HOW TO BUILD ROOFSPACE ANTENNAS AND GET THEM TO WORK
Most books and articles on antennas assume that the reader has a garden or outside space to be able to install the design. Not everyone is in this fortunate position and as a result a number of amateurs restrict themselves to either VHF or close down and give up the hobby. There is a solution, as long as you have a roof over your head, you have room for an HF antenna which will enable you to make many good long distance contacts.
In a restricted enviroment, the first task is to ascertain the degree of restriction. The local authorities may have decreed no outside antennas, but the internal roofspace of the house may be available. In other cases, the amateur may be a flat dweller, with or without a balcony - a far more difficult proposition.
In each and every case though, there will be a solution to the problem which although not necessarily capable od DX operation, will permit European contacts and when conditions are good be capable of transcontinental contacts.
In the first case, loft antennas are quite a viable proposition. At the higher end of the HF spectrum, on the bands between 14 to 28Mhz, the space available may be well sufficient for a dipole or even a small fixed beam. The element lengths may have to be modified slightly to compensate for the proximity of house wiring, water tanks etc., but even outdoor antennas should be individually tuned to allow for the effects of nearby trees and buildings. some advantage may be gained by using a folded dipole made from 300 ohm ribbon, and antenna matching network (AMU), for thesae tend to be less affected by nearby objects.
If the orentation of the roofspace is suitable, it may be possible to erect a reversible, two driven element beam made from 300 ohm feeder.FIG.1.
FIG.1. A SIMPLE TWO DRIVEN ELEMENT REVERSIBLE BEAM
It is when the decision to operate on 40,80 and 160 metres that the problems become interesting. It is no longer possible to run a half wave antenna.
So how do you achieve the maximum radiated signal from the space available?.
The solution is helped by the fact that the majority of the radiated signal is from 2/3 of the antenna carrying the most current and the remainder of the antenna may be left in random fashion or substituted by inductive or capacitive loading without ill effect.
On 40m however, except in the smallest houses, no loading is necessary. The antenna may be mounted adjacent to the rafters around the roofspace to form a halo antenna, similar to that used in the past by 2m operators before vertical polarization became the norm with FM.
When errected, the length should be optimised by use of a grid dip oscillator. Even then, it will probably be found that the match to the feeder gives a high VSWR, but with a short feeder, the losses will not be excessive and you can use a AMU to achieve a match to the transceiver. It may be possible with a valve transmitter to achieve a match with the pi output circuit. (not recommended, always use a AMU).
If the feeder run is significantly longer than a half wave, it may be worthwhile to fit a gamma match to the antenna to reduce the VSWR on the feeder
Few, if any, normal houses have sufficient room within the roofspace to install a resonant antenna for either 80 or 160 metres and a loading system therefore becomes necessary
This may take several forms, the general priniciple being the same as that employed by mobile operators in the hey - day of 160 metre mobile operation. In the present case, three factors make the task far easier.
.1. The antenna is much longer
.2. The antenna does not have to survive a 70mph slipstream
.3. The use of an AMU removes most of the antenna -to - transmitter compatibility problems
In designing the antenna, the first reqirement is to ensure that the section carrying the most current is a high as possible, for this is the source of radiation. The main length of wire should therefore be run just below the apex of the roof and then down to the ceiling joist at either end. The most convenient of these ends should then be extended into the shack and the antenna loading system to the other.
The antenna loading system comprises an inductance and a 'fan' of wires which act as a 'top capacitance' to the antenna.
As every installation will be different, it is suggested that the inductance could take the form of a coil of wire, two to three inches diameter (a length of plastic water pipe makes a good former), tapped alternate turns. Thirty turns, spaced one wire diameter for 80 metres and 60 turns for 160 metres make good starting points. Under certain conditions, and particularly if high power is being used, a considerable voltage will be developed across this coil, so it is advisable to mount it on an insulated base, well clear of the woodwork.
The fan of wires which form the top capacitance comprises three or more wires, each 8 - 12ft long, resting across the ceiling joists. Again, under certain conditions, high voltage could be present and the necessary precautions should be taken. The use of insulated wire is necessary. In addition, as these are primarily a capacitance (the other plate being ground), the greater the diameter, the better. Old coaxial cable is ideal for this purpose, the connection being made to the braiding.
Connect the antenna to the top of the inductance, using a croc clip and the 'fan' to the other. Fit an antenna current indicator to the centre of the span of wire along the inside apex of the roof.
Switch the Tx to transmit and adjust the AMU for correct loading. Note the Tx power and also the reading of the antenna current indicator in the roofspace.
Now move the croc clip down the coil by two turns and again match the Tx to the antenna. If the antenna current increases with the same transmitter power, repeat the process, taking the antenna down the coil taps until you reach a point where the current decreases, now move up the coil to locate the current peak.
Should the current decrease as soon as you move down the coil from the top, the loading is obviously insufficient and either the number of turns on the coil will have to be increased or the number of wires in the fan increased. When the current has reached a maximum, the antenna is ready for operation.
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