## Info

The Tempo Line, now more than ever...solid value

Tempo VHF-ONE Full 2-meter band coverage (144 to 148 MHz for transmit and receive • Full phase lock synthesized (PLL) • Provision for accessory SSB adaptor * 5-digit LED receive frequency dispfay • Automatic repeater split * Solid state • 10 watts output . . $495,00

Tempo SSB ONE SSB adaptor for the Tempo VHF-ONE • Selectable upper or lower sideband * Noise blanker built-in • RIT and VXO $225.00

Tempo/CL-146A A mobile transceiver for the 2 meter amateur band * Compact and rugged * Solid state • 144 to 148 MHz(any 2 MHz without retiming), 12 channel possible. One channel supplied, two channels of your choice free ^ ** i..* .*»»■>> ^ ■ > $239,00

Tempo CL-220 Same general specs as CL-146A,

### New from Tempo

Watch for the announcement of Tempo's brand new advanced design digital display transceiver.^the Model 2020, 80 through 10 meters, SSB. Another solid value in the Tempo line.

but operates 220 Ao 225 MHz (any 2 MHz without returning) $299,00

Tempo FMH Two watt VHF/FM hand held • 6 channel capability • Solid state • 144-148 MHz • Includes 1 pair of crystals, built-in charging terminals for ni-cad batteries $199.00

Tempo 6N2 Power amplifier for 2 and 6 meter operation # 2000 watts PEP input on SSB & 1000 watts input on CW and FM • II

Solid state power supply,, $795.00

Tempo 2002 For 2 meter operation $695.00

Tempo 2006 For 6 meter operation $695.00

Tempo VHF/ UHF Amplifiers A wide range of solid state power amplifiers for use in most land mobile applications.

11240 W. Olympic IBM Los Angeles. Calif 90064 213/477-6701 931 N Euclid, Anaheim, Calif 92801 714/772 9200

Sutler, Missouri 64730 816/679-3127

optimum design, though, so that you end up with a compact size, efficient antenna, instead of a ''heating element1' for rf. Don't worry -no higher math will be used,

A DDRR can be designed (or use in just one ham band, or as a model covering all amateur frequency assignments in the HF region. Once the design of a single band element is understood, there wii! be no problems in adding other band coverage elements, We will assume that the prospective user lives ¡n a typical cramped-space, urban QT11 where the installation of a really effective artificial ground plane system is completely out of the question for many practical reasons; to make the design even more attractive, a total vertical height of only six feet will be used in the example here, together with a selection of just one outside diameter conductor size to be employed in all conductors of the DDRR, The design relations given, however, are in such form that other conductor diameters, antenna heights, and frequency bands may be substituted as desired. Later, details will be given for adding the other elements to the one band design example to convert il into an all-bander.

Fig. 3(a) shows a dual post DDRR like the Wheeling model, crccted over a continuous surface, highly conducting ground plane, We will temporarily retain such a super ground plane for our discussion purposes here; later, we will discard it completely in the practical home QTH model. Also, we will end up with a one post DDRR, as the two post model does not afford the minimum si/e design we wish for ham use. In the drawing lhe rf currents are shown (lowing in both the overhead conductors and as image currents in the ground plane at a single instant of time In the rf cycle. It is noticed that the directions of the currents in the ground plane arc not radial like those produced by a simple vertical monopole antenna. In Fig, 3(b) ¡ust one half circumferential section of the two post DDRR antenna is shown in "-straightened out'5 fashion. Because the part of the DDRR element of Fig. 3(b) includes a vertical post conductor, a horizontal conductor elevated above and parallel to the ground plane, a tuning condenser (C), as well as input feed terminals, it will function as a DDRR antenna element itself in our finished design.

Immediately, it would appear that all a DDRR really boils down to ts a "one wire/' unbalanced rf transmission line parallel to ground at a height h, and "shorted" to ground at one end by a vertical post, "¡"his ought to be easy! Now, we will agree with you that the horizontal conductor of total length S° parallel to ground does indeed form nothing more than a "one wire" rf transmission line "stub/' But we are going to insist that you unlimber your imagination and go along with us in considering the vertical "shorting" post at one end of such line as another separate and different rf transmission line also. Any good amateur antenna handbook gives the formula for finding the characteristic impedance of the "one wire" line above ground in terms of its mean height (h) and conductor diameter (d), U is,, merely, lic ■ |3F1 voyui Ohms

(1 lift

Armed with equation (1-1.0), let us begin by selecting the 75 meter band for use in our example. We will start the design at Lhe upper frequency limit band edge of 4,0 MHz. At 4,0 MHz, wavelength X in air is 984/4.0 MHz, or 246.00 feet. We said we would use only a vertical antenna height of six feet. We will not be precise here and take into account the conductor diameter in determining the electrical length (h°| of the vertical post element at 4.0 MHz. Instead, we will arbitrarily select 4.0 inch O.D., thin wall, aluminum alloy tubing (type 6061 T6 or other weldable alloy) for both the post and horizontal conductor. Taking the post height (h) as 6,0 feet, its diameter of 4.0 inches as 0.33 feet, and its radius as 0/I7 feet, we find the following "electrical dimensions" at 4,0 MHz:

Knowing these dimensions allows us to use (1 -1,0) to get the characteristic Impedance (Kc) oT Lhe "one wire" over ground horizontal transmission line section as,

Turning now to the vertical post itself, it appears that we face a problem in determining its characteristic Impedance as an rf transmission line. Tor example, we know that another way to define the characteristic impedance of ordinary rf transmission lines is in terms of the ratio of the distributed series inductance (L) of the conductor to its distributed shunt capacity (C) between the conductors, Such a i elation is written as ZD = n/L/C Ohms. We know we would get Kc equals 256.70 Ohms for the horizontal line section by this alternate formula if we could just measure the distributed series inductance (L) along our 4.0 O.D. conductor and its distributed shunt capacity (C) to ground per unit length. Such characteristic impedance is constant along the entire length (S^) of the horizontal DDRR transmission line, because its conductor diameter (d ) and height (h i is constanl per unit length and thus gives constant L to C per unit length. Just looking at the vertical post we see this cannot be the case for a cylindrical conductor mounted vertical to a fiat ground plane, Anyone can see that if we sawed out a given width slice from the vertical post conductor at a height of, say, 14 inch above ground, and measured the shunt capacity of this insulated section to ground there, and then repeated the same procedure at a height of 36 inches above ground, and then at 72 inches above ground, shunt capacity would be maximum at % inch above ground, less at 36 inches, and least at a height of 72 inches, Because shunt C varies with length h°, the ratio of L/C cannot possibly be constant; therefore, the

"characteristic impedance" of the vertical post when considered as an rf transmission line - would have to be a variable function of height h\ At the same time you have a suspicion that the vertical post in the DDRR is something more than just a "shorting post/1 A grounded, vertical monopoie antenna, maybe?? You may wonder what we are up to here.

Well, you are perfectly right. Not only does the characteristic impedance of the vertical post rf transmission line change with height, but it is also a grounded, vertical monopole antenna. How do you find the "characteristic impedance11 of a monopole antenna? Well, thanks to a brilliant antenna man, Dr. S. A. Schelkunoff of the Bel! Telephone Laboratories,2 we can do just that:

Km ffl I 2.3M&I0SID 10 I

n.ioi

The above equation gives the average characteristic

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