HAM Radio 3/hamradioversion3.0examsandprograms1992.iso/ant/antennas/antennas.1

Text File | 1989-09-29 | 14KB | 219 lines

HamNet Antenna Tutorial - Part 1


        Through the courtesy of the Teleconference Radio Net and net manager
Rick Whiting, W0TN, we've received a transcript of the June 2nd
Teleconference by Joe Reisert, W1JR. Joe spoke on the subject "Antennas and
Antenna Systems, Where is the State of the Art Going?"

        Joe's talk did an excellent job of covering the whole spectrum of
antennas - from HF through UHF - and made heavy use of references in the
amateur and professional literature.

        For the next couple of weeks, we'll be serializing Joe's talk here in
the Ham Radio Tutorials section of the Message Board. The initial series of
messages will be left shortly - to be followed by four remaining sections of
the talk in the coming weeks.

        After the series is presented on the Message Board, it'll also be
found archived away in the XA7 database - so that you'll be able to readily
refer back to it in the future.

        We hope you find the series interesting and useful. It covers a broad
range of technical material on a subject of interest to all amateurs. Your
discussion of the Joe's text is welcomed!



PART I - Overall Summary and Definitions

        Good evening, my fellow amateurs. It is a great pleasure to be here
tonight. I feel honored to be selected to speak at this very large and
possibly largest ever teleconference. Many thanks go to the Honeywell Amateur
Radio Clubs and, in particular, Dave Meldrum, KA1MI, and Rick Whiting, W0TN,
for the confidence they have placed in me and also for their helpful hints
and suggestions to make this presentation a success.

        Tonight my talk will be about "Antennas and Antenna Systems, Where is
the State of the Art Going?" I will divide the talk into four separate
segments. The first part will deal with general terms and definitions which
will set the stage for the rest of the talk. The other three segments will be
the "low HF" (40 through 160 meters), the "middle HF" (10 through 30 meters)
and finally VHF/UHF and EME antennas. In most cases I will be talking about
the top of the line, state of the art or future antennas and antenna systems.

        There is probably no other amateur radio topic that inspires such a
vigorous line of conversation as the subject of antennas. Virtually every
amateur has some interesting story to tell about his or her favorite antenna
or antenna system or one yet to be fully tested. In reality there is no
better place to spend you time improving the performance of your station
since if the same antenna is used for transmitting and receiving, every dB of
improvement in gain yields a two dB overall station improvement, one dB on
transmit and one dB on receive. The old saw still stands... "If you can't
hear them you can't work them."

        If there is one big complaint that can be leveled against the amateur
community it is that they almost never use true gain standards when
evaluating antenna performance below 30 MHz. Typical performance is measured
in dBs above my last antenna or above a long wire in the trees of how many
guys I beat out in the pile up on XZ2XX. Seldom regarded are changes in radio
propagation, power of the competition, or operator savvy not to mention good
luck in being at the right frequency or timing of the call. Another problem
is the sheep following the wrong leader copying an antenna that Joe Blow uses
because he works more DX than I do without regard for his physical setup,
power or operating ability. The trouble with this kind of approach is that
you never really know what you have. You even may have built a real good
antenna and replaced it with a poorer one. The situation I've described is
not hopeless or beyond even the novice doing antenna tests if you understand
your limitations and make a few basic tests.

        Typical amateurs can only measure a few antenna parameters such as
VSWR and in the case of a rotary beam, front to back ratio. Therefore, the
commercial manufacturers make sure that these parameters are good.

        Let me be more specific about these parameters and first look at
VSWR. All kinds of myths have evolved such as the one that says a 1:1 VSWR is
necessary for an antenna to be working properly. This is entirely false. More
recently, there has been an upsurge in the belief since many of the modern
solid state rigs will only put out power if the VSWR is below 1.5:1. The
lowly antenna tuner is now enjoying a big comeback. We have gone to solid
state rigs that don't require tuning but now we have to add an antenna tuner
that takes more time to adjust than the old pi-networks we all used to use
that would literally load into any VSWR.
        To further muddy the water, the typical VSWR measuring gear used by
amateurs is patterned after the monimatch, a breakthrough in its time but an
instrument that is sadly lacking in accuracy when compared to a good
directional coupler like the Bird Model 43 thru-line wattmeter and its
equivalent toroid directional hybrid VSWR meter. Whenever I hear someone tell
me their dipole covers the full 80 meter band with a 3:1 VSWR I just laugh to
myself and wonder if they have a lossy feedline, a dummy load or just another
incorrect reading VSWR bridge.

        Probably the most important antenna parameter is gain. Gain is an
antenna property dealing with its ability to radiate power in a desired
direction or conversely to receive energy preferably from a desired
direction. It is a relative quantity, not measured in watts or ohms, etc.
Hence gain must be referenced to something such as a dipole or an isotropic
radiator, a theoretical antenna that radiates power equally well in every
direction. HF'ers rarely measure this very important parameter and usually
blindly accept the manufacturers claims especially if the front to back looks
good. Many persons wrongly feel that if an antenna has good front to back it
has good gain. This is not always true and will be discussed later in this
teleconference. In many cases the antenna manufacturer can't even measure
gain or compares his antenna to the competition and inflates his gain figures
by adding a single finagle factor to the competition's claims. Now don't
misinterpret my remarks as a slap at the manufacturers. I have noticed a
significant improvement in this area in recent years but we still have a long
way to go and I hope to give some guidance on this subject later in this
teleconference.

        VHF'ers realized this problem years ago when they organized a antenna
measuring parties. Some of the setups have gotten very exotic but the
accuracy can be quite good. The main things that must be taken into
consideration are an accurate gain reference antenna, a well illuminated
source antenna, accurate measuring instrumentation and a reality as to what
to expect. Two excellent articles on the subject are "Antenna Performance
Measurements" by Dick Turrin, W2IMU in November '74 QST pgs 35 thru 41 and
"UHF Antenna Ratiometry" by Dick Knadle, K2RIW in February '76 QST pgs 22
thru 25.

