Rotatable Flag

by Larry - W7IUV

The "FLAG" and "PENNANT" antennas optimized and publicized by K6SE on the topband reflector show great promise for those of us who live on less than full section farms. While the Pennant is easier to build, the Flag is symmetrical and therefor adaptable to a rotatable configuration.

This design is presented only as a suggestion; one form of construction of many possible choices. This design, like most of this writers designs, was an iterative process. (I broke a lot of stuff before this one stayed up!) Most of the design compromises centered around free or cheap material. If cost was no object, this thing would look a lot different.

The choice of boom material was easy; a friend had some chain link fence top rail laying in his horse pasture. I volunteered to clean it up for him. I wanted to use bamboo for the spreaders, but none could be found for free or even cheap. Several materials were experimented with before settling on wooden clothes poles. This material is almost as light as bamboo and is readily available in most lumber yards and home improvement stores. It's also pretty sturdy; I dropped the thing trying to put it up by myself and nothing broke!

The spreaders were attached to the boom with square steel tubing welded to the ends of the boom. The tubing formed nice "sockets" for the wood poles to slip into. One through bolt holds it in place. See drawing for details. If welding is not your thing, consider making a spreader mounting plate from a square piece of aluminum about 12x12x1/4 inch. Drill for U-bolts in appropriate places.

The insulator for the feed point is fabricated from polycarbonate sheet (Lexan). Acrylic could be substituted. See drawing for details. The transformer is just glued on. Mount a TV type F connector on a small bracket. F connectors are real cheap and great when using RG-59/U or RG-6/U 75 ohm coax. If you need to weatherproof it, paint the whole business with PVC pipe cement, several coats, and spray with black Krylon paint. The insulator for the termination resistor is made the same way.

As you may recall, the top and bottom wires are exactly 29 feet long. The two vertical sections are exactly 14 feet long, with the insulators at the 7 foot points. Cut the two wires to length, each 43 feet long plus what you need to attach to the insulators at each end. Mark the points where the bends are made. Attach the marked points to the poles while pulling taught. You may have to juggle the attachment points to get it all to line up correctly. I used a staple gun to make temporary attachments, and went back after it was all square and made permanent attachments.

Here is a photo of the finished antenna installed in the yard.

TRANSFORMER:
The transformer is a topic requiring a whole paper by itself. It is necessary to use an "isolation" type transformer rather than the more familiar bifilar wound un-un or auto-transformer type. I started with a FT50-43 core with 7 turns of #29 AWG for the primary, and 28 turns of #29 AWG for the secondary. The primary and secondary windings are separated as much as possible on the core. Although that worked OK, I wanted to experiment a bit and wound up with a Amidon BN73-202 binocular core with three turns for a primary and 12 turns for a secondary winding. This transformer was patterned after a W8JI suggested design.

I suggest that you wind the transformer with the low impedance winding first, and put the high impedance over it. This will allow you to wind an extra turn or so, and using an impedance bridge, pull off turns until you get the exact impedance ratio you need. Here are some photos of the transformer construction: a single turn through the core, the primary winding, and the complete transformer. Be careful when winding so as not to damage the enamel insulation. I had one transformer the developed leakage between the windings and created IMD products all over 160! The transformer is probably the most critical single component in this RX antenna system. If you have a problem with a flag or pennant, look to the transformer!

PREAMPLIFIER:
Flags and Pennants have about 35 dB or so of signal loss compared to your TX antenna, so some installations may need a "preamplifier". Here is a schematic of an amplifier that I have used for a number of years. It's based on a CATV transistor and gives pretty good IMD performance and a gain of about 20 dB. This circuit provides good performance from 100 KHz to at least 30 MHz.

Through the years I have modified the basic schematic for specialized applications. Recently I made a change to the bias and increased the idle current to around 75 mA. This causes the transistor to run a bit warmer, but also increased the third order intercept point. The "new" design measures 18 dB gain and +20 dBm power output for 1 dB compression. I was also able to run a third order intercept for this circuit and the IP3 is reasonably good at +39 dBm. While the "old" circuit was good, I could just detect AM broadcast IMD products on a quite band. With the "new" circuit, I can detect no IMD products any place on topband when using any RX antenna I have. Remember the transistor will dissipate a little more power and really needs a small heat sink.

The 2N5109 transistor is the device recommended for this application. However, the 2N3866 is useable if the 5109 is not available. I also recommend that you shield the pre-amp very well and provide adequate de-coupling on the power supply leads. Noise from switching power supplies in local computers, as well as AM broadcast signals, can cause problems in overall performance if they get into the pre-amp power supply.

I have had a number of requests for vendors who could supply the 2N5109. Since I made my buy years ago, the vendor I used no longer stocks that part. However, Mouser Electronics (http://www.mouser.com) does carry it now.

For more information on ground independant receive antennas, see K3KY's "Flag and Pennant Antenna Compendium".

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