Do I need a balun for this?

[Bill2k]

If I'm only using a yagi to receive, do I need to worry about the input impedance? Using the dimensions in the pic below, could I wire this directly to 52 ohm coax, or would I need a balun? Would this effect the forward gain at all?


Thanks,
Bill

 

[kf4lne]

Since the driven element is just a dipole you should be able to feed it with 50 ohm coax without any problem. What you can do is to make a balun using a 1/2 length of coax, connect the grounds together and the center cond ot the feed and the balun to one leg of the driven element and the end of the balun to the other leg, keeping in mind that a 1/2 wave length of coax will be smaller than a 1/2 wavelength based on the coax velocity factor, if the coax has a 66% vel. then you will need a coax balun that is 66% the length of 1/2 wave. But, its easier to feed it with the 50 ohm coax and you will get about the same results.

 

[Al42]

Dan, the feed impedance of the antenna is 15j0 and you're suggesting a 4:1 balun, making this a 3.75 ohm antenna fed by 50 ohm cable. That's turning a bad situation into a worse one.

My suggestion would be to use an antenna with a reasonable feed impedance. While this situation isn't as bad as using a 15 ohm antenna at the end of an unknown length of cable for transmitting, the mismatch, and the SWR on the cable, with the balun will be better than 10:1, resulting in about 99.9% of the signal being lost. The rubber ducky will probably do better. Even without the balun the mismatch is about 3.3:1, resulting in a pretty high SWR.

 

[Bill2k]

I'm only using this for reception. Does the swr matter that much for reception? If it helps anyone, I'll only be using about 10' or so of coax.

 

[kf4lne]

Al42: Look at the frequency in question, it is in the middle of a band where nobody who is going to be making an antenna and asking about a balun will be using it to transmit, so its kind of obvious that this antenna is for receiving. Yes, I am aware of the impedence mismatches already present and what using a balun will cause, but its a receive antenna so just connecting the shield to one side of the driven and the center to the other side should work sufficently for receive. Last time I looked because of the ranges of frequencies involved a typical scanner antenna impedence was from just a few ohms to several thousand ohms depending on frequency. Bill2k, your antenna will work for what you want it to without a balun, I have a 860MHz yagi I built that is somewhere around 4 ohms but works just fine, however its just a 3 element yagi. Bill2k, the 10 feet of coax will be helping you, at that frequency range coax loss becomes an issue with even short runs of coax so the shorter the coax the more signal makes it to the scanner.

 

[Bill2k]

Thanks for the help