        The National Bureau of Standards in Boulder, Colorado has done alot
of work in gain measurements and NBS Report 5539 entitled "Methods of
Accurate Measurement of Antenna Gain" by H. V. Cottony is well worth reading.
NBS developed an antenna gain standard which consists of 2 full wave dipoles
mounted 1/4 wavelength in front of a 1.6 by 2 wavelength reflector that
yields an accurate gain of 9.31 dB over a dipole. This standard was later
redesigned for the EIA (Electronic Industries Association) by Richard Yang, a
consultant to the Andrew Corporation to a simpler and smaller reference which
consists of 2 half wave dipoles space 1/4 wavelength in front of a 1 by 1
wavelength reflector and yields 7.7 dB gain over a dipole. The EIA adopted
this smaller reference and incorporated it into EIA Standard 329 entitled
"Minimum Standards for Land Mobile Communications Antennas, Part 1, Base and
Fixed Station Antennas." This reference antenna standard is the one most
commonly used by amateurs and is often referred to incorrectly as the NBS
Standard when it is actually the EIA Standard.

        While on this subject, the EIA has issued another standard, RS-409
entitled "Minimum Standards for Amateur Radio Antennas, Part 1, Base and
Fixed Station Antennas." It uses a half wave dipole for reference and is more
apropos to HF antennas. This standard is very specific about the range itself
including the reference antenna height, the source antenna height, the
minimum distance between antennas and the minimum gain of the antenna under
test. It should be noted that the the formula for minimum separation distance
between the source and reference antennas is often quoted in the popular
literature as 2 D squared/lamda where D is the largest aperture dimension of
the antenna under test. This minimum can introduce an error of up to 1 dB.
A better antenna separation standard and the one used by the EIA is 10 D
squared/lamda which is accurate to 0.2 dB.

        There is a crude but informative method of gain measurement that can
be done on rotary beams using simple amateur techniques. It relies on the
fact that gain results by redirecting the power radiated in many directions
into a single direction or directions. If the half power bandwidth of the
radiated signal can be measured, the gain can be calculated using the formula
41253 divided by the product of the beamwidth in the horizontal and the
beamwidth in the vertical plane. Using a locally generated low power signal
(such as a local amateur), you first measure the half power bandwidth of your
antenna (the points where the signal is down 3 dB from the direct heading).
The vertical beamwidth can be estimated to be 5 to 15 percent greater than
the horizontal beamwidth. To give a numerical answer, a typical well designed
3 element Yagi has a 60 to 65 degree beamwidth in the horizontal or "E" plane
and 70 to 75 degrees in the vertical of "H" plane. Dividing 41253 first by 65
and then by 75 yields a gain of 8.5 or approximately 9.25 dB. This is
isotropic gain which is approximately 2.15 dB above the gain of a dipole.
Therefore the gain of a typical 3 element Yagi is roughly 7.1 dB over a
dipole. By measuring your beamwidth over the frequency bands of interest you
can estimate the gain. The wider the beamwidth, the lower the gain and vice
cersa. The only restriction to this formula is that all side lobes and read
lobes should be at least 15 dB below the main lobe. If not, the gain may be
lower than calculated. For further information on this subject, I refer you
to "Antennas" by John Kraus.

        Always be aware of the beamwidths quoted for a specific antenna. This
parameter can usually be accurately measured and tells you if the gain is
true gain or specsmanship. Also, some manufacturers list half beamwidth. To
convert, just multiply by 2 and proceed. Check the gain reference carefully.
Some sources quote isotropic gain which is 2.15 dB above the gain of a
dipole.

Transmission Lines and Baluns:

        No antenna talk would be complete without at least a few words on
transmission lines and baluns. Time does not permit a long segment on this
subject. A few general rules apply. Use good low loss non-contaminating types
of coax cable such as RG 213. Most RG-8 is poorly shielded, contaminating and
deteriorates rapidly meaning your feed line becomes a big attenuator after a
few years. The CATV foam coax is low loss, inexpensive and 75 ohms. It will
require special connectors and matching transformers (such as synchronous
transformers) if you want to go between 50 ohm sources. Make sure no water
gets inside as some of the older types of this line will draw water through
like a sponge and then cause discontinuities. I prefer Andrew Corp. Heliax or
its equivalent at VHF and UHF. It costs more but has long life and is very
low loss making it less costly in the long run. Oper wire lines are great and
low loss but require special handling techniques and are particularly
vulnerable when humidity or rain is present.

        Baluns are a subject that invokes strong arguments. Suffice it is to
say that a balun probably doesn't help much on wire antennas and dipoles. On
directional antennas it can prevent re-radiation which will reduce front to
back ratio. I prefer the balun types that do not require extra wires or
windings that interrupt the feedline. My article in September 1978 Ham Radio
and the one by Walt Maxwell, W2DU in March 1983 QST discuss this balun type
in detail.

Part 1 Summary:

        To summarize this portion of the teleconference, we need some
accurate antenna gain references. EIA Standard RS 409 may be a step in the
right direction. A good directional coupler type of VSWR indicator is a must
for the serious amateur. As a side benefit, it may be used to measure output
power if the FCC changes the amateur regulations to PEP output power as
discussed in the recent Notice of Proposed Rulemaking. Amateurs can determine
gain if they make the effort to measure or study the beamwidths and antenna
patterns on their antennas and calculate gain as I have described. Hopefully
in the not to distant future there will be general agreement on amateur
antenna standards so we can objectively compare antennas.

[End of Part 1]