HAM Radio 1/HamRadio.cdr/ant/anttutor/antenna.4

Text File | 1989-07-16 | 11KB | 173 lines

.Part 4 - VHF and UHF Antennas:

   The spectrum above 50 MHz has special significance to the development
of antennas and antenna arrays.  This is the frequency range where you can
build a really high gain antenna without owning a large piece of real estate.
It is also the region where antennas can be tested easily in preparation for
scaling them to the HF region.  At the upper end of our frequency spectrum the
antennas are more aking to optics.  I'll divide this segment of the spectrum
into two parts, the VHF and UHF regions.

   The two major types of antennas used in the VHF spectrum (50 to 225
MHz) are the collinear array and the Yagi structure.  The collinear array
usually consists of a group of 1/2 wavelength dipoles in front of a screen or
set of half wave reflectors.  In the later case, it technically could be
called an array of two element Yagis.  The unique thing about the collinear is
the simplicity of the feed system which usually is an open wire line.  The
collinear is usually quite broadband, unlike most high gain antennas, and
efficiency and gain can be quite high.  The extended expanded collinear is a
stretched out version that has less elements and was described in an article I
wrote in Dec. '74 QST.  Both the conventional and the extended expanded
collinears were widely used in the days before good Yagi designs were
available and are still in use by some 144 and 432 EME operators.  This type
of antenna has two main drawbacks:  1. It is large and hence it can be large
enough, an expression the late Sam Harris, ex W1FZJ, used to use for antenna
that couldn't stay up under adverse weather and 2.  Its size usually prevents
mounting other antennas on the same mast.

   The workhorse in the VHF spectrum is truly the Yagi antenna. The first
high gain VHF Yagi designs were published by Carl Greenblum (QST, Aug/Sept.
'56), J. Kmosko, W2NLY and H. Johnson, W6QKI (QST, Jan. '56) and Dr. Hermann
Ehrenspeck and H. Poehler (IEEE, PGAP, Oct. '59, pp 379-386). Unfortunately,
these Yagis weren't always as good as claimed and had only fair cleanliness in
the side lobe and front-to-back ratio. In Jan. '72 (QST pg 96 and March pg 101
corrections), Don Hilliard, W0EYE, now W0PW, published his 4.2 wavelength 15
element Yagi based on the unpublished works of Peter Biezbicke at NBS. Don and
I urged Pete to publish his work and he finally did so in Dec. '77 in NBS
Technical Note #688, now out of print. This publication was the result of
extensive studies done by the NBS in the 1950's to develop high gain arrays
for ionospheric scatter and included models with boomlengths of 0.4 to 4.2
wavelengths plus new information on scaling and boom corrections. In August
1977 "Ham Radio" I published a full length article on the NBS report including
all the necessary details to build your own Yagis and sketched several models
for 50 thru 432 MHz. There are some errors in the NBS publication which are
corrected in my article. Not correct was the gain of the 2 element Yagi which
should be approximately 5.0 dBd, not 2.6 as reported by NBS (they must have
had some measurement errors). The NBS Yagis are not the only Yagi designs
available but they are easily duplicated and near the maximum gain attainable
for the appropriate boom lengths. They have excellent patterns and are easily
stacked for additional gain.

   One more point in passing. The trigonal reflector system in NBS 688
definitely is no good on the 3.2 wavelength and shorter booms. It actually
reduces gain by up to 1.5 dB! By lengthening all three elements in this
reflector system, I have been able to recover all the gain but no real gain
improvement over a single reflector. I have not tested the trigonal reflector
on the 4.2 wavelength designs.

   In Feb. 1978 QST, Wayne Overbeck, N6NB, published an antenna he named
the Quagi. It is basically a Yagi using a quad driven element and reflector.
It is low in cost using a wooden boom and fed directly with coax cable. DL9KR
and others have done further optimization on the Quagi and have used arrays of
16 to do 432 MHz EME. This design could still use some optimization in gain
and only a limited number of designs are available.

   Other versions of the Yagi have also been used including the log-
periodic fed Yagi developed by the late Oliver Swan and now manufactured by
KLM (See Ham Radio, Jan '76, pg 46). The log periodic antenna discussed
earlier in this talk has never found much favor with amateurs since there is
no need for the bandwidth and it has less gain than a well designed Yagi.
Along these lines, we can now make high gain Yagis with clean patterns using
the NBS designs. These antennas seem to stack well in larger arrays yielding
the 20 plus dBs required for 144 and 220 MHz EME. One EMEer, Dave Olean,
K1WHS, is using an array of 24 of the 2.2 wavelength NBS type Yagis stacked 8
feet apart for EME and he has worked stations all over the world who are only
using single Yagis and moderate power.

   Most recently, with the help of a large computer, a special program
and a local person interested in the design of VHF antennas, we were able to
develop a very unique Yagi, an 8 element one on a 12 foot boom for 144 MHz
that had extremely high gain (greater than 11.5 dBd true gain) with excellent
pattern (all lobes down 20 dB). It worked so well that I made 8 copies and
first tested them on a 144 MHz EME DXpedition to Rhose Island where 25
stations were worked off the Moon in two nights of operation. Computers will
undoubtedly be useful in the future as this work continues.

