T-25 Vertical
Working Principles


The base of the 7.5 m / 25 ft long radiator is mounted on a sturdy flat aluminum plate and insulated by two UV-protected polycarbonate rings. Mounted parallel to the radiator is a Teflon® insulated copper wire. Also mounted on the plate are two HF transformers (feeding and tuning transformers) and the radials.

The entire structure can easily be mounted to a 40-50 mm / 1.5-2" mast using the two stainless steel U-bolts provided. Should mobile operation or an approaching storm or other reasons make it necessary to take the antenna down, this can be done easily by one person. The radiator can be dismantled and removed from the mast base in a matter of minutes.

The working principle of our new all-band vertical antenna is similar to the rhombic antennas, terminated long wires, and the T2FD (terminated folded dipole) or W3HH antenna. These systems, called "wave antennas" (see ARRL Antenna Book, all editions), have a very wide frequency range and a terminal resistor, which absorbs the unradiated energy.

40 m and above
The antenna is fed by a 50-ohm coaxial cable (feeder line) of any length. A coaxial broadband transformer (T1) produces a 1:4 50:200-ohm match at A, providing a power radiation efficiency of 85-98% at 7 MHZ and above (40 through 10 m bands).

75-80 and 160 m bands
The 7.5 m / 25 ft radiator is far from full-size on the 75/80 and 160 m bands. When operating in these frequencies, parts of the non-radiated rf are fed through a "tuning line" (B-C). The return path goes through tuning transformer T2 to a 50-ohm coaxial cable connected to a dummy load at C (typically a 50-ohm terminal resistor).

Using this arrangement, the "return loss" on 80 meters is about 30%or about 1.5 dB (1/4 s-unit). On 160 meters, the "return loss" is about 60% or 4.5 dB (3/4 s-unit). For additional information on gain, please see the facts on vertical gain at the end of this page or at the end of the TCS 25 information.

The terminal resistor should be connected directly to the 50-ohm tuning line and be capable of handling 30% of the average output power. For example, a 1.5 KW output signal on cw would have an average output of 750 W and the terminal resistor must be capable of handling 30% of that power, or an average of 250 W.

It is interesting to note that there is no measurable SWR reading at the antenna. Unfavorable electrical surroundings result simply in a higher return to the terminal resistor. (If you want to check your return power, just insert a wattmeter in your tuning line.)

The antenna is ground-independent and can be mounted almost anywhere. If mounted on the ground, it should be installed at least 2 m / 7 ft high, for safety reasons (heads!). Other options might be a roof, a balcony, etc.--almost anywhere.

The antenna can also be mounted at any angle, even in a horizontal position (e.g., in an attic).

Suggestion for the technically minded, experiment-loving ham
Instead of the real 50-ohm resistor, an L/C system (e.g., a matchbox) can be used at the tuning cable. This can serve as a selective tuning system, achieving increased efficiency--especially in the 80 m and 160 m bands. See multi-position switch at C.

Some vertical gain antenna information

A typical vertical antenna with 0 (zero) dB/D gain in space is the Sleeven 1/2-wavelength dipole. Other 0 dB/D gain systems are 1/2-wavelength antennas like the Zepp-Windom-Fuchs and vertical radiators such as Ground Plane-Discone (all in space).

Antennas shorter than 1/2-wavelength (tuned by a loading coil or traps) have much less gain. Typical: 1/2-length = 1/4 "power gain" plus loss of traps/coils. Following are some examples:


Its efficiency is 7%; its gain is -12 dB/D (compared to a full-size system without losses). This means that only 7 W of a 100 W signal will be radiated. 93 W are wasted in this antenna.
--See Ham Radio Magazine, Sept., 1982, p. 18 ff, by W1GV.

* A full-size 40 m 1/4-wavelength ground plane is 34.6 ft / 10.5 m high; it has 0 dB/D gain.

*160 m GROUND PLANE 33 ft / 10 m HIGH, 12-OHM GROUND

				         Gain	     Efficiency

1. top loaded (big capacitor)           -10 dB          10%
2. center loaded, 2-ohm coil            -19 dB          1.2%
3. base loaded, 2-ohm coil              -20 dB          1.0%
Note: All gains compared to a full-size radiator and 0-ohm ground. With 2. and 3. only about 1 W of a 100 W signal will be radiated--99 W is wasted.
--See Ham Radio Magazine, May, 1983, p. 36 ff, by W7DHD.

* A full-size 160 m 1/4-wavelength ground plane is 136 ft / 41 m high! It has 0 dB/D gain.

[Working Principles]
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[Mechanical/Electrical Data]

Sommer Antennas

P.O. Box 710
Geneva, FL 32732
Phone:( 407) 349-9114 | Fax: (407) 349-2485