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P O L Y T E C H . N U

Moonraker discone antenna

imageRecently I bought a Icom IC-R8600 receiver. Since I didn't had a suitable (omnidirectional) wideband antenna, I was looking for a wideband receiver antenna. The discone type antenne is the proven method for wideband receiving. The idea of this antenna is that two stacked cones act as a wideband antenna. This is known as a biconic antenna. To reduce mechanica wind load to the antenna, the cones may be replaced with radials. The electromagnetic effect is similar and the mechanical load is a lot reduced. It's also possible to eliminate one cone and replace this with a disk that acts as an 'electrical mirror'. Also this disk may be replaced my radials in the same plane. The wind load is also reduced and the construction is easier. This explains the typical shape of the discone antenna and this enplanes also the name: discone = disk + cone. For reception, this construction is fine. The antenna can be used for transmitting if the vertical rod is added as the driven element. The vertical element may be removed if the antenna is only used for reception. The vertical part is needed in this case so the antenna can transmit on the 6m, 2m, 70cm and 23cm bands. This makes the discone antenna rather common for wideband reception. The antenna will receive signals above 25MHz and below 2GHz, but the reception performance is compromised. The antenna after assembly is shown on the photo. There are more discone antenna's of several brands. They're all a 'variation on a theme'. There's also a version that receives up to1,3 GHz or 3 GHz for example. I chose for this brand/model due to the price and local availability.

brand: Moonraker
description: Scan-King Royal Discone 2000
intended application: VHF/UHF base aerial for scanners 25...2.000 MHz
type: discone
RX frequencies: 25...2.000 MHz
TX frequency bands: 6m/2m/70cm/23cm
impedance: 50 Ohms
connector: N-type (F)
maximum TX power: 200 Watts
gain: 2,15 dBi
VSWR: <1,5:1
height: 153,5 cm
weight: 1,6 kg
radials: 8 + 8
mounting tube diameter: <50 mm

build quality
The build quality is rather good. The radials are made of stainless steel and the quality of machining is great. There's also a allen wrench supplied for fitting the vertical rod to the base of the vertical driven element. I would have expected that the mounting clamps would have been made of metal since this is rather common. But maybe this is a strategic choice for antenna performance as a result of electrical isolation of the clamps. The clamps are made of good quality plastic, so no problem there. All the parts fit well and are well machined. Only the 'center piece' doesn't fit perfect in the tube. My choice would be to make the center piece a little wider. But since these parts are bolted together, this is no problem. Basically the antenna is a good quality antenna. The photo's of the kit and a detailed photo of the center piece is shown below.



Antenna's wil last much longer when there's good care taken during assembly and maintenance is performed. And by good care during assembly, the needed maintenance can be reduced. Therefore I greased all the threads with copper grease. Stainless steel has the tendency of 'welding' in time. So to prevent this welding of the nuts and radials, this copper grease is mandatory. Don't use thread locker since there's a jam nut and thread locker is therefore not necessary. And removing the radials when grease is applied is much easier and saves damage to the parts. Even if the grease is gone, there's a thin layer of soft copper that acts as a lubricant. So my advice is to grease all the threads using copper grease or similar ceramic grease. You will thank me later during disassembly of maintance. ;-)


imageThe driven element is inserted into tie cavity at the top of the antenna. This is a perfect spot for collecting water resulting in corrosion eventually. My advice is to fit the jam nut with a lof of grease and to seal the connection with self vulcanizing tape. This tape wil 'melt' together in time to create a perfect watertight seal. (Also perfect for coax connector protection.) The best way is to apply the tape by stretching the tape a lot to create mechanical tension and work from the bottom to the top. The result is a perfect watertight seal. Expect this antenna to be rather maintenance free now. My advice is to (copper) grease the threads of the mounting clamps. Instead of using grease Tectyl is even better! But the problem is that this makes a mess if this has to be removed again. Since my antenna's are easy reachable my choice is to add some grease every one in a while to the threads and use Tectyl only on construction parts like nuts and bolts of the mast.