HAM Radio 1/HamRadio.cdr/ant/anttutor/antenna.3

Text File | 1989-07-16 | 8KB | 119 lines

Part 3 - Upper HF Frequencies:

   This is probably the frequency region where most amateurs are really
concerned about their antennas and probably where the most $$$ are spent. Here
the rotary antenna is very popular and being competitive in the pileups is
very important.

   Other than the usual ground planes, dipoles and long wires, the most
often used antennas in the 10 to 30 meter frequency range are the Yagi and the
cubical quad.

   First my thoughts on the quad. It may be a fine antenna for some
amateurs. It is surely an inexpensive antenna but is difficult to keep in the
air. It really has high Q in that it's front to back ratio detunes rapidly
versus frequency. I strongly doubt that it has greater than 1 dB gain over a
properly designed Yagi (more on this later). The usual way a quad is tuned is
to maximize the front to back ratio. This does not necessarily mean maximum
gain. I personally feel the quad is popular because its construction is simple
and low cost. A quad using aluminum tubing would probably do much better but
would obviously be unwieldly. One big plus for the quad, and I may add its
original invention was for this reason, is its lower static reception level
during rain and snow storms. This is unquestionably true. I've gone the quad
route twice. Despite 2 years of work on a 3 element quad, it never could
compare with a well designed 3 element Yagi and hence was finally scrapped in
favor of the Yagi. Tests at VHF and UHF on scaled quads have never
successfully shown the gains claimed except on the loop Yagi which I will
discuss later in this teleconference.

   In this frequency range, the Yagi is King especially among the DXCC
Honor Roll members. This antenna has been around the amateur community since
the late 1930's. Many people have used Yagi antennas but few have really paid
any attention to proper element lengths. Recent work on scaling and especially
on element tapering have been thoroughly discussed by the late Jim Lawson,
W2PV, in a series of articles in Ham Radio from August 1979 thru December
1980. These articles show that on 20 meters the elements may have to be
lengthened as much as 12 inches and more to equal the free space length of an
equivalent untapered element. The results of not performing this extension are
lower gain and poorer pattern than expected!

  This same series of articles is probably the best collection of
references on Yagi design to date. W2PV meticulously explored all details. Of
greatest interest are his charts and patterns showing what can be done and how
to do it. This is must reading for the serious HF'er.

   W2PV also shows how to use computer aided optimization, a technique
that is presently beyond those without access to a large computer, but surely
something that will be within the realm of home computers in the not too
distant future. The principle advantage to computer-aided Yagi design is the
ability to optimize gain, front-to-back ratio or side lobes. One big problem
is bandwidth, typically 250 to 300 KHz. maximum at 20 meters. This is true
because of the feed systems we amateurs use and the cutoff of the first
director causing the pattern to break up at the top of the band. As a word to
the wise, design your Yagi antenna on the high side of your favorite operating
frequency since this type of antenna cuts off rapidly above the design center
but drops performance slowly as the frequency is decreased.

   All things being equal and optimum, the boom length, not the number of
elements, is the important parameter when determining the gain of a Yagi
antenna. A larger number of elements than required insures a good pattern over
a wider bandwidth but more elements can also be a negative since there are
more things to go wrong both electrically and mechanically! Maximum gain on a
one wavelength boom is about 10 dBd! Compare this with the high gains you hear
amateurs bragging about on the HF bands.

   Another interesting phenomenon on Yagi's is the improved pattern at
certain boom lengths. This was first discovered by Peter Viezbicke and
reported in NBS Technical Note #688 and later confirmed by W2PV. The NBS data
showed slightly higher gain at certain boomlengths which is probably due to
some pattern cleanup. Suffice it to say that for best pattern and gain, the
boomlength of a Yagi antenna should be an odd number of quarter wavelengths (
eg .25, .75, 1.25, etc.) long. (The only known exception is the famous W2PV
0.575 wavelength boom published in the Yankee Clipper Contest Club Bulletin.
However, this design is asymmetrical about the axis and uses very close
reflector spacings and is believed to be a special case).  We will discuss the
NBS Technical Note in depth in the VHF/UHF portion of this talk because the
antennas in that report are mostly longer than the typical designs used on HF.


   Boom resonances can be a problem especially at HF and where mono-band
Yagi's are often stacked Christmas tree fashion for multiband operation.
Again, computer optimization has shown that these effects are real.  Gain and
front-to-back ratio can be significantly decreased when one Yagi is placed
close to another one even though they are on different frequency bands.
Computer techniques have been used to reduce these effects by re-tweaking the
element lengths to offset the detuning effect but even then the results show
bandwidth may be decreased by up to 50% of the original design.  Some amateurs
have used insulated boom mounting clamps in an attempt to offset this effect.
Another technique but an ungamely one is to rotate the offending antennas at
right angles to the lower antenna.  If you place one antenna in close
proximity (1 to 2 meters at HF) to another, check the VSWR carefully before
and after the change.  If the pattern or the VSWR shifts or changes, it is a
possible sign of an interaction problem.

   Let's not forget the log periodic array!  "The Log-Periodic Dipole
Array" by Peter Rhodes, K4EWG, QST, Nov. '73, "The Log-Yag Array" by K4EWG and
J. Painter, W4BPP, QST, Dec. '76 and "The Log-Periodic V Array" by K4EWG, QST,
Oct. '79 articles are must reading. The addition of the new WARC bands in the
future will make log-periodic antennas much more practical.  Their main
andvantage is good gain, VSWR and pattern over a very wide frequency range
rather than the usual narrow bandwidth of the conventional Yagi antenna.  One
amateur antenna manufacturer presently employs a log-periodic feed system to
some of their antennas to increase bandwidth. The log-periodic structure
forces current and therefore pattern by its unique feed system and I think we
will see more antennas of this design in the not to distant future.

   Summary: We've come a long way in the HF region. There will be a swing
towards wider bandwidth and perhaps LPA's will find their way into the
amateurs bag of tricks as more spectrum and bands become available (eg. 18 and
24 MHz.).  We are getting more discriminating and will demand good patterns
and gain at the same time!  In the future I see the use of computer aided
design to improve patterns and gain as well as bandwidth.  Wider bandwidth
feed systems are needed. The LPA is one example, the use of the open sleeve
dipole is another.  Amateurs have notoriously ignored the feed systems and
consistently used narrow band feed systems.  We must develop wider feed
systems and consistently used narrow band feed systems.  We must develop wider
bandwidth feed systems in the future.

(End of Part 3)