Thursday, November 3, 2011

K8RRT's Dual-Flag Receive Antenna

The photo at the left shows Tim, K8RRT, standing behind his new Dual-Flag receive antenna. The antenna is made up of two rectangles 30-feet long by 20-feet tall. The telephone pole is set up to accommodate a total of 3 or 4 Flags aimed in different directions. The feedpoint is at the bottom corner next to the far support. That support is made up of military surplus telescoping fiberglass tubes available on eBay. You can see a close-up of the feedpoint in the small photo on the right. Click on any photo to enlarge it then Close the open photo window or use your BACK button to return to this page. The photo of the antenna was adjusted so that the wires showed up a little better against the bright sky.

Below is a Posting by Charlie, N8RR, who has been leading the use of this type of antenna here locally. He gives a fuller description of the antenna.

Yesterday in about 4 or 5 hours W8TN, K8RRT and I built a dual flag receive antenna for the low bands in Tim's field. We continue to refine the process of building these. The next one will have a couple of minor changes to the construction method. It occupies about the same length as a 40-M dipole.

These are pretty effective RX antennas on 160, 80 and even 40 meters. For those without space for a significant beverage, they might be a solution. My 60-degree dual flag held it's own with a 450' NE beverage, and was a bit less noisy most of the time. These antennas can be built to a lot smaller dimensions than this one used, and will still have great directivity. The smaller the antenna, the less output signal it has, and the more pre-amplification that is needed in the receive chain. I would not hesitate to build a small version of this for limited space, as rx pre-amplification is not expensive or difficult to obtain.

The flag antenna has been around a while. Some have even built rotatable flags, which by necessity are a lot smaller than this one. This current design is from the work of others, and I deserve and take no credit for it.

Tim's antenna models to have maximum gain response at 25 degrees elevation, with a f/b ratio of nearly 30 dB. The beam width at -3 dB points is 107.3 degrees. What this means is that Tim's antenna, which is centered on 60 degrees azimuth, will cover from 6.35 degrees azimuth around to 113.65 degrees azimuth before the gain response falls off 3 dB from maximum.

Although the antenna max gain is at 25 degrees above the horizon, it has significant response at lower angles. The local power lines are behind this antenna. At 2 degrees Azimuth, the model shows over 34 dB front to back ratio, so the antenna should really do a good job of rejecting noise from the powerline.

Now for the real world testing. I am anxious to see how this antenna performs for Tim in the real world of DXing!

The horizontal wires are 30', the vertical wires are 20'. VI in the drawing is the matching transformer. The antenna is fed with RG6 TV cable coax.
The terminating resistor is 1222 ohms. The antenna is directional toward the terminating resistor, away from the feedpoint.

If someone wants to build one of these in less space, it can be done at reduced dimensions.

One can also build a single flag version of this. The beam width is broader, the signal output is higher, but the front to back ratio is poorer. I built one of these single flags last year and at times it was useful.

73 Charlie N8RR

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