We are setting out our roof-space as an attic, and after decorating, there will be a triangular void under the house ridge, which will be about 36" wide, and 18" high. I can easily fit a standard folded dipole loop antenna for VHF FM, but the DAB (Digital Audio Broadcast) antenna is vertically polarised, and its dipole will not fit.
I am toying with the idea of using a slot antenna, with a horizontal slot in this space, and am wondering how to do this. I have seen what I think is a 'skeleton slot' which I thing is a short-circuited horizontal folded dipole, which is shorted where the normal folded dipole feed is, and is fed instead, as if it was a slot.
As a blessing, the roof ridge is pointed almost exactly at the transmitter, both for VHF FM, and for DAB. The shape of the space though limits the size of the plane into which the slot is cut. So I thought, could I put in place, a horizontal ground plane, and build half a slot antenna on top of it, with the centreline of what would be the full slot, coinciding with the surface of the ground plane. The plane surrounding the half lot would of course be electrically continuously with the horizontal ground plane.
Would this be an effective antenna, and what would be the characteristic impedance, would it be half of the impedance of the full slot, being connected between the point on the horizontal ground plane under the centre of the lot, and the midpoint of the opposite side of the half slot? I would expect it to be half of the typical slot antenna impedance.
For the DAB antenna, I settled for the slot. Having seen a double YAGI antenna, with a skeleton slot as its main feeder, in place of a pair of dipoles, I concluded that the plane does not need to be infinite, or even large.
I settled for the largest piece of fibreglass reinforced foam roof insulation, which was aluminium foil coated on both side. This turned out to be, a pentagonal shape, starting with a rectangle, 37" x 19.5", with the top two corners removed to fit under the roof membrane, and a notch cut into the top point, to accept the ridge beam.
About 4" from the bottom of this sheet, using a Stanley knife, working from both sides, I cut out a slot 25" wide by 2" high, carefully saving the cut out piece.
Then using self adhesive copper screening tape, I lined out all the cut edges, both of the slot, and the periphery of the board. and finished both, with more tape, to fully short-circuit the front face to the back face of the board on all the edges, giving a generous overlap on the tape. (The tape is reputed to use a conductive adhesive.)
Then I carefully removed all the copper foil from the piece cut out of the slot.
Next, I measured to find the midpoint of the inside long faces of the slot, and carefully tinned, and made a nice solder blob on both face centres. Taking some braid off some scrap co-ax, two pieces about 6" long, I dressed, and tinned one end of each, and soldered them to the blobs on the midpoints. These I dressed to exit the slot on what was to be the access surface, and carefully refitted the the now naked piece of foam, which I had cut out for the slot.
Using parcel tape, I taped across the slot, with some tension, to keep the packing piece tightly in place, and to add some mechanical strength to what would otherwise be a very fragile article.
Then, taking a 300 ohm to 75 ohm BALUN, Trimming the braid ends to fit, I connected it in place, and bedding it in quick-setting epoxy, I fitted the BALUN onto the naked filler piece, at the midpoint of the slot. Realising that I had got epoxy on the co-ax securing screws, before the glue had time to set, I connected a generous length of 75 ohm co-ax.
When the glue had had ample time to set, I carefully installed the antenna in the roof ridge, where I had prepared a place for it.
This installation is not yet properly installed, and the FM antenna is not yet prepared, so my connection to the antenna lead is a very crude twisting, and taping of wires. Nevertheless, at least on DAB, it seems to work OK, but without the proper FM antenna, FM reception is very poor.
Is there any special care required to unite the FM antenna to the DAB antenna feed to reduce any standing wave effects?
