fields. To avoid this:
3a) Keep the distance between the center of the FCP's third folded wire and the relay points or switch as short as possible.
3b) After 3a), 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.

3c) At a site where the works are contained in a project box at the center of the FCP, 3a), 3b) requirements bear on wiring layout inside the box. When picking wire and insulation and routing these wires, remember the 8kV p-p RF running around. For these connections do not use wire with unknown insulation characteristics. Some lengths of wire/sleeving left over from trimming the isolation transformer leads may be a good choice. Or use #14 AWG (2.5 mmsq) bare solid copper wire in standard wall #12 PTFE tubing.

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 two ranges in 1.8-2 MHz and up to five ranges in 3.5-4 MHz. Each desired range on 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 guesses below:

Please advise us of changes in supply and/or new sources of supply for identical or equivalent material.

80M Capacitor

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 capacitor's current rating is just as important. The current rating must be known and is not always available. Do not use capacitors without a known, adequate current rating. Other than being destroyed by heat from I²×R loss, the capacity change with heat will cause the tuning to wander. 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. Vacuum capacitors are the most stable, generally with high current ratings, if one owns or is able to procure them. Russian surplus transmitting doorknob caps are available that show a kVA reactive rating, Russian lettering looks like kBAp or KBAP. Caps with these labels usually include a kB rating which is kV. Divide the kBAp by kB which will give you the current rating. One such 470 pF cap has 15kB and 40kBAp and therefore has a current rating of 40/15 = 2.7 amps. Some of the Russian caps look heavy duty, with the same outward appearance, but are not marked with a kVA rating, only voltage. Do not use these. We have reports of destroying these with 1500 watts.

Probably the least price "stock store-bought" 500 pF transmitting capacitor can be made with three High Energy Corp (HEC) format transmitting doorknob caps in parallel. Three HEC-style 7.5 kV 170 pF doorknob caps in parallel will produce 510 pF rated 12 amps (1 MHz) at 7.5 kV.

At this writing the caps are available from Surplus Sales of Nebraska as their part (CFC)HT580170-75 , Look down the page under HT-50/58 Series. MFJ sells them as their 290-0170-7 , RF Parts as their 580170-7 . See links for prices.

As of (V.2019.03.15), the bargain-priced JCSL-500-5S fixed value vacuum cap previously mentioned here is no longer for sale at MaxGain. Such items will appear on Ebay from time to time, or on the sources above with limited quantities.

80M Inductor

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 tinned copper is available as "coil stock" (see left) from MFJ, their part 404-0024, used in the output networks of their Ameritron AL-82,-1200,-1500 amplifiers, and repairs of same. See link for price. It's a good choice for the 160-80 dual-banding project, with just enough space between turns to support taps. It is supplied in 11 inch lengths at far less than Barker and Williamson charges for the same diameter/wire size product.

A number of important information points apply to use of the MFJ coil stock. Please read all below:

(A) There can be a back-order interval on coil stock until a manufacturing run. You should order this item well in advance of your need and be thankful we have a source.

(B) This project will only use three of the eleven inches stock length. MFJ will only sell the eleven inch manufactured length. You may wish to share part of the length with others to save on cost. However, this writer is forever needing coil stock for something or another, and bought two 11 inch lengths the last time, just for himself. It has a near infinite shelf life.

(C) The coil stock itself is a bit fragile as manufactured. For this strengthening task only use Krazy Glue™ or an exact equivalent very thin consistency cyanoacrylate glue. Put a tiny drop of glue on both sides of the wire at all points where the wire is melted into the support rods. The very thin glue will soak into the air bubble spaces where the wire embeds the plastic. Allow 24 hours for full curing before cutting the coil into smaller pieces. When you have done this, the coil will have a distinct rigid solid "feel" to it.

(D) When you are cutting a piece of the coil stock to use, leave extra length for forming mounting wires. To do this, leave an extra entire ¼ turn for connection on either end. Do not attempt to use the short turn piece for connection. For three inches count 12 full turns plus two additional full ¼ turn sections, a total of 50 ¼ turn sections. In the middle of the 51st ¼ turn section cut the wire with a Dremel tool. Keep the angle of the cutting disk to the wire very small, almost parallel to the wire, to keep from scoring the adjacent turns. You can clean up the coil after you separate the 12 turn section.

Cut the wire before you cut the clear support rods. Otherwise you risk separating the wire from the support rods, which you will find amazingly difficult to repair.

Before cutting the support rods, twice carefully recount the ¼ turns, and twice carefully verify the cutting points. Mark the verified cutting points with a sharp tipped marker. It is very easy to get the cutting points mixed up if they are not marked.

Once the coil section has been separated, free the outside ends of the ¼ turn end wires from the support rods, cutting with the Dremel tool right next to the outer wire. You can then clean up the outer end of the ¼ turn worth of end wire with the Dremel tool.

Important: When forming the bends for the connecting ends of the coil, do not use the clear support rods as the rigid clamp for bending the wire. Use an end tip of a bench vise or vise-grip pliers to solidly grab the ¼ turn end wire right next to the clear support rod. Then make your bends, cuts, etc, with the vise protecting the support rod from any wire twisting or bending.

