HAM Radio 1/HamRadio.cdr/ant/anttutor/antenna.2

Text File | 1989-07-16 | 9.9 KB | 166 lines

Part 2 - Lower HF Frequencies

    In the past few years we have enjoyed some of the greatest radio
propagation ever. Now the sun spots are declining and the fervent DX'ers and
those looking for a challenge are heading for the lower frequencies. All kinds
of new or improved systems are evolving and I will now attempt to cover this
frequency range and development.

Simple Antennas:

1. 1/2 wave dipole is hard to beat. It has good directivity, very efficient
and the ground reflection in the far field is the only real loss (and that we
have no control over!). The biggest problem is broadbanding especially on 80
meters. The open sleeve dipole invented by H. E. King and J. L. Wong (IEEE
PGAP, pg 201-204, March 1972) is now being explored for HF. If it can be
successfully scaled down from 225-400 MHz., it could improve bandwidth by a 2
to 5 factor.

2. Inverted Vee - radiates equally poor in all directions. Not really my
favorite antenna!

3. Verticals: There are many articles on this antenna type by Jerry Sevick,
W2FMI, Paul Lee, ex W3JM and now K6TS (?) etc. There are several popular
lengths - 1/4, 3/8, 1/2 and 5/8 wavelength. See Ham Radio September 1981 for
an interesting article on the 1/2 wavelength vertical by VE2CV. The main
problem is ground losses. The ground plane is an exception since it has 3 or 4
resonant radials and hence is very efficient. Typical resistance for the
conventional vertical 1/4 monopole is 30 to 36 ohms. Top loading, especially
with a top hat is recommended to improve efficiency especially on shortened
verticals. Also bandwidth can be very narrow especially on shortened verticals
since they are highly reactive. I am somewhat against verticals for QTH's
where ground conductivity is poor or where there are lots of local
obstructions. A good vertical has most of its radiation near the current point
which is usually the base! Absorbtion by trees, local objects, houses, etc. is
very detrimental. Also we have very little control over the far field unless
we live on or near a salt marsh or alkaline flat in the prairie.

4. Loops, Quad, Delta, side-fed Delta loop and Bi-Square. Great antennas if
you have the space. The most popular seems to be the delta loop apex up fed on
the lower corner up part way up the side.

5. Slopers: This is typically a 1/4 or 1/2 wavelength antenna that hangs off
a tower and in a semi-vertical fashion and therefore may have some directivity
(due to the tower acting as reflector) and a low angle of radiation. I prefer
the G5RV antenna (June 1977 Ham Radio Horizons) since it is shorter than a 1/2
wavelength dipole. It consists of 51 feet of wire each side of the center
insulator fed with 30 feet of 300 ohm feed line which then connects directly
to a 50 ohm coax line. It does have poor VSWR over most of the band but never
infinity. Advantages are multiple band operation (eg. 80/40/20/10) and it acts
like a collinear (with gain) on harmonic bands. At my station I use three G5RV
antennas as slopers spaced equally around a 97 foot tower and hence get good
coverage over most of the world on multiple HF bands with fair directivity.

6. Beverage or traveling wave antenna is especially good for receiving despite
its low efficiency. This is true because the outside or ambient noise is very
high and hence compensates for the loss. Use a trifilar wound transformer and
a low noise high dynamic range preamp to make up for the losses. Keep the
height up at say 10 feet so no one walks into antenna and files a law suit
against you. This happened locally when a horseback rider was knocked off a
horse by a local's beverage antenna! The length should be greater than a
wavelength at the operating frequency but 2 wavelengths is probably the
maximum recommended length. To keep noise down, use a wire with at least 30%
copper and is PVC coated. I have used beverages for transmitting and John
Belrose, VE2CV, has recently written an article on same in a recent QST.

Guys and Guy Wires: They must be tested for resonances especially if they are
not broken up with insulators. The difficulty is testing. One test is to
monitor VSWR carefully and remove or change a guy. Any changes indicate
problems. Likewise, the front to back ratio carefully monitored on a local
controlled station can give a feel for the problem. In some rare cases such as
sloper arrays, etc., they can actually be part of the array such as working
like reflectors, etc.

ARRAYS:

1. Yagi: Very large at HF, especially if full size! Bandwidth can be a big
problem. One 75 meter fan (W2HCW) had problems hearing the Russian SSB
stations operating on 3640 KHz, despite the fact that he was very strong over
there when transmitting in the US phone band at 3800 KHz. When he turned his
beam 180 degrees he could hear them but now they couldn't hear him. It turns
out that the front to back ratio flipped over below 3700 KHz!

