Tips and Tricks
Coaxial 4:1 Baluns:
The best method of connection to
most antennas is by using an impedance matching device called a balun.
Commercial baluns are available that work across a wide range of frequencies
and channels, however, at the higher UHF frequencies, they tend to be rather
To obtain the lowest loss and best match to the UHF antennas found on this website, use of a coaxial 4:1 balun is recommended.
Construction method is shown here, and it is advised that the whole assembly is carefully taped up to prevent the ingress of moisture.
Note that it is important that semi-air spaced coaxial cable is used, otherwise the loop length will be incorrect.
The purpose of this balun is to provide a transformation of impedance from a balanced 300 ohms at the antenna dipole to an unbalanced 75 ohms on the cable.
Coaxial 1:1 Sleeve Baluns:
The ZL Special FM Antenna also needs
a balun for best performance, however, due to it’s lower impedance, a different
kind of balun is required.
This one doesn’t transform the impedance at all, but rather prevents the signal from travelling down the outer braid of the coax, (which is unwanted in any system as it leads to excessive losses.)
The end of the 75ohm cable is fed through the 12-14mm aluminium tube. This end of the cable is prepared as shown, for connection to the antenna. Weatherproof this end with tape.
The bottom end of the tube is “crimped” onto an exposed section (5-6mm) of the cable’s outer braid. Weatherproof this end as well.
Take care not to “crimp” the tube into the braid too much, otherwise excessive deformation of the cable’s centre dielectric may occur.
One of the problems with large antenna
arrays is boom sag caused by the weight of the aluminium.
Allowing the boom to sag won’t have an adverse effect on the performance of the antenna, but in practice, allowing the unsupported antenna to flex excessively in the wind may cause premature mechanical failure.
As a rule, antennas over 4 metres in length should have additional support. This support should ideally be (in the case of a 4 metre antenna) at the 1 metre and 3 metre points.
One type of support is shown below.
In this case, the vertically polarised antenna (like an FM ZL Special) is mounted on a standoff with the support frame which will be mounted onto the mast or tower.
In the case of a horizontally polarised antenna, (like a Band 3 Parabeam) the antenna is mounted directly onto the mast or tower, with the support frame underneath.
The support can be made from heavy duty 25mm thick wall aluminium tube. The two little support arms need only be 75mm long and bolted to the antenna boom and support frame with 1/4” x 3” bolts. Then, both the antenna boom and support frame are clamped to the mast/standoff using heavy duty U-bolts & saddle clamps.
TV Transmitter Locations:
It makes little sense constructing
and installing a high-gain antenna system and then neglecting to ensure
that you are actually pointing it towards the transmitter from which
the signal is transmitted.
As a general rule of thumb, high-gain antenna arrays have a much narrower horizontal (and vertical) beamwidth than low gain antennas, such as is in use for receiving channel 1 and 4.
SBS and channel 1 & 4 are transmitted from "The Bluff" just north of Port Pirie. Channel 7, 9 and 10 are transmitted from "Mt Lofty" in Adelaide. Hence, aim your SBS and channel 1/4 antennas towards "The Bluff" (approx 90 degrees East of Whyalla) and your Band 3 antennas towards "Mt Lofty" (approx 146 degrees S/East of Whyalla). Final adjustment for Adelaide TV could be carried out by actually using a portable TV set outside and within view of the person adjusting the horizontal direction of the antenna, until the clearest picture is obtained.