T-25/S Vertical
Working Principles


The T-25/S uses the same electrical working principle as the T-25. It has, however, some significant electrical and mechanical advantages:

Compared with the T-25, the T-25/S has about 0.5-2 dB higher gain, depending on frequency.
(Fig. 1)

Working Principle:

A vertical radiator can be matched to a 50-ohm feedline at its base by either an L (inductive), a C (capacitive), or an L/C system. Fig. 1. A-G show several matching systems.

An HF-line with its end either open or shorted (see Fig. D and E) can be an "L" , a "C", or an "L/C" matching system, depending on the ratio of its length to a given wavelength. (See also ARRL Antenna Book, all editions.) Most matching systems can therefore  be replaced by a "coax stub."

A very long coax stub, several wavelengths long, can be used as a "multiband-tuning-system" in the 3-30 MHz range. This long line almost always offers a correct L or C or L/C match for each frequency, and a good match of the vertical radiator to the feedline.

If a low SWR over the entire frequency range is desired, a folded monopole is the best solution. This arrangement needs two matching transformers within the feeder and the tuning line (FT and TT or T1 and T2).

The T-25/S radiator is a "coaxial-folded monopole." This means low wind load and few problems with snow and ice. The necessary coaxial cable PTFE (Teflon) is inside the radiator tube. The electrical length of the T-25/S is approximately equal to radiator length S1 (25ft/7.5m) + radiator length S2 (8ft/2.5m) = 33ft/10m. The T25/S is similar to a 3/8 wavelength "W8JK-folded monopole" on 20m (the "tuning line" is not active on 20m).

The T-25/S is fed similarly to a "Windom Antenna." The tuning line is a small diameter coaxial cable (e.g. PTFE coax cable).


The base of the 25ft/7.5m self-supporting radiator S1 is mounted on two sturdy aluminum plates and insulated by two UV-resistant polycarbonate (Lexan) insulators. The feeding transformer and the tuning transformer (part of the kit) are mounted on the base. The T-25/S with its unobtrusive plain aluminum radiator can be a well-disguised "antenna-flagpole."

The radiator part S2 consists of a 600-ohm line that runs parallel to the coax feedline, as in a "Sleeven" feeder system.

The tuning line is about 250ft/75m long. It is part of the antenna kit and is delivered on a spool approximately 1ft(30cm) in diameter, which can be either attached at the mast or laid out as a long radial in any shape (e.g. a big "8" or "double-8" or any other shape possible at your particular location).

The tuning line can be used as described above (basic tuning line),
and/or as a radial to improve performance on the lower frequencies (5MHz and below).


Where is the best mounting place for the T-25/S?

The ideal mounting for the T-25/S is 6-10ft(2-3m) above ground on a wood or metal pole, as far away as possible from reflecting metal objects. A flat roof on a concrete building is also good. This guarantees low radiation angles. See also ARRL Antenna Book, Vertical Antennas.

Mounting too close to a house (1/2 wavelength or less) or on the roof is less recommended. This results in an inferior radiation pattern and, more importantly, a heavily increased noise level from house installations, power lines, etc.

Mounting on a tower higher than 1/4 wavelength results in high radiation pattern (up to 60 °). No DX performance! See ARRL Antenna Book.

The T-25/S is a very effective antenna. As with all good antennas, it reacts to nearby objects such as other antennas, lightning protection systems, etc. This is particularly pronounced with items that are closer than one wavelength and longer than 1/2 wavelength. Under these circumstances, the antenna can receive its own signal back, resulting in increased SWR.


Our multiband verticals T-25 and T-25/S will work very well in any location where a monoband full-size vertical antenna (such as a ground-plane, vertical dipole, etc.), would work well -- on each band.

The T-25/S is grounded through the tuning line and at the feedline in the shack. This reduces static noise. A spark gap is provided at the antenna base. The base needs to be well grounded.

[Working Principles]
[Parts Supplied]
[Mechanical/Electrical Data]

Sommer Antennas

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