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

Friday, February 5, 2010

Bird Watch

Working satellites was something which took shape some time around September 2009. It was chance encounter with VU2RMS from Bangalore at VU2AF home QTH at Nerul, Navi Mumbai. I had gone to test a home brewed 4 element UHF QUAD with Adolf (VU2AF). Was introduced to VU2RMS & an update on the satellite communications(also referred as bird watching) was provided by VU2RMS. The idea was planted. Then came an input by my friend VU2SGW about VU2GUR from bangalore and his expertise in the satcom. Did have couple of telephone sessions with VU2GUR to understand finer aspects of SATCOM. Received an SMS from VU2GUR about the availability of VU2SWG (SHANKAR) from bangalore at Mumbai in January 2010. Had an eye ball and understood the practical basics of communicating through satellites. Also was informed that a horizontal di-pole and slimjim should be good enough for the satcom. Homebrewed following antennas for the satcom:-

1. Cross Yagi (Arrow) [2m & 70cms]
2. Slimjim antenna for UHF (70cms)
3. Dipole mounted horizontally East west direction for UHF(70cms)
4. Egg beater for UHF using a 12 gauge bare copper wire.

Currently using 5/8 antenna for 2m.

First success on the Satellite was on VO-52 on 23rd Jan 2010 during its pass duration 21:55:04 to 22:08:04, copied VU2SWG (SHANKAR) from Bangalore and VU2GPS (PARTHSARATHY) from Chennai. The signal report was 5/9. Could not transmit ( the uplink freq was 235.259 Mhz), the antenna used was an egg beater.

The same story repeated for 24th Jan 2010.

Copied VU2GUR on VO-52 in the morning pass.

Copied VU2SWG & VU2GPS on 31st Jan 2010 on VO-52.

Copied VU2GUR on VO-52 in the morning pass. Copy report 5/9

All through the copies on down-link through my 5/8 antenna and ICOM 2200 was working wonders. The issue was uplink through my Yeasu FT-60R handy. The battery was getting drained after 3 to 4 transmissions, the antenna was egg beater for UHF.

Finally two developments or experimenting took place. Changed the antenna for UHF, installed the slimjim made of brass rods 5mm. And got a step down voltage supply of 7.5 volts dc for my Yeasu Handy made with the help of VU2AF. It worked fine and i could transmit with the input from this supply uninterrupted.

The evening of 5th Feb 2010 was a great moment in my journey of HAM radio. Made the 1st DX contact. The stations contacted were E21EJC from thialand, copied 5/9 and for more than a minute. VU2GUR, VU2ZUB.

Waiting for this weekend to see the progress and success.

Also thanks to all the HAMs from Mumbai & Bangalore who helped me in this venture by supporting and motivating all around. And some HAMs in particular for bearing with all my experimentation :-