A big thank you to Frank, VO1HP, St. Johns, Newfoundland, whose successful version of the L + FCP + short tower is the genesis for this method.
FCP refers to the 5/16 wave single wire folded counterpoise for low bands, described at http://www.w0uce.net/K2AVantennas.html At some point an illustrated version of this post will be available on that web page. Caveat: ----------------------------------------------------- I don't have Frank's setup, nor does anyone close by (all using trees), so I haven't been able to do personal hands-on independent validation. But the NEC4 ** modeling supports both Frank's experience in Newfoundland and the "suck out" degradation I have experienced with vertical wires close to resonant/near-resonant towers. Since we've had so many inquiries about doing L+FCP off a tower, we decided to release this information anyway and let people give it a try. I expect we will gain more experience rather quickly. We will keep readers posted on developments. *** Super short version: ---------------------------------------------------- This is a simple solution of limited application range for reducing lossy interaction between a 160 meter inverted L and a short tower supporting the bend. The method does not involve tower detuning, or complex anti-resonance schemes. Up top, support the bend in the L 1-3 feet from the tower. Locate the FCP feed and isolation transformer box 13-15 feet horizontally from the tower. Pull the vertical run straight from the isolation transformer box up to the bend. The wire up to the bend will be at an angle to the tower, 10 or more degrees off vertical. ===> There are some nasty devils in the details, ways to lose all your gains. Read everything carefully. <=== Short version: ---------------------------------------------------- Some installing an FCP under an "L" must support the L's bend with their short tower because it's the available high point on a small lot. This tells how to do that without losing gains from FCP to losses where the tower goes into the ground. It requires the tower plus top load being resonant well above the 160 meter band. Performance *improves* with *poorer* RF connection between the tower and ground. The method consists of : a) how to run the "L" and locate the FCP in relation to the tower. b) removal of existing tower base radials, ===> BUT lightning ground rods and connections remain. <=== c) breaking up destructive resonances in other close antenna + cable connections by using or adding balun(s) and common mode choke(s) WITH FULL RATINGS ON 160 METERS. If your tower is bristling with large antennas up top this almost surely won't work for you. If it's taller than 65-70 feet and has an HF beam, this solution most likely won't work for you. This solution is specific to a single wire antenna, commonly called an "inverted L", run vertically, then bent to a horizontal direction with the bend supported by a tower. The far end of the "L" away from the tower can be very miscellaneous, even a partial inverted U, without affecting the solution. Long version: ------------------------------------------------------ Frank has a small lot in St. Johns, Newfoundland, with a layer of dirt on the big rock that is the island. The bend in his "L" is supported by a 55 foot crank-up tower topped with a small tribander. Short miscellaneous radials were not working with his "L". So he converted to an FCP. He kept the tower supported bend, but the FCP had to be located away from the tower due to yard circumstances. Frank then put up surprising summer numbers at EU RBN sites, and worked (unprecedented for him) new top band countries in summer season. That REALLY got my attention. Low band vertical wires closely supported by a tower have always have possibility for severe interaction loss. One can model a driven vertical wire next to a tower where the tower's induced current exceeds the driven current on the wire. For all practical purposes the vertical wire was just another way to load the tower. Modeling a correspondent's actual tower/tribander/L/FCP dimensions always came out middling to poor to awful. On small lots the tower was just one more way to push power into a lossy medium. Until VO1HP. Frank gave me the exact layout for antennas and back yard and some data about his local geology. I modeled Frank's setup and monkeyed with it a while. I finally figured out why it worked and that led to a method for shorter tower/L/FCP users that minimizes the usual demons of tower-supported vertical wires. Frank's setup matched the method in all but one detail. His FCP itself is per the web page. Keeping the bend close to the tower up top, and slanting the wire away from the tower going down reduces current in the tower, in turn reduces tower induced ground loss. The method doesn't eliminate all tower base loss, but reduces it significantly. A two to one tower current reduction is a four to one reduction in power loss, three to one current reduction a nine to one loss reduction. I have done many NEC4 runs on variations of the VO1HP solution, looking for defeating sensitivities, including modeling wires and coax attached to the tower and nearby. There is a common, particularly destructive sensitivity that requires treatment: A 160 or 80 or 40 meter inverted vee or dipole, fed with coax, either on the tower or nearby. Nearby vertical coax runs for other supported antennas may also need treatment. See the text in 8) and 9) below. ------------------ The Essentials of the VO1HP small tower L+FCP method, as optimized. ------------------ 1) This solution is for a short to medium tower, supporting a small to medium triband beam just above the top of the tower with mast and boom not insulated from the tower, a very common situation for 1/4 wave L+FCP+tower inquiries. For a tower 60' and less with a small or medium tribander up top, the solution seems equivalent to a tree supported 1/4 w L/FCP of the same size, shape and ground characteristics. The solution works for towers up to about 70 feet with *small* tribanders up top. With a lot of top aluminum at 65 or 70, or by 80 feet regardless of top load, the performance has already fallen off the cliff into steeply increasing loss and the method can't be used. The problem is the tower plus top-loading getting closer and closer to resonance and increasing tower current. 