is a balun really needed?

[kc9neq]

i built a j-pole and a slim jim antenna out of copper pipe. i made both of these antennas for railroad freq 160.500. since these antennas will be used on a scanner, is a balun really needed. i saw some post on the internet that said to wind some of the coax and hold it in place with tape or a wire tie. i have see large loop and small loop baluns made from coax. i really need some advice.

should a balun be used on an antenna that is used for a scanner?

if so, should a large loop or small loop be used?

thanks

 

[jonny290]

Not necessary - that's to stop unbalanced currents from getting on the coax when you're transmitting. It'll play nicely with no balun.

 

[Don_Burke]

I do not use them on my receiving j-poles.

If I was getting noise from the antenna I might give one a try.

 

[RevGary]

Just as a construction project, you can make one out of RG58U cable JUST TO TEST. The formula for loop lengths is in the ARRL Antenna handbook. Nice 45 minute project if you have absolutely nothing else to do. (Actually, a properly constructed balun can sometimes help lessen SOME intermod under SOME circumstances.)

 

[kb2vxa]

Hi all,

Just to close out this thread, both antennas are unbalanced feed so no balun is needed. I've been working closely with him via PM, he has bigger fish to fry and it looks like he's making some great progress.

 

[n4voxgill]

coils in coax can become chokes thus reducing signal strength. The size of coil tomake a choke depends on frequency. That's why you should not coil up excess coax in an installation like a mobile. Either cut to correct length on layer the excess. So for receive I would not use a balun and not coil up any coax.

 

[Don_Burke]

Strictly speaking, that thing should be called an "un-un" or an RF choke.

Since it seems to make little difference in many applications, what we call it is academic.

 

[N_Jay]

coils in coax can become chokes thus reducing signal strength.

No they don't!

The size of coil to make a choke depends on frequency.

First it depends on the inductance desired, and it only applies to current traveling on the shield, not the transmission of RF within the cable.

That's why you should not coil up excess coax in an installation like a mobile.

Wrong for the above reasons.

So for receive I would not use a balun and not coil up any coax.

I would use a balun for connecting a balanced antenna to unbalanced line or for connection an unbalanced antenna to balanced line. (What it is designed to do)

 

[kb2vxa]

Hi again,

I thought we'd come to the end but N4 got confused. What Gary described is a choke balun, now you need to check the Handbook he wrote about.

Say goodnight Gracie.