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BUILDING ROOFSPACE ANTENNAS AND GET THEM TO FUNCTION Most publications and articles on antenna’s believe that audience has actually a yard or outside area to be able to install the look. Not everyone is within this fortunate position and as a result some beginners limit on their own to either VHF or close down and provide up the hobby. Discover a remedy, so long as you have actually a roof over your face, you've got area for an HF antenna that may enable you to make many good long distance associates.

In a restricted enviroment, initial task would be to ascertain the amount of constraint. The area authorities might have decreed no outside antenna’s, although internal roofspace of the home can be available. Various other cases, the amateur might be a-flat dweller, with or without a balcony - a far more tough proposition.

In each and every situation though, there will be a solution towards the issue which while not fundamentally capable od DX procedure, will permit European contacts so when circumstances are great manage to transcontinental associates.

In the first situation, loft antenna’s are very a viable idea. In the upper end of HF spectrum, on bands between 14 to 28Mhz, the area obtainable are well enough for a dipole as well as a little fixed ray. The factor lengths may have to be customized a little to compensate when it comes to proximity of house wiring, water tanks etc., but also outdoor antenna’s should be individually tuned to allow for the results of nearby woods and structures. some advantage might be attained simply by using a folded dipole made from 300 ohm ribbon, and antenna matching community (AMU), for thesae are generally less suffering from nearby things.

If orentation for the roofspace is suitable, it may be feasible to erect a reversible, two driven element beam created from 300 ohm feeder.FIG.1.

FIG.1. AN EASY TWO DRIVEN ELEMENT REVERSIBLE BEAM
Its if the choice to work on 40, 80 and 160 metres the problems come to be interesting. It really is no longer feasible to perform a half revolution antenna.

How do you achieve the most radiated sign from room available?.
The clear answer is aided because of the undeniable fact that most of the radiated signal is from 2/3 of the antenna holding the most present and rest for the antenna could be left in arbitrary fashion or replaced by inductive or capacitive running without ill result.

On 40m but except in tiniest houses, no running is essential. The antenna can be mounted right beside the rafters across the roofspace to make a ‘halo’ antenna, much like that used in the past by 2m providers before straight polarization became standard with FM.

When errected, the space must be optimised by use of a grid dip oscillator. Also after that, it will probably oftimes be discovered that the match on feeder gives a higher VSWR, but with a short feeder, the losings will not be exorbitant and you can use a AMU to reach a match on transceiver. It might be feasible with a valve transmitter to quickly attain a match aided by the pi production circuit. (not recommended, always utilize a AMU).

If the feeder run is significantly longer than a half-wave, it may possibly be worthwhile to suit a gamma match on antenna to cut back the VSWR on the feeder couple of, if any, normal houses have actually enough space within the roofspace to install a resonant antenna for either 80 or 160 metres and a loading system for that reason is needed

This might just take a number of types, the overall priniciple being exactly like that used by mobile operators within the hey - day of 160 metre mobile procedure. In the present instance, three elements result in the task much easier.
.1. The antenna is a lot longer
.2. The antenna need not endure a 70mph slipstream
.3. The usage of an AMU removes all of the antenna -to - transmitter compatibility dilemmas

DESIGN

In creating the antenna, initial reqirement is make sure that the section carrying many current is a higher that you can, because of this could be the source of radiation. The key amount of wire should therefore be operate just beneath the apex of the roofing then down seriously to the ceiling joist at either end. Many convenient among these stops should then be extended in to the shack as well as the antenna loading system to the other.

The antenna loading system includes an inductance and a 'fan' of wires which work as a 'top capacitance' towards the antenna.

As every set up will be different, it is suggested that inductance could take the form of a coil of line, 2-3 inches diameter (a length of synthetic water-pipe tends to make a great former), tapped alternative turns. Thirty turns, spread one wire diameter for 80 metres and 60 turns for 160 metres make great starting things. Under certain conditions, and especially if high power has been used, a considerable voltage will undoubtedly be created across this coil, therefore it is better to mount it on an insulated base, really free from the woodwork.

The fan of wires which form the most notable capacitance comprises three or higher wires, each 8 - 12ft long, resting over the ceiling joists. Once again, under specific circumstances, high-voltage might be present plus the required safety measures must be taken. Making use of insulated line is important. Additionally, as they are mainly a capacitance (one other plate becoming surface), the greater the diameter, the better. Old coaxial cable is great for this function, the bond being designed to the braiding.

Link the antenna towards top of the inductance, using a croc video additionally the 'fan' to the other. Fit an antenna present indicator into the centre associated with span of cable over the inside apex associated with roofing.

Switch the Tx to transfer and adjust the AMU for correct loading. Note the Tx power as well as the reading of the antenna existing signal in the roofspace.

Now move the croc clip down the coil by two turns and once again match the Tx to your antenna. If antenna existing increases with similar transmitter power, repeat the method, taking the antenna along the coil taps unless you achieve a place in which the existing decreases, now move up the coil to find the current top.

Source: www.qsl.net
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