The Minooka special

by Barry Boothe, W9UCW, QST December 1974

Mobile operators and those who reside on property of city-lot size should findthe author's treatment of physically shortened 160-meter vertical antennas of considerable interest. Construction details are given here for making his Minooka Special from readily available inexpensive materials. Performance is reported to be excellent with both the mobile and fixed-station versions of his design. Data are given also on basic antenna types in use by some of the leading "top band " DXers.

THE MAIN ACTIVITY at W9UCW for the past 10 years has centered on 160-meter operation. The focus has been on antennas. Much of the study has been directed toward comparing the performance of vertical and horizontal antennas for various objectives. A further study has been made on portable, mobile, and small-lot antennas, including suitable antennas for use at DX locations. Three trips by the author to HK0 and HK1 locations motivated further investigations of portable antennas for use on 160 meters. Improved mobile operation was sought also, seeking solutions to the problems of efficiency, durability, bandwidth, ease of construction, and cost.Inspiration for the design offered here resulted in past from the author's survey of antennas in use by 160 meter operators, worldwide. An extensive report was compited.

It filled a loose-leaf notebook! A boil-down of the most useful information resulted in an 11-page report which was made available to numerous amateurs. Subsequently, the writer was encouraged to submit some of the data for publication. Approximately 60 operators were polled. The following information from that inqury should be of interest to those who are curious about "preferred" antennas for topband use.

Question 1: if you could put up any antenna for 160, what would it be?

Result: 60% favored verticals, 30% said horizontals, and 10% indicated mixed feelings. (The term "vertical" includes various configurations 1/4, 1/2, and 5/8 wavelength, vertical arrays, and inverted Ls.)

Question 2: "Comparing antennas that an average ham could build, do you prefer verticals or horizontals for 160?

Result: 70% said vertical, 17% favored horizontal, 5% inverted L, 2% horizontal and vertical, 2% inverted V and 3% had no opinion. Reason; given for theresponses were, "Because of signal comparisons and past experience. A backyard-compatible vertical is more effective than a back-yard-compatible horizontal."

Question 3: "Do you operate mobile on 160? If so, what is the antenna used?"

Result: 51 used base-loaded verticals 8 used center-loaded ones and one employed a Heliwhip. Some of the operators used capacitance hats. Other questions in the surrey dealt with receiving antennas, types of soil and terrain, besides requesting complete details of the present transmitting antenna.

Minooka special

The following is a description of an effective vertical antenna for 160 meters, designed with these objectives in mind.

1) Highly effective for 160-meter DX and local work.
2) Easy to build and adjust.
3) Very economical.
4) Fits neatly into back yard.
5) Reasonable bandwidth.
6) Good for portable and DXpedition work.
7) Can be scaled down for mobile operation.

The resultant antenna (Fig. 1) is top loaded inductively and can be built by anyone from readily available material. Only a dip meter and SWR indicator are needed for tune-up Man versions were built and tested, ranging from 7-foot mobile types to 60-foot backyard or DXpedition models. They have been used with good results from 20 foot wide backyards in cluttered Chicago, to vast beaches on Carribean islands, and in South America. The fixed-tuned bandwidths vary from 10 kHz for mobile versions, to 50 kHz for the larger fixed-station models (SWR 2:1 or better).

Fig. 1 Electrical details of the Minooka special.

Table 2 gives specific information concerning dimensions X, Y and Z. L2 may vary in size from 1 to 20 turns, and L3 will contain between 5 and 10 turns, typically. L2 and L3 are made from No.18 wire spaced 1/8 inch between turns. The coil diameter is 1-1/2 inches. Refer to text for tuning instructions.

X	5 ft.	8 ft.	4 ft.	4 ft.	19 ft.	3 ft., 3 in.
Y	2 ft.	15 in.	3 ft., 6 in.	4 ft.	11 in.	3 ft.
Z	*	*	*	***
Wire size	No. 20	No. 19	No. 18	No. 16	No. 19	No. 22

* mean as long as possible

Construction

The physical layout of the antenna is centered around the use of 3/4-inch-diameter rigid PVC water pipe. This sturdy tubing has an outside diameter of 1.038 inches. The inner diameter provides an "interferencefit" for 1/2-inch EMT conduit (thinwall). A 3/8-24 nut is driven into a 5-inch length of conduit, then aligned and braized into position. Next, the conduit is taper-ground and polished on the opposite end, then driven into the PVC tubing (see table of dimensions).

