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Small City-lot 160,75,40,20
As an older ham having lived on a small city lot in an older neighborhood, I have only 45' x 55' of back yard for antennas. Fortunately, my neighbor has a tall tree with a branch overhanging my driveway. I have a nylon rope and pulley (used sling-shot) about 40' high on this branch. I have done a considerable amount of experimenting over the years, including wasting time on the infamous CFA (Crossed-field-antenna) or "EH" antenna. I recently posted my surprisingly good results with a mobile configuration [search "Built Mobile using Lewallen "fat dipole" concept"] and decided to see if I could emulate these results by using a "surrogate vehicle". I happened to have a 4' x 11' sheet of aluminum so I placed it on top of 1/2" styrofoam (non-foil) so that nothing touches ground. (This is my substitute "truck"). The feed coax shield goes to the exact center of the aluminum sheet, and of course the center of the coax to the insulated vertical antenna going up toward the pulley. This flexible pulley system conveniently accommodates 160, 75, 40 and 20 meters and works very well on all....especially causing some head-scratching among the long established 160 meter groups who consist mostly of high-power and optimally large antennas. Of great help is the "Capacitive Hat" I use. Actually, it is dual purpose, because it is a 6' diameter Hi-Q horizontal loop (Halo-type) made from 1/2' aluminum tubing (surplus coax) with concentric tin cans with teflon spacers as capacitor to fix-tune to resonance. A trim tab is available when lowered. Its 5' BNC/RG58-leader is connected to about 75' of RG58-U to reach the radio shack, and is hoisted with nylon struts to about 35'. (I have been using it with 1KV SSB peak for about 10 yrs) and it works very well because of decent low-angle omni-directional pattern. So this was very convenient to subtstitute as capacitive hat on 160, 75 and 40 meters. On 160 meters, a very Hi-Q loading coil is used at the 5' leader of the loop mentioned above. I used the same loading coil that I described in the Mobile article mentioned above....with a few more Hi-Q turns added. For the vertical conductor, I used the shield of RG-8 (tied to inner conductor) for low-loss. I didn't measure the shunt matching capacitor required to match feed point to 50 ohms, but it was probably about 1000 Pf . On 75 meters, I jumpered out (shorted) most of the 160 meter loading coil for resonance, and reduced the shunt matching capacitor to probably 400 pf, or whatever is required for very low SWR. On 40 meters, no loading coil is used, and actually the capacitive hat had to be lowered for verical height of only about 15' to reach resonance, and very small shunt capacitance is needed....also it covers the whole 40 meter band. I used RG58 shield because I didn't have RG8 in my junk pile. I may later reduce the 160/75 to RG58 also to reduce the weight on the small pulley that I installed earler...I'm wondering if too much up-and-down might ruin the pulley. I usually take my transceiver and SWR bridge out near the aluminum plate for matching on 160 meters, which is more critical for top performance. As in the Mobile discussion, I still pass the feed coax (I use RG58) with several turns thru some large ferrite toroid cores to minimize currents from coming back on the outside of the feed coax...thus being readily coupled to ground as loss. My speculation about this good performace, is that the aluminum sheet represents an infinite number of radials of equal and opposite horizonal currents that minimize propensity to couple to earth thru the 1/2" styrofoam insulator, and the voltages at the end of the "radials" have very little corona-type loss as the voltage is distributed around the sheet edges, and touch ground nowhere. For those who find these performance claims credible, there could be alternative ways of experimenting, such as figuring ways to make good low-loss connection to "foil-sided" styrofoam sheet laying on a "non-foil" insulator sheet. Or perhaps a series of aluminum-foil spokes connected well at the center and laid on (or between) styrofoam. |
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