Saturday, February 13, 2010

SLIM JIM Antenna

Working through satellites is a field in HAM radio, which has given a new dimension to the HAMs interested in working with DX stations using their VHF/UHF set.

After few initial failed attempts to work through satellites, Gururaj (VU2GUR) from Bangalore advised me go start with separate VHF and UHF rigs and fixed antennas for both the rigs. I was already using 5/8 antenna for my VHF communications, through ICOM 2200 rig. This part of setup was ready.

I had a dual band handy YEASU FT-60R (vhf/uhf), max power 5 watt. We zeroed down on the slim jim antenna for the rig. The material for the same was lying around in my shack. Decided to use 5 mm (dia) brass rods. Slim Jim is an modified version of J pole antenna. The Slim Jim is a vertically polarized omnidirectional end-fed antenna having considerable "gain" and this is concentrated almost parallel to ground toward the horizon rather than skyward making it more efficient than a ground plane type antenna by about 50 percent better. I hung one of mine using a nylon wire from the non-conductive mast.

Due to it's SLIM design, there is very little wind loading. It is fed with 50 ohm coax (i used RG-58). It uses a 'J ' type matching stub (J Integrated Matching = JIM), hense the name SLIM JIM. Credit for the original design goes to F.C. Judd, G2BCX. Since the vertical angle of radiation is so narrow, about 8 degrees toward the horizon, it usually out performs 5/8 wave or groundplane type construction due to their much higher angle of radiation.

Construction details

It should be totally insulated from it's mount, mast, tower, etc with at least 1/4 wavelength of "freespace" distance. Formulas are provided below for all the measurements including the freespace distance.

The Slim Jim should be constructed from 5mm Brass/Copper/Aluminum rod. However precaution needs to be exercised if using a brass rod, specially during the the 90 degrees bending or the U turning of the rods (see the slimjim photo). You should bend the rods only after heating it (other wise it may break).
The antenna for UHF ( 434 to 438 Mhz) for a center frequency of 436 Mhz shall require a length of approx 3.5 ft of brass.

Best point of working with brass is ease of slodering from the tapping point. As this antenna for UHF is fairly small you also do not need to worry about the separators for the arms of the antenna. For 2 meters you need to use separators to hold the arms of the antenna steady.

Make sure you make a balun using a 6-9 inches of cpv and coiling around 9 rounds of RG 58 coax.

The Feed Point

The center of the coax is connected to the long and continuous arm of the antenna. The outer sheath is connected to the smallest arm of the slimjim ( see the photos)

Parts List 1. Brass Rod:- 3.5 ft
2. RG 58 coax - 10 ft. ( depending on you rig position)
3. PL-259 connector ( to connect it to the end of the coax feeding to the rig)
4. Fishing Gut (uv protected)- 2 feet ( to hang the antenna from the mast.

While testing the antenna along with OM VU2AF, the results were 1:1 .

Many contacts made through this antenna.

Original Design by F.C. JUDD, G2BCX
Image with description; courtesy, N4UJW

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