UHF:

   The 420 MHz and up area is in a transisition region. Long Yagi
antennas can be made with high gain such as the NBS and Guenter Hoch, DL6WU,
types. The later designs are an extension of the Greenblum designs mentioned
earlier and can be designed up to 20 wavelengths (see VHF Communications, #3
and #4, 1977, and #3, 1982). These designs show an increasing gain of
approximately 2.2 dB for every doubling of the boom length which is about the
maximum so far reported. Indeed I built a 9.25 wavelength (21 foot) 432 MHz
Yagi using this design material and achieved a verified gain of almost 17 dBd
at the 1981 Central States VHF Conference in Sioux Falls, SD.

   Long backfire ("A New Method For Obtaining Maximum Gain from Yagi
Antennas", IEEE, PGAP, Vol 7, Oct. '59) antennas have been tried by the EMEers
but gains have failed to live up to claims. The short backfire ("The Short-
Backfire Antenna", H. W. Ehrenspeck, Proc IEEE, Vol 53, Aug '65) has been
duplicated by myself and others and gains of approximately 15 dBi have been
achieved. Perhaps more work should be done in this area as an array of short
backfire antennas has the potential of higher gain without the problems of the
surface tolerances on the parabolic reflector.

   Loop Yagi: Another popular UHF antenna is the loop Yagi developed in
1974 by Mike Walters, G3JVL (Radio Communications, RSGB, Jan '75 and Sept '78).
Although it looks like a quad, it is distinctly different in that it uses
wide but thin metal scraps for elements. Mike started out with wires but could
never achieve high gains (like discussed earlier on quads). He recons that the
wide but thin strap improves bandwidth and hence gain. The loops are bolted
directly to a metallic boom thus solving the mechanical problems of mounting
elements at UHF. It is a very practical antenna for 902 MHz and above and has
worked well for me on 902, 1296 and 2304 MHz. G3JVL has even designed and
tested to specifications a 10 GHz model. The principle designs use 26, 38 and
45 elements. The gain on the 45 element model (which is 16 wavelengths long)
is 21 dBi! G3JVL has also published correction factors so that the loop width
thickness and boom size can be scaled.

   Dishes: There is something esoteric about the parabolic dish antenna.
It just has to work but the typical dish only has a 55% efficiency at best.
Furthermore, it has a large wind surface. Therefore, it is not too popular
except at frequencies where loop Yagis are no longer economical and for EME
where it can often be mounted close to the ground. More on this subject later.


   High Performance Arrays: I'd now like to turn to the subject of high
performance arrays and more specifically EME (Earth-Moon-Earth) antennas. EME
affords a unique property, viz. that due to the approximately 2-1/2 seconds it
takes a radio wave to traverse the 450,000 mile path to the Moon and back, the
EMEer can make improvements to his antenna system and actually hear the
difference by listening for his own echos. Furthermore, EME antennas have such
high gain (typically greater that 20 dBi) that you can listen to the noise
generated by the sun to measure beamwidth, patterns and hence determine actual
antenna gain (see "Requirements and Recommendations for 70-cm EME", J.
Reisert, W1JR, Ham Radio, June '82) as well as system noise figure.

   Large Yagi arrays are becoming increasingly popular especially for
EME. WB0TEM has 24 5.75 wavelength 19 element Yagis on 432 while K1WHS has 24
14 element 2.2 wavelength Yagis on 144 MHz. Both stations have big signals and
are able to work small (1 or 2 Yagi) stations off the Moon.

   However, the really big EME stations use parabolic dishes up to 40
feet in diameter!  The advantages of a dish for EME operation are numberous
despite the low (55%) efficiency (some commercial antenna manufacturers have
claimed up to 80% efficiency but use cassegranian feed systems that are quite
complex). First off, the feed system can be changed to permit multiband EME.
Circular polarization is also possible by using dual dipole feeds or the W2IMU
multimode horn. Dish type antennas are usually much quieter on reception
because of low side lobes and hence are very desireable with the low sky
temperatures experienced on 432 MHz and above. On 432 MHz where linear
polarization is still predominant, the most efficient dishes are using the EIA
symmetrical "E" and "H" plans and works well with a dish with a 0.45 to 0.5 F/
D ratio. VE7BBG has such a feed with a W2IMU horn built into the center and
has made cross band (23 to 70 cm) EME QSO's. A single dipole in front of a
splasher plate is definitely not recommended due to its unequal "E" and "H"
beamwidths! We still have a long way to go to improve efficiency and the
offset parabola recently introduced to EMEers by W2IMU from Bell Labs has
considerable advantages if the construction can become feasible for amateurs.

   Summary: The VHF/UHF frequency region is a good test bed for
developing and improving antennas. Recent developments in the Yagi and loop
Yagi have greatly advanced the state of the art in VHF/UHF communications.
Antenna patterns have improved and hence the noise temperature of the antennas
used is now more compatible with the state of the art preamplifiers. The NBS
Yagi data now gives everyone interested a recipe for a suitable antenna
without guesswork. EME antennas have taken a big leap forward in performance
and made EME operation almost commonplace. We still need to do more work in
the area of low loss feed systems especially for Yagi arrays.

(End of Part 4)