|I have made various models of the Flag RX
antenna using MININEC (i use NEC4WIN)
to study the effect of height and size on the pattern.
The VE6WZ QTH is on a small city lot and various rx antennas have been tried: magnetic loops, short stealth beverages, on the ground beverages and the delta RX loop. (usually these antennas are phased against a noise antenna to further eliminate man-made noise). All of these have been an improvement over the TX ant. The Flag antenna was investigated because of good reports from Larry W7IUV using a full size rotatable version see here.
SIZE: MININEC models show that the dimensions can be scaled to make these antennas easier to build rotatable. The gain is down, but the pattern seems unaffected. See models here . The 3/4 scale version should be easy to build: 22' long by 11' high rectangle. The half scale version should fit into most small city lots and could be turned by a small rotor. (good preamps will be essential since the gain is way down)
ELEVATION: The modeling also shows that if the antenna is raised to close to 1/4 wavelength, a very desirable low angle pattern develops. See models here . In a noisy city location, having the rx antenna elevated above the man-made noise sources should be beneficial. The high angle lobe suppression should improve S/N. Such an installation could be built using a 3/4 scaled flag mounted on top of the HF tower or 160m TX tower.
INTERACTION: Modeling shows that these antennas will not work if in close proximity to a resonant TX antenna. The pattern shows to be destroyed even when up to 1 full wavelength away from the resonant TX antenna!! If the antenna is de-tuned however the pattern can be restored (according to the models). A complex model of the VE6WZ shunt-fed tower, 40m yagi and 80m dipole show that the flag on top of the tower retains an acceptable pattern if the tower is de-tuned from 160m resonance. (the flag will not work on 80m at 100' because of a nasty high angle lobe see here). This pattern degradation near or on top of a TX antenna could prove to be the biggest problem to solve in most practical installations. Maybe further modeling and testing will provide solutions.
These small scale or elevated flags have NOT yet been tested at VE6WZ. Construction of a half-scale rotatable flag near the ground, and a 3/4 scale flag installed at 100' on the TX tower are planned for this summer. A homebuilt MOSFET preamp with a 160m bandpass filter will be installed at the flag with suitable remote switching for protection from strong TX fields. Common mode chokes will be installed at the feedlines.
more great rx antenna links: K3KY flags and pennants
Please e-mail me if you have some comments or suggestions firstname.lastname@example.org
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