(E) Until you have performed (C) on the coil stock, do not store or use this coil stock anywhere the air can be damp (including outdoors in an enclosure). Where the wire melts into the support rods, if the tiny bubbles get filled with water, carbon arc paths can develop under high power. The Krazy Glue fills the pockets where the tiny bubbles touch the wire.

Band and Band Range Switching Relays

For the main relay that switches in the 80M circuitry, a single 12 volt DPDT Deltrol 20852-81 suffices well. You can find these at Allied Electronics . These are also stocked at Array Solutions at a markup, but may get you one in time when Allied is back ordered.

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.

We had also been evaluating the Deltrol relay for shorting the FCP to flip it to 80M operation.

To understand the voltage requirements for the FCP shorting relay, in EZNEC Pro running the NEC4 engine on an L over FCP model, we inserted an EZNEC "load" in the FCP shorting wire, and set R and L to 0 (short) and set C to values up to 20 pF to represent misc capacitances across an open relay. Clicking LOAD DAT on the main EZNEC window, NEC4 calculated as much as 3540 volts RF RMS across the open circuit. RMS voltage is needed to calculate power, etc.

To calculate the DC breakdown voltage most often quoted in relay specifications, we have to convert the RF RMS voltage to RF peak-to-peak. RF PP = RF RMS × 2.88. 3540 × 2.88 = 10195 VPP. That means that we could expect a relay with a DC breakdown voltage of 8 kV to arc often or continuously if 1500 watts makes it to the FCP feed point. A minimum 12 kV, or better 15 kV DC breakdown gives a rounded figure plus a safety factor.

Modifications to the Deltrol DPDT relay produces an SPST normally open shorting relay with .160 inch (4 mm) of total contact gap space. Both Normally Closed contacts are bent outward to increase the gap between the armature leaf contact and the Normally Open contact. The new gap is 80 mils (2 mm). The connection between the armature leaf and the base connection pin is removed on both poles. The two armature leafs are then wired together, placing the two normally open contacts in series.

The 3.0 kV/mm dry air gap breakdown rule says each gap provides 6 kV isolation.

When measured with a Hi-Pot, either gap tested by itself breaks down at approximately 6 kV DC, according to the rule. However, when the two gaps are wired in series, the combined gap breaks down at only 9 kV DC. This is less than the possible 10 kV calculated operating voltage at the shorting point, as indicated in NEC4 model. So 9 kV is inadequate at 1500 watts, but is enough to handle 500 watts (6 kV at the shorting point), or a barefoot transceiver.

It is clear from measured results that one cannot add the individual breakdowns when gaps are put in series. So to handle 1500 watts, this application would take a pair of modified Deltrols, their coils in parallel, and their four contacts in series to handle QRO with 6+3+3+3 = 15 kV. This makes vacuum relays less expensive relatively, and intuitively far longer-lived in this actual use.

An installer may have a suitable 12+ kV vacuum relay on hand or reasonably available. Several stations originally considering Deltrol for the FCP shorting relay have managed to obtain 12+ kV vacuum relays from various sources.

Before purchasing a vacuum relay verify the manufacturer specs for the model of vacuum relay. Being called "HV" by the seller may mean anything. The most common vacuum relays, RJ-1A, HC-1, etc, at 2.5 or 3 kV DC are woefully short of the needed 12 kV. There are many vacuum relays with 5, 8, 10 kV DC ratings inadequate to this purpose.

Another source of confusion on voltage specs is some suppliers listing load switching voltage instead of voltage rating between contacts. When you see a vacuum relay with a 1000 or 1200 VAC rating it is probably the maximum under-load switching voltage that can be frequently exercised without a huge decrease in operational life. See this relay spec for an illustration of both voltage types on the same page.

SPST single throw vacuum relays may require making modification to the dual band circuit above to accommodate NC or NO contacts. Many available suitable 12+ kV 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.

At times, we have seen Jennings RD6A vacuum relays at Max-Gain at a very decent price. 15 kV DC, 24 volt coil, SPST Normally Closed. Potential users of the circuit will need to keep their eyes on the various sources for vacuum relays. They come and they go. K2AV bought an RD5A from Surplus Sales for his FCP shorting relay. The RD5A is SPST, 15 kV normally open contacts, and will allow his preferred power-off-selects-160M.

The vacuum relays above are usually used, pulls from equipment, sometimes NOS. The only ham price new manufacture vacuum relays we are aware of are the Taylor brand VC2F and VC2T series sold by RF Parts . These are SPDT vacuum relays rated 15 kVDC, with 24 and 12 volt coils and both flange and threaded mounts. They will support either power-on or power-off selects 80m by chosing the NO or NC contact of the SPDT relay.

The RD6A's normally closed contacts will require that power-off-selects-80M, and 24 volts applied selects 160. The Deltrol 20852-82 is the 24 volt version of the 20852-81, and if used for the tuning circuit switch will allow a single selection voltage for operation. Note that several contacts of the diagram above need to be reversed for power-off selects 80M.

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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.