“Saturday 22-Feb-2014 was a beautiful day at my QTH. A good time to do some antenna experiments, as I am still looking for a small DX-antenna for 80 mtrs. I had been reading up on umbrella antennas, as they supposedly can provide decent performance for their length (height).
An “umbrella antenna” is basically just a vertical monopole antenna that is top-loaded with a large “end-hat”. The hat consists of a number of radial wires that slope away from the top of the antenna. These spoke-wires resemble the ribs of an umbrella, hence the name. The beneficial effect is significantly increased (2x) by connecting the bottom of the sloping wires with a perimeter “skirt” wire. A properly dimensioned hat will make the current distribution along the vertical radiator more uniform. That is, it will taper off much less towards the tip of the radiator. Ref. 6. This increases the radiation resistance of the antenna, and also the bandwidth – both highly desirable.
Umbrella antennas are nothing new, they have been used since the early 1900s. My favorite umbrella antenna system was the one built for the German navy in WW2, for their “Goliath” 1 megawatt VLF (15-60 kHz) transmitter. That is a wavelength of 5-20km (3-12 miles). The vertical radiators were only about 1% of the wavelength, but the efficiency of the complete system was an impressive 47% at 15 kHz and 90% at 60 kHz.
From other antenna experiments, I have two telescopic fiberglass fishing poles at my disposal. One with a total length of 6.5 meters (21 ft), the other 9.5 meters (31 ft). And I had a spool of thin (≈0.5 mm, 24 AWG) multi-strand wire (unfortunately teflon coated, which is a little hard to work with). I decided to go with the 6.5 meter pole, and attach four loading wires about 0.5 m below the tip. The top section of the telescopic antenna is just too thin. The wires should not descend by more than about 40% of the length of the vertical radiator (see ref. 1, 3). In my case, about 2.4 m. Given the size of my terrace (incl. potential attachment points for the guy wires), I decided to slope the loading wires at an angle of about 40°. This makes the wires 3 m long (10 ft), and they can be interconnected with four skirt wires of the same length. I added four radial wires of the same length as the vertical radiator (6.5 m). There length is primarily driven by the dimensions of my terrace, and is not necessarily optimal for the length of the radiator + umbrella………..
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