A few notes on Yagi (and similar) antenna
PVC pipe (especially white PVC) is not a
for antenna construction for the following reasons:
PVC of the sort used for pipes has poor mechanical
It has rather poor longevity when exposed to direct
Being intended for use underground or inside buildings,
to direct sunlight will cause it to discolor rather quickly,
Under stress and when exposed to heat (e.g. hot sun) it
This can skew element alignment for longer antennas.
If you must use plastic pipe for a boom, you can
it - to
protect it from UV, or you can use UV-stabilized PVC.
ABS pipe as an option as it is somewhat more resistant to the
prolonged UV exposure. (It's still plastic pipe and
For antennas that are not used outside for long periods of
(such as portable or "emergency" use) PVC may be a reasonable
Soft-drawn copper tubing is, electrically speaking, quite
is not a very good choice for use in elements, mechanically
The material is very soft and is easy to bend.
Being hollow, it tends to accumulate insects (especially
This is more of an annoyance rather than a performance issue.
In northern climates, if left uncapped, they tend to
and ice - especially if they are full of spiders/webs.
them to become heavy (as they fill) or even split. (A
silicone in each end of the element will help prevent this
When building any antenna - especially a Yagi - the
elements has an effect on how long the element should be.
consider two antennas that are identical except for the
elements. Given two elements that are the same length,
the larger-diameter element will be electrically shorter (e.g.
higher frequency) than a smaller-diameter element. This
means that if an
designed with, say, 1/8" diameter elements is built using 1/4"
elements, it will be slightly high in frequency. In
general, larger-diameter elements will have a wider
operational bandwidth than smaller-diameter elements.
An antenna really does need to exhibit a
If the return loss of an antenna is too low (that is, the
then one cannot efficiently transfer power into and out of the
The design of any antenna should really be checked for
and repeatability. For instance, a return loss of 5 db
over 56% of the power sent to the antenna gets reflected back
correlates with an SWR of at least 3.5:1. This sort of
only yields poor performance, but it has the potential to
connected to it.
As mentioned above, a matching section should be
repeatable. Long experience has shown that there are
basic types of matches
that are simple and effective in Yagi construction. One of
in common with all of these matches is that they have
of impedance transformation - which makes sense because a
would not be necessary if there were no need for it!
A gamma match. This is a single-ended matching
quite simple, mechanically, and can be adjusted to accommodate
of matching conditions. It's complicating factor is that
requires a series capacitance which must be protected from
A "T" match. This is a balanced matching network
half of the element equally. Its main complication is
uses a 1/2 wave delay line to feed the other half of the
element out of
A "J-pole" type match. This is, essentially, using a
wave element as the driven element. The J-pole has the
that it's simple to construct as it has a good range in its
capabilities. While the initial adjustment of the two
for proper match may be tricky, it can be repeatable when
As a driven element in a vertical Yagi, it can also be used as
support if the Yagi is fairly short. It potential
problem is that
it can have a pattern symmetry problem - but that's usually
for small-ish Yagis. It also isn't well-suited for
polarization as the weight balance can be offset by the added
hardware and cable connection.
The matching network should be fairly rugged.
of the matching network can change dramatically if it is
Also, the accumulation of rain/snow can greatly affect the
as can degradation of the materials used upon exposure to the
Connection to the feedpoint.
be lossy, such a short length of it is typically used so that
its contribution to
losses is usually negligible. Generally, open-wire line
on Yagi antenna (with the possible exceptions of some
collinear arrays) and when used, open-wire line will require
of impedance transformation to make it usable at 50 ohms - but
the open wire itself may be part of that transformation
Connector mounting should be well thought-out.
When mounting the coaxial connector, one must be wary of
in such a fashion that there are too few threads exposed to
(especially a UHF connector) to be fully tightened. This
can be a
particular problem when single-hole chassis-mount UHF
Typically, these are intended to be used on a fairly thin
- but when used on a thick material (especially plastic -
which must be
rather thick for adequate strength) too much thread is taken
up and the
body of the cable's connector cannot be cinched into place by
resulting in a loose ground connection.
Another consideration is clearance between the connector and
other antenna structures. It is often the case that the
is placed very close to the boom, making waterproofing with
and/or tape very difficult. If possible, space the
farther away from the boom and/or mount it at an angle such
can more-easily wrap the connector.
Even a bad outside antenna may work better than a
Typical construction materials
in a house can easily result in 10-15db (or more) of
attenuation - and that amount of loss is difficult to make up
reasonably-sized indoor antenna. It should come as no
then, when even a very poorly
antenna is placed outside - even when fed with a lossy coaxial
it is likely to outperform nearly any indoor antenna to which
it may be
compared. In other words, it is possible that a rubber
antenna on the roof may work better than a "real" antenna
Placing an indoor antenna near a window can help the
- if you happen to have one that faces the right direction -
aware that some energy-saving window coatings can be very
lossy at RF.
Beware coax loss and "false" matches.
It should also be noted that at higher frequencies -
as 70 cm - even a few 10's of feet of a small cable such as
very high losses. These losses not only "eat" both
signals, but they can cause a false sense of security: Even
SWR on the far end of the cable can, when such losses are
to be perfectly acceptable, thus masking problems
If you choose to build antennas, you can find good (and bad!)
all over the web. For a handy reference, I would strongly
recommend obtaining a copy of the ARRL Antenna Book (even an old
version!) as it has a reasonable balance of practical antenna
plus the theory and formulae required to design your own!