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Building a 4:1 Balun

For some time now, after hearing about the wonders of a LOOP antenna. I decided to build one that I could erect easily in the available space that I have here. I decided on building one for 40 Meters. Theoretically, I would also be able to use it on other bands. So I read as much of the available information I could find on LOOP antennas.

With only one central point to hang wire antennas, I decided to make a delta loop hung and fed at the top ( support point ), let the wire hang down vertically from the center and tie it off as best I could. Next decision was how to match the loop's characteristic impedance of approximately 100 ~ 120 Ohms so I could use RG58 feedline going to the shack. The conventional method is to use a quarter wave transformer made from 75 ohm coax. However, I decided to use a 4:1 balun at the feedpoint. Baluns, coax, open wire line, and other items used by Amateurs to construct antennas are hard to come by locally. So building a 4:1 balun was the best option I had.

The wire used for winding the balun ( 14 Guage ) was scavanged from an old transformer.

I wound 10 bifilar turns of the 14 guage wire on a small piece of 1 inch O.D. PVC pipe. This was then enclosed in a small piece of 1 1/2 inch O.D. PVC pipe and end caps for weatherproofing. Two stainless steel eyebolts are used to attach the antenna wires and another to support the balun.

So here is the construction of the balun in pictures.


Schematic of 4:1 Voltage Balun.

1:- Balun just finished winding coils.

2:- Second view
Balun just finished winding coils.

3:- Terminals added
Coated with sealant.

4:- Second view
Terminals added.
Coated with sealant.

5:- Balun and PVC end cap with
hole for Coax fitting.

6:- End cap with hole for coax fitting.

7:- Balun with coax fitting soldered in place.

8:- Second view of Balun with Coax fitting.

9:- End cap fitted over Balun.

10:- Another view - End cap fitted over Balun.
Note drain hole.

11:- Another view - End cap fitted over Balun.

12:- Completed Balun.

There you have it. The completed Balun.

I do not have any test equipment so I am not able to show you any emperical results. Loop was cut to size and balun attached. Antenna was then erected. I set the output of my Icom 756 Pro2 to 5 Watts as measured with my Bird 43 wattmeter into a dummy load and started to take SWR readings with the SWR meter of the IC-756.

The results are as follows: -

SWR Readings
Frequency 7.000 Mhz 7.100 Mhz 7.200  
Over 3
Frequency 10.100 Mhz 10.150 Mhz    
Over 3
Over 3
Frequency 14.000 Mhz 14.100 Mhz 14.200 Mhz 14.300 Mhz
Frequency 18.068 Mhz 18.120 Mhz 18.168 Mhz  
Frequency 21.000 Mhz 21.200 Mhz 21.300 Mhz 21.400 Mhz
Frequency 24.890 Mhz 24.930 Mhz 24.990 Mhz  
Frequency 28.100 Mhz 28.300 Mhz 28.500 Mhz 28.600 Mhz

The antenna works very well on 40 / 20 / 17 Meters with the automatic antenna tuner in the IC-756 Pro2. It does not work well on 30 meters although I was able to use it with the tuner in the radio to make about 40 QSO's. I have not tested it on 15 / 12 / 10 Meters either on receive or transmit. That will be done at a later date.

I have not as yet tried pruning the antenna to get a better SWR on 40 Meters . It will be interesting to see what happens when that is done. But for now, it is a keeper antenna especially for 40 Meters which is the band it was built to work on.

More when other tests are carried out.

Time In Grenada

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