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ANTENTOP- 01- 2005, # 007

Broadband Indoor Antenna




Igor Grigorov, VA3ZNW





It was in winter 1994. In that time heavy winter winds destroyed my outdoor antennas. I could not do repairs of the antennas at cold winds and snowdrifts so I decided to try indoor antennas. I tried several antennas at the times. The article describes one of them, a very simple antenna that can work at all amateurs bands.


Figure 1 shows the design of this antenna. It is a loop of a wire that installed at the perimeter of a window with sizes 210 x 140 centimeters. The loop is loaded to 600 Ohms from one side another side of the loop is connected to a transformer 9:1.

So this transformer provides matching 600 Ohms to 75 Ohms. At the time I used a home made transceiver plus a home made broadband transistors PA that worked well to load of 75 Ohm. The antenna worked from 160 through 10 meters, however, at 160 and 80 meters the antenna worked not satisfactorily. But at 40 and at the others higher bands the antenna did not bad job.




Figure 1 Broadband indoor antenna


For the antenna I used a home made RF transformer. Figure 2 shows the schematic and design of the transformer. Winding of the transformer was made by a twisted wire, 2 twist per 1 centimeter, each wire was 0.7-mm in diameter (21 AWG), each wire was covered by Teflon. For form of the transformer I used a ferrite core from a yoke of a color 27 TV.


The numbers of turns of the transformer depend on the sizes of the yoke core and on to working frequencies. I could not manage to make a transformer that would provide job of the antenna at 160 through 10 meters. Three different transformers were used to cover the range of 160- 10 meters.

I switched needed me transformer when I changed a band. At my case, for 160- 80 meters the transformer contained 18 turns, for 40- 20 meters the transformer contained 12 turns, for 15- 10 meters the transformer contained 8 turns. The turns were evenly distributed on the ferrite ring. The transformers were placed in a box made of PC- board stuff. The box was located near antenna terminal. See photo.


Figure 3 shows the circuit for testing of a working range of the RF transformer. Output of the transformer is connected to 600 Ohm load, input to a RF- Bridge. When working range of the transformer is determinate, you can change the range in needed side. It is possible to lower frequency range of the transformer by increasing number of its turns.

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