2) Support and insulate the L's bend 1-3 feet from the tower. This allows a rigid support which maximizes the height of the bend. Because neither end of the wire moves, this keeps wind stress from trees from affecting the wire at the isolation transformer box, or changing the spacing between wire and tower, and allows pulling the wire straight from the bend to the transformer box, required for best results. 3) The FCP feed point and isolation transformer box should be 13-15 horizontal feet from the nearest tower surface. This means you can't support the FCP feed point on the tower, or directly below the bend. This results in the wire running down from the bend at 10 degrees or more off tower vertical. Less separation than 13-15 feet is lossier *and* more separation than 13-15 is lossier. This is true with the shorter tower heights we are talking about. 4) The azimuth direction from the tower to the FCP feed can be any angle relative to the horizontal wire of the L. There is only a very mild advantage to placing the FCP feed in the quadrant underneath the horizontal. Practically, it doesn't matter. Let the necessities of the lot dictate. The FCP feed can be on the other side of the tower from the horizontal or out to the sides from the horizontal, in addition to being underneath the horizontal. Support the bend 1-3 feet to the side of the tower going past to use an FCP feed away from the horizontal. 5) The distance from the FCP to the tower at any point along the wires should be no less than the half the distance from the FCP to the ground. The combination of (2), (3), (4) and (5) should allow a great deal of flexibility for FCP placement to deal with physical issues on the small lot. 6) If you have radials attached to the tower base, they should be removed or disconnected. You want the tower to appear to RF like an uncooperative resistive short non-resonant conductor. IMPORTANT: Do not remove connections to grounding rods. These are required for lightning, and to satisfy building and electrical codes. Since they are notoriously poor RF connections, removing them gives you no RF advantage and does not help this method. 7) This method also minimizes induced current in miscellaneous tower conductors, thus *not* requiring special treatment of tower conductors *just* for the FCP. Exception: See 8). *Other* reasons to treat tower conductors still apply, such as grounding, blocking common mode noise on conductors from the house to keep it from going up the tower and getting into other antennas, etc, just as they did before. 8) IMPORTANT: If you have a 40, 80 or 160 meter dipole or inverted vee, and possibly other 40/80/160 antennas, either nearby or with its feed coax going up the tower, you must treat them or they may defeat your L+FCP+tower setup. Both sides of the dipole or inverted vee must be connected to feed coax via a common mode blocking balun RATED FULL PERFORMANCE ON 160 METERS, regardless of the normal band of use. On 160 meters neither wire of the dipole or vee must be able to "see" the common mode looking through the wire-to-coax connecting device. If either wire of the dipole or vee has an "easy ride" to the common mode on the feed coax shield on 160, it becomes the equivalent of a massively top-loaded small tower, and defeats the method. Other 40/80/160 meter antennas may present the same problem. If the vee/dipole center is mounted on the tower, the 160 rated balun at the top will suffice. If the vee's apex is not supported by the tower, and the coax is not running down the tower, but is nearby, the induction to the coax from the L's vertical wire is NOT minimized with the slant wire trick, and a 160 meter rated common mode block is required at the ground, before any grounding, in addition to the 160 meter rated common mode blocking at the feedpoint. 9) Other nearby coax fed antennas with a vertical coax drop not on the tower need a 160 meter rated common mode block at the ground, before any grounding. When the RF coming off the L is looking around in the near field, you don't want it to see any ad hoc 160 antennas coupling to the ground. You want the RF to get a bad taste in its mouth if it decides to use the tower. 10) Metric values: Tower roughly 20 meters or less and FCP feed 4 to 4.5 meters horizontally from tower. Bend in the L supported 0.3 to 1 meter from tower. ----------------- Other considerations ----------------- Don't start on a program of changes related to this posting unless you are within the size limits. A resonant or near-resonant tower/beam combo supporting an L/FCP can be remarkably lossy. If you don't fit the limits, there may be a different technique for you later, but we don't have anything proven. VO1HP on the other hand, has a proven installation of this method. Many have tried loading RF to a tower on 160 using whatever short, sparse, miscellaneous radials they could manage in the cramped confines of the small lot. If that really worked, they would be happy with their loaded tower and we wouldn't be having this discussion. If one is putting up an L/FCP, the radials are not needed. DISCONNECT THE RADIALS FROM THE TOWER! They are lossy if connected to the tower. Radials lower the impedance to ground that a non-resonant tower otherwise presents against induction from the vertical wire, increasing the induced current, and increasing lost power. You either do tower radials well, or you do an FCP. But not both. No combos. A ground rod(s) for lightning may be required by building or electrical code, or a UFER ground (controversial) may suffice. I will not weigh in on that. But the lightning ground rods are notoriously bad at RF. DO NOT REMOVE lightning grounds -- a lot of risk for NO payoff. DO NOT AVOID INSTALLING LIGHTNING GROUNDS to "help" this project. You won't help it. Follow your building and electrical codes. If you do this, please let me know how it goes. 73, and GL Guy K2AV ------------------------ ** Modeling this circumstance requires the ability to specify conductors below ground. NEC4 can process underground conductors, but NEC2 based processes can't. Only users with the NEC4 program itself or running the NEC4 engine in a shell (e.g. EZNEC Pro), with the appropriate NEC4 license, can currently model this problem. *** I am trying to get in touch separately with all those who have corresponded with me in the past, mentioning both "tower" and "FCP" in the same email and without header reference to TopBand. I have no way to know if any of these receive TopBand. Most correspondents have not posted here on TopBand reflector, that I have been able to tell, though many read and do not post. _______________________________________________ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