    Many stations on 75/80 meters are using wire Yagi beams quite
successfully even at low heights (30 to 50 feet). They do work but there is
much tuning needed to determine correct lengths, etc. The problem of narrow
bandwidth mentioned above must be considered. Loaded Yagi antennas have even
narrower bandwidth.

2. W8JK: This antenna has been around a long time and is very successful at
HF but it is bi-directional.

3. The ZL Special and KB9CV modern version of same is seldom considered but I
think a worthwhile antenna. It is essentially a 2 element log periodic
invented over 10 years before the log periodic! It has excellent gain (like
the W8JK), directivity and is uni-directional. The feed system forces the
pattern so it does not have the limited bandwidth and pattern reversal
problems as severely as the Yagi does. See Ham Radio, May 1976, "Understanding
the ZL Special."

4. LPA (log periodic array): It is essentially a wide-band uni-directional
antenna. It has a sort of cardiod pattern at its lower frequency end so a
reflector is worthwhile. Make the low frequency cutoff a few % below the
lowest frequency of interest to enhance the lower frequencies. The best
references are George Smith's articles is 73, Ham Radio and QST. Other good HF
articles of interest on the subject are:
    - "Log Periodic Antenna Design," Ham Radio, Dec. '79 by P. Scholz
W6PYK and G. Smith W4AEO.
    - "Vertical Monopole Log-Periodic Antennas for 40 & 80 Meters," Ham
Radio, Sept. '73 by G. Smith.
    - "Feed System for Log Periodic Antennas," Ham Radio, Oct. '74, G.
Smith W4AEO.

5. The bobtail array: This simple array has recently enjoyed a comeback. It
consists of three 1/4 wavelength verticals spaced 1/2 wavelength joined at
their tops by a single wire. Usually a high impedance antenna tuner is used at
the base of the middle vertical to match the high impedance to coax. This
antenna has 3 to 5 dB gain and is bi-directional. Recently articles have
appeared in 73 magazine on how to feed the array directly with coax at the top
of the array.

6. Vertical Arrays: In the last decade or so, many amateur radio state of the
art advances have been made in vertical arrays by the late Jim Lawson, W2PV (
QST, March and May 1971), Dana Atchley, W1CF et al (QST April 1976), "Updating
Phased-Array Technology," W1CF (QST August 1978) and Richard Fenwick, K5RR and
R. Schell, PhD (QST April 1977). They have used computer aided techniques to
design optimum 2, 3 and 4 element arrays using triangles, squares and lines of
verticals. Their work has considerably improved not only the gain but also the
front to back and patterns of arrays.

    More recently, Roy Lewellen, W7EL (QST, Aug. 1979 pgs. 42/43) and
Forrest Gehrke (Ham Radio, May, June, July 1983 and other articles to follow
which will tell all!) have shown how to improve the feed systems of such
arrays to guarantee that the mutual coupling between elements will not
deteriorate the gain and patterns in the real world. This work and computer
aided work in the future will have a big effect on operations in the lower HF
region.

7. Other Arrays: Don't forget "V" beams and Rhombics. They can yield high
gain. The principle problem is patterns which are not always very good (side
lobes, etc.). These types of antennas are particularly good if you have lots
of real estate and only are interested in one or two directions. I think the
sloping terminated "V" beam is particularly worthwhile.

8. The active antenna array: Last but not least let us explore the active
array. This usually consists of a small (0.5 to 1.5 meters) vertical monopole
feeding the high input impedance of a low noise high dynamic range FET preamp.
Arrays of these are in commercial service and can provide extremely high
directivity. I am presently working on one for myself for solving some HF
receiving problems. The chief advantages of such a scheme are that it is small
and doesn't need an elaborate grounding system. Phasing is easy since the
outputs are not reactive and mutual impedance affects are low compared to a
conventional full-sized array. Also don't overlook ferrite loaded antennas and
loops. A good reference for HF DXing and antennas is ON4UN's book on 80 meter
DXing.

Summary: There is lots to be done. Computer aided design will help. We must
explore optimum topology for vertical arrays (2, 3 and 4 elements etc.) to
find best layout. Maybe we should look at the Mill's Cross! The sloper system
used today can probably be improved. The software just emerging in the last
few years will greatly help in the design of high performance arrays. Don't
overlook the log periodic or the ZL Special. The biggest problem to solve may
be the wideband feed system. Only now is the open sleeve dipole by Howard King
and J. Wong (IEEE PGAP March 1972) being explored. If it can be successfully
scaled from the 225 to 400 MHz spectrum, it could potentially yield a 2 to 5
times bandwidth increase over the present half wave HF dipole!