I was recently asked to help out a newly licensed radio ham with his VHF & UHF aerial system, he was finding that a Disc-cone aerial which he had been using for a number of years as a Short Wave listener was not doing a good enough job of transmitting his VHF l and U.H.F. signal. Here is a suggestion of how you can improve your Ham Radio aerials.
The Disc-cone is a well-known wide band aerial used by many people who enjoy scanning the VHF and UHF part of the Radio Spectrum for commercial traffic and Ham radio transmissions. The Disc-cone works well if you are nearby to radio masts that repeat the broadcasts from commercial or Ham Radio bands but they vary in their bandwidth and sometimes fail to give adequate matching at ham allocations. I have seen Disc-cone S.W.R. (Standing Wave Ratio) readings at 5 to one on two metres or 144 Megahertz, this can severely impair both your output signal and received signal.
Most Ham radio Transceivers are designed with 50 ohm output connections using SO-239 sockets, these allow easy connection to coaxial cable used by many operators as their means of feeding their transceiver power into an aerial. A 5 to one impedance match at VHF is not desirable; it indicates the load you are transmitting into is either 5 times 50 or 50 divided by 5. 250 or 10 ohms will not deliver all of your power into your aerial and conversely it will not gather the voltage required by your receiver for optimum efficiency. Radio transmissions need to be correctly matched, maximum transfer of power only happens when the feed line sees the correct load, in our example 50 ohms.
Here is a simple aerial, which can be built inexpensively from a length of square wooden post. The aerial will transfer power correctly and at the right load, I.E. 50 ohms. Using this aerial I have seen my signal meter improve on two metre transmissions at 144 Megahertz and the rest of the frequency bands it was designed for.
Wooden posts are available in different lengths, from four metres and above down to 2.5 metres, these posts are available from most high street Do It Yourself outlets and are not too expensive to buy. Being square they have four sides; these sides are used to hold a dipole for the bands you choose to operate on. A three metre length of post comfortably holds `four Radio ham bands, namely six metres, four metres, two metres and seventy centimetres. These bands work out at 50 Megahertz, 70 Megahertz, 144 Megahertz and 432 Megahertz respectively.
The aerial is very similar to a nest of dipoles that are commonly used by radio Hams for use on the H.F. part of the Radio spectrum. This allocation is between 1.8 Megahertz and 30 Megahertz.
Using the formulae to calculate the length of a dipole, I.E. 468 feet, divide the lowest frequency into this number. In our case we are going to use 50 Megahertz. The six metre band or 50 Megahertz works out about at about 9.36 feet for our dipole, depending on what part of the band you wish to use, this length is the longest that can be accommodated on a three metre length of wooden post.
Using the remaining three faces of the wooden post, attach the other three dipoles which you have calculated from the above formulae. Attach you 50 ohm coax to the centre of the four dipoles weather proof using duct tape, the inner or Braid of the coax is attached to the lower part of the four dipoles and the inner is attached to the upper part. Both the inner and Braid parts of the coaxial cable are secured with an additional piece of wire. If you intend to use this aerial in the horizontal plane, make sure that the feeder falls away from the post at 90 degrees. If you intend to use the aerial in a vertical position and additional piece of wood is attached near to the feed point at ninety degrees, running the coax parallel with the dipoles makes tuning very difficult because the dipole elements an coax couple with each other and disturb the aerial radiation pattern.
The aerial is completed by trimming the highest frequency for the lowest S.W.R. reading, in our case this will be 432 Megahertz. Mounting the post on brackets on the side of your house as high as possible will give you the best performance, however if your loft or attic space is high enough you can mount it indoors. Turn the aerial through 90 degrees for horizontal polarization, used mainly for S.S.B. and C.W. transmissions. Complete the aerial by winding about nine coils of the coax cable and place it near to the feed point; this choke prevents the braid from radiating your signal.