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, prolonged
to direct sunlight will cause it to discolor rather quickly, and it
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. Also
ABS pipe as an option as it is somewhat more resistant to the effects
prolonged UV exposure. (It's still plastic pipe and rather
For antennas that are not used outside for long periods of time
(such as portable or "emergency" use) PVC may be a reasonable choice.
Soft-drawn copper tubing is, electrically speaking, quite
is not a very good choice for use in elements, mechanically speaking:
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. This can
them to become heavy (as they fill) or even split. (A plug of
silicone in each end of the element will help prevent this problem.)
When building any antenna - especially a Yagi - the diameter
elements has an effect on how long the element should be.
consider two antennas that are identical except for the diameter of
elements: The antenna with skinny elements will have shorter
than the one with rather fat elements. This means that if an
designed with, say, 1/8" diameter elements is built using 1/4" diameter
elements, it will be high in frequency.
An antenna really does need to exhibit a reasonable
If the return loss of an antenna is too low (that is, the SWR
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 means that
over 56% of the power sent to the antenna gets reflected back - and
correlates with an SWR of at least 3.5:1. This sort of mismatch
only yields poor performance, but it has the potential to damage a
connected to it.
As mentioned above, a matching section should be effective
repeatable. Long experience has shown that there are several
basic types of matches
that are simple and effective in Yagi construction. One of the
in common with all of these matches is that they have some
of impedance transformation - which makes sense because a matching
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 a wide
of matching conditions. It's complicating factor is that it
requires a series capacitance which must be protected from moisture.
A "T" match. This is a balanced matching network
half of the element equally. Its main complication is that it
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 attraction
that it's simple to construct as it has a good range in its impedance
capabilities. While the initial adjustment of the two tap points
for proper match may be tricky, it can be repeatable when accurately
As a driven element in a vertical Yagi, it can also be used as the
support if the Yagi is fairly short. It potential problem is that
it can have a pattern symmetry problem - but that's usually not
for small-ish Yagis. It also isn't well-suited for horizontal
The matching network should be fairly rugged.
of the matching network can change dramatically if it is mechanically
Also, the accumulation of rain/snow can greatly affect the performance,
as can degradation of the materials used upon exposure to the elements.
Connection to the feedpoint.
be lossy, such a short length of it is used that its contribution to
losses is usually negligible. Generally, open-wire line is
on Yagi antenna (with the possible exceptions of some log-periodics and
collinear arrays) and when used, open-wire line will require some form
of impedance transformation to make it usable at 50 ohms, anyway.
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 allow the
(especially a UHF connector) to be fully tightened. This can be a
particular problem when single-hole chassis-mount UHF connectors are
Typically, these are intended to be used on a fairly thin metal chassis
- 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 the
resulting in a loose ground connection.
Another consideration is clearance between the connector and
other antenna structures. It is often the case that the connector
is placed very close to the boom, making waterproofing with sealant
and/or tape very difficult. If possible, space the connector
farther away from the boom and/or mount it at an angle such that one
can more-easily wrap the connector.
Even a bad outside antenna may work better than a good
Typical construction materials
in a house can easily result in 10-15db (or more) of additional
attenuation - and that amount of loss is difficult to make up with a
reasonably-sized indoor antenna. It should come as no surprise,
then, when even a very poorly
antenna is placed outside - even when fed with a lossy coaxial cable -
it is likely to outperform nearly any indoor antenna to which it may be
compared. In other words, it is possible that a rubber duck
antenna on the roof may work better than a "real" antenna inside a
Placing an indoor antenna near a window can help the situation
- if you happen to have one that faces the right direction - but be
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 RG-58 can
very high losses. These losses not only "eat" both transmit and
signals, but they can cause a false sense of security: Even
high 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!) advice
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 designs
plus the theory and formulae required to design your own!