Building an Antenna on the Roof

(and having fun)

IZ1QZE Giuseppe Ciaccio
[email protected]



june 2010


The basement of my current antenna. The roof is common property of all people living in the building, but the Italian law gives right to any inhabitant of a building (including hams) to place their antennas on the roof regardless of the opinion of the roof's owners. Of course it is always better to find a peaceful agreement, and this is exactly what I did.
Click on the pic for enlarging.
picture of antenna basement

For my first serious antenna I chose an inverted vee. The original reason in favour of the inverted vee was the lack of practical ideas on how to keep both arms of a dipole to the same height as the center. The center is 10 meters high on the roof plane, and other two masts of that height for the two arms of the dipole seemed too much. The inverted vee has also other advantages over the straight dipole: it has a better match with the RG58 cable impedance (50 ohm) and its radiation pattern is a bit more uniform around.

A picture shows the home made balun, obtained as a bare (but mathematically dimensioned) coil of RG58 coaxial cable enclosed in a plastic bottle. This too was fun to build!

The antenna basement is made by three crossed planks of wood (the kind used in construction) coated by water-resistant black paint, and comfortably resting on the roof without concrete. As apparent in the picture, the stability of the basement is improved by two cans full of stones.

You can also see the lower part of the main antenna support, an iron telescopic mast for TV antennas, 10 meters high, secured by 4 + 4 guy wires. The two lateral masts are much lower, just 4 meters, and made of flexible PVC pipe. They have no guy wires, but are secured by the nylon wire carrying the antenna arms.

Results, problems, and solutions:

The paint is not really resistant. Once a year it must be repainted.

The two PVC pipes are too flexible with wind, so the antenna waves too much during storms. With time and sun, the two pipes become permanently curved and thus the vertex angle of the inverted vee tends to close gradually, and this is bad! I've tried with thicker PVC pipes, same result. So I would not recommend this solution anymore. See below for a much better solution!


august 2012


A composite view of my antenna in summer 2012. You can see the central telescopic TV mast with its guy wires, and two fiberglass poles hinged to the lower part of the mast and secured to the guy wires by means of additional nylon wires (almost invisible in the picture). The small bottle hanging near the top contains the home made balun; the two arms of the dipole are clearly visible.
Click on the pic for enlarging.
picture of antenna as of august 2012

The two fiberglass poles (``Delta Pro'' from LineaEffe, Italy) were 10 meters long originally, but the top 2 or 3 meters have been removed in order to let the antenna arms bend towards ground. I tried with the full size poles initially, obtaining a wonderful straight dipole, but the impedance mismatch was quite large (SWR was in the vicinity of 2, and the resonance dip was barely detectable). So I preferred a wide-angle inverted vee, as you see in the view.

This is to suggest how the support cable of the antenna makes through the top of each fiberglass pole.
Initially, the fiberglass poles were hinged directly to the central mast. A bolt passed through a hole at the plastic reinforced base of each fiberglass pole.
But the above solution was criticized (in the ARI Fidenza forum). Basically, it was dangerous for the flexible fiberglass pole to be rigidly hinged at a single point (the bottom end, which is not so resistant after all). A better solution can be seen in these pictures. The idea is simple: the bottom hinge should allow a degree of flexibility. On each branch of the antenna, this is achieved by hinging a chunk of thin PVC pipe, 1 mt long, to the central mast; the pipe is then inserted into the bottom of the fiberglass pole without any bolts, as shown. The PVC pipe prevents the pole to escape laterally, and is long enough to prevent it to escape from the top (if properly secured to the guy wires). In case of wind, the pole can oscillate but the torque is largely absorbed by the flexible PVC pipe.


I'm curious to see to what extent this configuration will resist to the hot sun in summer and the storms in winter...


august 2014


A composite view of my antenna in 2014, taken from the bottom up. It is a triangular loop (a "delta") fed at bottom. The triangle is made of three fiberglass poles (the one vertical in the center and the two diagonal ones) hinged together at the bottom apex. A horizontal PVC pipe in the middle enforces the poles to keep a fixed angle at the bottom; that angle determines the antenna impedance (50 ohm). Two of the three poles were in use with the previous inverted vee, which has been dismantled after two years of successful and satisfactory usage and after having resisted to the strong northern winds typical of this town in winter.

The wire is only visible in the top part of the picture.

Click on the pic for enlarging.
picture of antenna as of august 2014

The exact geometry of the loop, in meters, is as follows: 8.09 x 5.30 x 8.09, with the bottom apex 2 mt above the roof plane. With this geometry, a SWR minimum of 1.2 with 50 ohm cable is reached at 14.1 MHz. Pruning the wire and regulating the bottom angle in order to achieve a reasonable SWR dip at the desired frequency was rather tedious, and the EZNEC simulator was of no help (according to EZNEC, a loop with this geometry should resonate at 15 MHz...a software bug, probably).

This antenna seems to perform better than the previous inverted vee on DX, worse on short range, a bit less noisy, and a bit more directive. A great advantage is that the feedline attaches at the bottom; with the inverted vee, the input point was about 10 meters high and thus the feed system (cable + balun) was much more vulnerable to wind.

A more distant view of the antenna, showing the triangular shape and the four nylon guy wires anchored at the middle crossing between the horizontal pipe and the vertical pole.

The fiberglass poles are flexibly anchored to the bottom mast using a technique similar to the one already in use with the previous antenna: two thin PVC pipes are hinged to a central PVC mast, then the poles are simply inserted on the pipes from top, and kept in place by their own weight (plus two small strings for the two diagonal poles, to prevent accidental escaping). The central PVC mast is then inserted into an iron tube secured to the basement (and thus becomes invisible). The triangle can then be rotated to the desired direction; when done, two bolts in the iron tube (not visible in the picture) can be tightened in order to block the inner PVC mast and prevent further rotation.

The white bottle behind the poles contains the home made balun.

Another advantage of this antenna is that two persons can easily mount and unmount it in less than half hour. I might unmount the antenna in autumn, thus avoiding the risks of those frightening storms...


june 2015


Unfortunately, the triangular antenna was less practical and much less resistant to wind than expected. The central horizontal mast got broken after an episode of rather powerful southeast wind. So I ultimately decided to rebuild the previous inverted vee, which was more stable and resistant thanks to the iron central mast secured by 4 + 4 guy wires.
Initially the antenna took the shape of a wide-angle inverted vee. As you can see in the picture at left, the angle was so wide that it was a nearly straight dipole.
The impedance of straight dipoles is not a good match for a 50 ohm feed line and rig. It is also different from the usually advertised 72 ohm. In the graph at right, you can see how the impedance of a dipole changes with the height from ground. Especially when the height is slightly less than half wave, the impedance can grow to nearly 100 ohm. If your rig and feed line are 50 ohm, you may then see a SWR in the vicinity of 2. It is commonly held that a SWR of 2 is not a big concern, but it is also known that the loss on the feed line increases with the SWR, so the actual value is larger. My personal experience is that, with this amount of impedance mismatch, the behaviour is bad: fewer QSOs, and more noise on receive.
So, in the end, and after quite a number of trials, I found that the best compromise is similar to my antenna of two years ago: an inverted vee with a not-so-wide angle (picture at left).