In the construction of mobile antennas another piece of conduit is driven into the bottom of the PVC coil form and a 3/8 - 24 bolt is brazed into the bottom end. The bolt will mate with standard mobile antenna mounts. For larger versions of the antenna, the PVC material can be mounted in TV wasting, or whatever. A standard 8-foot stainless steel whip between 3 and 8 feet in length can be screwed into the top section. For home-installed versions of the antenna a three- or four-foot piece of thin-wall tubing can be used in place of the whip. This will save on the cost of materials, and will eliminate the need to have brazing done. The coil wire should be soldered to the conduit to assure a good electrical connection.

Adjustment

Pick a set of dimensions from the table which suits your application, but add a few inches more of coil turns (all turns close wound) than are recommended. This will allow leeway for pruning the system to resonance. Pick set dimensions from table which suits your application, but add a few inches more of coil (all turns close wound) than are recommended.

To simplify adjustment it is suggested that the system be assembled first with only the coil and top section (no base section). Place the antenna where it is in the clear (on the car or fixed-station site), and where it can be tuned against the proposed ground system - car body or ground radials.

A three-turn link should be connected temporarily between the lower end of the coil and the ground system. This will permit rough tuning of the system to resonance by inserting a dipper coil into the link and adjusting the coil turns on the antenna until a dip is noted in the desired part of the band. Upon completion of the pruning the constructor can, if he wishes, cover the coil with weatherproof tape or shrink tubing. The antenna should be tuned for roughly 2000 kHz if the entire band is to be used. Adjust the resonance for 1850 kHz if only the low segment will be utilized.

Erect the antenna with all of its parts - coil, top and bottom sections - and insert inductors L2 and L3 of Fig. 1 as shown. With L3 temporarily out of the system, adjust L2 for the lowest value of SWR obtainable at the desired operating frequency. Then, place L3 back in the circuit and adjust it for an SWR reading of 1. Addition of the base section later on will not affect the resonant frequency of the overall system materially, provided the base section does not exceed, say, 60 feet.

The foregoing steps are used also in adjusting the mobile version of the antenna. However, because of the small size it is possible to adjust the antenna in completely assembled form. Only L3 of Fig. 1 is needed for mobile antennas. The main coil, L1, is adjusted for resonance, then L3 is set for lowest SWR. L3 can be mounted inside the trunk or under the bumper in a weatherproof enclosure.

For fixed-station operation it is recommended that a good ground system be employed. One should use at least 10 radials of, say, No. 18 or larger wire, 10 to 50 feet in length. If you can manage 40 radials, 60 feet in length, all the better.

Concluding Comments

Three fixed-station versions of the Minooka Special have been tested and used at W9UCW. Each was compared against the regular antenna, which is a one-quarter wavelength vertical (130 feet high), and operates against a radial system that contains 12,000 feet of wire. The short verticals were always inferior to the big antenna by approximately 5 dB.

Using version No. 4 from the table (4-foot base section), the author worked Wls through W6s from the mobile setup. The rig was an HW-18 transceiver. Contacts were also made with VP9 and KV4 stations from the car.

While DXing from San Andres (HK0) reports were received of signal strengths exceeding S9 plus 20 dB from Maine to Washington. Good reports were received from Europe also.

Because the loading coil acts as an rf choke at 3.5 MHz and above, several versions of the antenna have been used successfully from 160 through 20 meters with an appropriate L-network matching section installed at the base of the system.

Acknowledgements

The author wishes to express his gratitude to all who helped make this article possible particularly K9SKX, WA9EYY, and W9YYS, who assisted with the initial experiments on the Minooka Special. The participants in the antenna survey (a list too large to include here) deserve considerable thanks for their help as well.

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