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Some Plain Facts about Multiband Vertical Antennas

Posted: 13 Jun 2020 04:54 PM PDT
http://www.iw5edi.com/ham-radio/4543...tical-antennas



By Lew McCoy, W1ICP
QST September 1972, pp. 14-16, 28




DURING DISCUSSIONS with newcomers, and old timers for that matter, it
becomes apparent that there is considerable confusion as to what exactly a
multiband vertical antenna is. The confusion concerns the method of feed,
how much mismatch one can expect, how many radials are required, how the
particular antenna is built for multiband use, plus some other points.




This article breaks the subject into simple language and provides the
reader with sufficient expertise to assure him that he wont wind up with a
system he really doesnt want Before going into a discussion of the
different types of multiband verticals we will offer some simple antenna
facts.








Some Basic Theory




The term multiband antenna has come to mean many things to hams. With trap
antennas, tapped coils, random wires, and so forth, there is plenty of
reason for the confusion. Simply, a multiband antenna is one that can be
used on more than one band. How we make it work on different bands is
another story.








Basically, any piece of wire of any length can be classed as a multiband
antenna. For example, a length of wire four feet long could be used on any
amateur band, from 160-meters on up. However, how well the piece of wire
would work is a completely different matter.




In the feed point of any antenna there is radiation resistance. The energy
supplied to an antenna is dissipated in the form of radio waves and in heat
losses in the wire and nearby insulating materials. The radiated energy is
the useful part, but so far as the transmitter is concerned it represents a
power consumption just as much as does the energy lost in heating the wire.
In either case the dissipated power is equal to I2R: in the case of heat
losses, R is a real resistance (ohmic losses), but in the case of
radiation, R is an assumed resistance. This fictitious resistance is the
radiation resistance. This brings us to our first important point about
multiband antennas.




Whenever one reduces the size (length) of an antenna physically, the
radiation resistance is reduced also. As an example, assume we have a
20-meter quarter-wave antenna, which is approximately 16 feet long. Lets
imagine we made it out of No. 40 wire, which has a resistance of about one
ohm per foot. The radiation resistance of a resonant quarter-wave vertical
operated against a perfect ground is on the order of 35 ohms. In this case,
the feed-point impedance of our antenna would be roughly 35 ohms in
radiation resistance plus 16 ohms in ohmic resistance. If we were to feed
51 watts into this antenna 16 watts would be dissipated as heat (lost
power) and the remainder 35 watts-would be radiated. Now, suppose we use
this same antenna on 80 meters. As mentioned above, when we reduce the size
of an antenna physically the radiation resistance is also reduced. On 80
meters our 16-foot antenna would have a radiation resistance on the order
of one ohm! However, we would still have the ohmic resistance of 16 ohms.
It doesnt take much figuring to realize that just about all of our power
would be lost as heat.








Of course we wouldnt use No. 40 wire for such an antenna. More likely the
antenna would be made from aluminum tubing and the ohmic losses would be
very low, but probably still more than the radiation resistance. There is
an old axiom in amateur radio that offers some pretty good advice: Always
make the antenna as long as possible, and erect it as high as possible.
Also, there is a joke that goes with that axiom if such an antenna stays
up, it is too small!




At this point we have only mentioned radiation and ohmic resistance in the
antenna feed point. These are the two resistances that exist when the
antenna is resonant. When the antenna is not resonant, there is reactance
present in the feed point. Reactance is also expressed in ohms, but it isnt
a real resistance in the sense that power can be dissipated therein. We
wont go into a long discussion on reactance because it would take up too
much space. An excellent explanation can be found in the League
publication, * Understanding Amateur Radio. Simply, reactance can be
likened to a gate or door that stops or hinders the flow of current into a
circuit. When an antenna is operated at some frequency other than the
resonant frequency there will always be reactance present. Keep in mind
that with any antenna, multiband or otherwise, we always have a condition
on some band or frequency where the antenna is not resonant. Therefore,
there will be reactance at the feed point.








