Vortex ‘Slim Jim’ Vertical Antenna – VHF 70Mhz (4m band) Amateur

Vortex 'Slim Jim' Vertical Antenna - VHF 70Mhz (4m band) Amateur

Vortex 'Slim Jim' Vertical Antenna - VHF 70Mhz (4m band) Amateur

£135.00 — £240.00

Product Code – 70-SLIMJIM

A Vertical Wideband Antenna for the 70MHz (4m) Amateur band.
The Vortex ‘Slim Jim’ Antenna consists of a single element, one half wavelength long radiator with a quarter wave parallel feedline tuning stub. The half-wave section is then folded back on itself towards the stub at a distance of approx 0.44 of a wavelength. At the base, the quarter wave matching stub is very similar to that of the J-Pole. The antenna is very easy to assemble and matching is easily accomplished by moving the feedpoint up or down the lower sections of the tubes. To also aid matching, we include a quarter-wavelength of Webro WF100 75 ohm coaxial cable. This has an added advantage that tuning can be varied over a fairly wide frequency range by adjusting the feed-point location which is easily done. This also gives the user finer control over the impedance and match.

The antenna is end fed and generally has an omni-directional radiation pattern although there is a slight bias in directivity towards the ‘Stub’ side of the array as shown on the EZNEC simulation. The antenna requires no ground plane or radials. Slim Jim antennas have two very desirable characteristics. Firstly the angle of radiation is exceptionally low. During modelling, an antenna at 10m in height above the ground had a radiation angle of only 5 degrees above the horizon making it an excellent base antenna for longer distance contacts. At 15m in height this reduced further to only 3 degrees. Some vertical antennas may exhibit higher gain figures, but the radiation angle is normally much higher – putting most of your power skywards rather than across the horizon. The measured 2 to 1 SWR bandwidth is also very wide with the 4m version running from 66 to over 73.25 MHz (which is over 7.25MHz of usable frequencies) making it very wide-banded.

Slim Jim antennas should include a 1 to 1 ‘Choke Balun’ close to the feed-point, which prevents feed-line radiation. A simple and cost effective solution is to wind a small length of coaxial cable around a former. There are many examples on-line, but we found wrapping 5 turns around a 30 mm plastic water pipe did the job. Vortex can also supply a larger commercial version of this if required using commercial grade ®Westflex 103 Coaxial Cable .
A Vortex Antenna Systems premium ‘Ferrite Core Choke Balun’ (Sealed to IP67 rating) is also available as an optional extra.

No Knowledge of Antennas? Our 70Mhz Slim Jim antenna is pre-tuned from the factory and all set-up measurements are included. However the system is fully adjustable should the user require to finely tune the system for their own requirements. We include a full set-up instruction sheet with each antenna.

All Vortex Antenna Systems Slim Jims’ use heavy duty components and hardware including M10 stainless hardware, heavy duty Stauff polyamide and aluminium clamps as also used in our lower frequency base antennas. The main element section of this antenna is supplied with genuine VORTEX 6082-T6 or *AA6082-T6 Aluminium with each section telescoping into its counterpart. The feed to the antenna is via 75 ohm cox stub positioned for best match which is weatherproofed.

See info below for mechanical and electrical details

  Vortex ‘Slim Jim’ Vertical Antenna – VHF 70Mhz (4m band) Amateur

Band(s):  70MHz (4m)
Element Material:  6082-T6/*AA6082-T6
Boom Material:  6082-T6 box section aluminium
Gain:   6.77dbi at 10m in height, rising to 7.35dbi at 15m
Feed:   Webro WF100 75 ohm coaxial stub (supplied)
Matching:   Webro WF100 75 ohm coaxial stub stub positioned for best match
Rating:   2kw
Typical Radiation Angle:   5 Degrees at 10m in height, lowering to 3 degree at 15m
Element Clamps:   Stauff (Germany) – heavy duty polyamide series
Antenna Weight:   4.5kg
Antenna Height:   2.75m
Bandwidth:   7.25MHz (2:1 SWR) Covers the whole band at 1.1:1 SWR or better
Hardware:   304/316 stainless steel

*If available