Summer 2003 Issue

Photo A. The 902–1296 MHz feed in use on the antenna range.

Multi-Band Dish Feeds

By Kent Britain, WA5VB

Multi-band dish feeds grew out of the need for something easier to haul around for my antenna range. These dish feeds have given me 12 years of service. Every July the Central States VHF Society holds an antenna contest, which gives the members a chance to compare designs on a one-on-one basis, try out their new designs, and tweak their current antennas. Marc Thorson, WBØTEM, measures 50–432 MHz antennas and I handle the higher bands up to 24 GHz (I’m working on 47 GHz). It’s not uncommon for us to measure 125 antennas during a morning. Hauling equipment around for my eight bands is a challenge, to say the least. Being able to combine antennas is a great help—a great help for me, and a great help if you’re a contest Rover station, or if you just want to combine several bands in one dish feed.

The dish was used as the signal source during a recent North Texas Microwave Society Antenna technical party. Yes, it was right on the ground. Putting the source antenna on the ground makes the ground part of the antenna. Basically, it gets rid of ground bounce and gives me a cleaner down-range signal.

You don’t have to use these feeds on two bands, because they work just fine as a 902 MHz, 915 MHz, 1260 MHz, 1296 MHz, 2304 MHz, 3456 MHz, WeSAT, ATV, AMSAT Mode L, AMSAT Mode S, or even 802.11 dish feed.

Electrically you should recognize this design as just a multi-band dipole like one you might use on 20 and 15 meters with two reflector elements tuned for the same bands as the driven element. It is just a very small version of many HF two-band, two-element beams.

Performance has been pretty good. Built carefully to the dimensions (see Table I), the SWR is less than 2:1 on both 902 and 1296 MHz. If you have test equipment, it’s easy to get better than –20 dB return loss (less than 1.1:1 SWR).

Construction

The driven element is made of bare No. 12 copper (2-mm) wire. I don’t suggest a different diameter wire unless you have the test equipment to measure SWR on 902 and 1296 MHz. Changing the wire diameter will change the tuning. The reflector elements are made from 1/8-inch (3-mm) copper or brass hobby tubing. No. 10 and No. 12 copper wire have also been used to make the reflectors. I formed the driven elements by bending the wire around the shank of a screwdriver. You want the elements parallel and 1/4 inch (6.5 mm) apart, measured from the center lines. For the boom I used 1/8-inch hobby tubing, which is the same as the reflector elements, but the 1/8-inch square hobby tubing is easier to work with.

The reflector elements are soldered to the boom and then soldered on one side of the driven element. Semi-rigid coax such as .141 works great, but Teflon® versions of RG-58 can also be used. I haven’t had very good luck with regular RG-58; soldering it to the boom turns it into a melted goo. The Teflon® types of coax solder pretty well, however. After soldering the free end of the driven element to the center conductor of the coax, hold it in place with a drop of RTV/silicon glue, or my personal favorite is a construction adhesive such as Liquid Nails. The blob of glue does change the tuning a bit. I measured a 10-MHz shift at 2400 MHz, and this shift is already figured into the final dimensions in Table I.

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