Types of Vertical Antennas




The basic and most popular type of vertical is one that is a quarter
wavelength long and is operated against ground or in a ground-plane
configuration. The antenna is usually made from tubing and the radials are
wire. An ideal ground plane (simulated earth ground) would be a sheet of
metal with a radius of one-quarter wavelength or more. However, this is
only practical at vhf so the customary method is to use wires as the
radials. Probably the number one question asked about ground-plane antennas
is, how many radials are required? The answer is simply, the more radials
used, the better the antenna will perform, at least up to a certain point.
This should not be construed to mean that an antenna with only two or three
radials wont work. Such an antenna will work, but for maximum performance
one should consider 40 or more radials. If the reader is interested in
performance data for a few radials versus many, he should read the recent
article in QST by Sevick 1




The feed-point impedance of quarter-wave ground plane is on the order of 35
ohms. The impedance can be raised by drooping the radials down until a
50-ohm match is obtained. Exactly how much droop is required depends on the
number of radials.




The quarter-wave ground plane is essentially a single-band antenna.
However, a 40-meter quarter-wave vertical can also be used on 15 meters, a
happy circumstance for the Novice. In this case, a 40-meter quarter wave
works out to be three quarter waves on 15 and any odd multiple of quarter
waves will provide a relatively low-impedance feed.




Multiband Verticals




When we get into the field of multiband verticals we find that considerable
confusion exists. As pointed out previously, any antenna can be called a
multiband antenna, but how we get power into such an antenna is another
matter.




Up until the 50s any amateur multiband antenna was a system that usually
consisted of an antenna, tuned feeders, and an antenna coupler. In the
early 50s more and more amateurs started to use coaxial cable for feeders,
along with band-switching transmitters. The next logical step was the use
of a multiband antenna system that required no adjustments and always
presented a matched condition to the feed line in other words, an antenna
that had a 50-ohm feed-point impedance on every desired band and frequency
within a given band. A logical development was the multiband trap antenna.




By inserting traps in an antenna it was possible to make an antenna look
like a resonant half-wave dipole in whichever band was used; or, in the
case of multiband verticals, making the vertical look like a resonant
quarter-wave antenna for the desired band. However, and this is important
as far as the newcomer is concerned, to our knowledge there is no multiband
trap antenna that will provide a perfect match on all bands, regardless of
what some antenna manufacturers may tell you. Many hams have spent
countless hours trying to adjust trap antennas for that perfect match when
actually, it is just about impossible to obtain such a condition.




Nontrap Multiband Verticals




Several antenna manufacturers sell multiband antennas that consist of a
vertical piece of tubing, usually 16 to 20 feet long. The tubing is used
with a loading coil at the ground end. By making appropriate taps and
adjustments on the coil the antenna can be matched (or closely matched) on
any given band. This type antenna has no traps. This in turn means that the
coil taps and adjustments must be altered when one changes bands. Some
misguided amateurs buy these antennas expecting all they need do is put
them up and the antenna will work on all bands, automatically. Lets make
one point clear: such an antenna is a multiband antenna, but requires
adjustment at the antenna when one changes bands.








Of course, the next question should be, If the antenna is that simple cant
I build my own? Yes, it is a very simple multiband antenna to make and
install. Two or three sections of inexpensive 10-foot TV mast sections can
serve as the vertical radiating element. The mast can be supported on an
insulator, such as a beverage bottle, and the mast guyed with nylon line.
Fig. I shows a diagram of the antenna system. L1 should be a coil made of
bare wire, No. 12 or 14, so that it can be tapped at every turn. A
convenient coil size is 2-1/2 inches in diameter, six turns per inch, such
as B & W 3905-1 stock. The number of turns required, assuming 80 meters as
the lowest band to be used, should be about 30 turns with an antenna length
of 25 feet.




Adjustment of the antenna requires the use of an SWR bridge. Connect the
coax line across a few turns of L1 and make a trial position of the
shorting tap. Measure the SWR, then try various positions of the shorting
tap until the SWR reaches its lowest value. Then vary the line tap
similarly. This should bring the SWR down to a low value. Small adjustments
of both taps should provide an SWR close to 1. If not, try adding C1 and
repeat the adjustment procedure, varying C1 each time until a match is
achieved. Radials will enhance the performance of the antenna. The number
of radials is up to the individual amateur.




Trap Verticals




As mentioned earlier, traps can be installed in a multiband vertical. These
traps are usually parallel-tuned circuits and the objective is to make each
section of the antenna work as a quarter-wave vertical or odd multiple
thereof on the desired band. Fig. 2 shows an example of this type antenna.








The purpose in using this type antenna is to provide a system that always
presents a matched condition for the feed line. Unfortunately, there is so
much interaction between various sections of the antenna that it is
impossible to come up with a perfect match on each band. What is an
acceptable match is another story.




Amateurs as a whole are inclined to attach too much importance to an SWR of
1.(see Reflections). They feel that. if their SWR bridge isnt showing an
absolute zero reflected power that something is horribly wrong and they
wont work out. The plain fact is that using a feed line such as RG-8/U
(assume a 100-foot length) one could have an SWR of as much as 5 to 1 and
have no appreciable loss in the system. However, there is one clinker in
this thinking!




In many instances commercially made transmitters and transceivers are
designed by the manufacturer to work into a 50-ohm load only. They dont
allow much leeway from this figure. When there is a mismatch in the antenna
system, it can become impossible to load and tune the final amplifier of
the transmitter. There just isnt enough tuning range in the tank circuit of
the amplifier to handle the reactance that may be present in the load.
There is a way around this problem however, and that is using a Transmatch
in the feed line to disguise the mismatch.2 The Transmatch can be adjusted
so that the transmitter sees a 50-ohm load regardless of the mismatch at
the antenna.




Elsewhere in this issue is an article by W1CQS/W4DWK describing the
construction of a four-band trap vertical. The system is recommended to the
Novice because it will work quite well on 40 and 15 meters.




The Harmonic Problems




Another consideration should be mentioned. As pointed out earlier, any
antenna can be a multiband antenna. By the same token any harmonics
generated in the transmitter that reach the antenna can be radiated. It is
true that a single-band antenna will reject harmonic energy, but not
completely. In the case of a multiband trap antenna there is no rejection
of some harmonics, simply because the antenna is designed to be resonant on
all hf amateur bands. The solution to this problem is the use of a
selective circuit installed in the feed line. A Transmatch is such a
circuit and should provide adequate harmonic rejection.




Some Other Thoughts




The question is frequently asked, should I mount my vertical on the ground,
or get the base up in the air? Getting the antenna up in the clear is
always better than having it mounted at earth level and surrounded by rain
gutters, house wiring, trees, power lines and so forth. However, getting
the vertical antenna up in the air also means that radials, as many as
possible, should be used. The average installation (if there is such a
thing) usually consists of three or four radials (or more) cut for the
lowest operating frequency. Such a system should give a good performance.




Another important matter is that of the earth ground. When verticals are
mounted at ground level the ground losses can be very important. Too many
amateurs buy their verticals, get a five-foot long TV-type ground rod and
drive it into the earth at the base of the antenna. They think this
provides a good ground connection. As a matter of fact, the TV-type ground
rods are practically worthless for amateur work. A good ground rod is the
type used by the power company for home installations. This is a rod that
is heavily galvanized, 5/8 inch in diameter, and about 10 feet long. The
amateur should be able to buy these rods from any wholesale electrical
supply house. If possible, tie your ground connection to the water-system
piping, assuming metal piping is used.




Youll hear the statement from fellow hams that verticals are poor antennas
and radiate poorly in all directions. This isnt true because a vertical can
be a good antenna, but you have to give it a fighting chance.




* Understanding Amateur Radio is no longer in print. A complete explanation
of reactance may be found in the latest ARRL Handbook Chapter 6 Ed.




1 Sevick, The Ground-Image Vertical Antenna QST, July, 1971.




2 A suitable Transmatch is described in the transmission-line chapter of
the 1972 edition of The Radio Amateurs Handbook. (Chapter 22 of the 2000
ARRL Handbook, Ed.)

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