do I need balun for receive?

[ffhelmet]

Hi!

I made a basic dipole for listening to 2m and aircraft among other things. I have directly connected it to ~20 feet of RG58 coax, which goes to the receiver in the house. There is some slack left and I have noticed that the whole setup is very sensitive to feedline position and orientation. If I grab some feedline in each hand, and move around just right, I can get a very strong signal. If I toss the feedline on the floor, the signal is very poor. At least on the weaker stations. Stations that are very strong and nearby are not affected.

Would a current balun at the antenna help?

 

[w2xq]

Assuming it is a half-wave on each leg of the dipole, no balun should be necessary. I assume you have checked it for shorts between the center conductor and shield. The antenna probably should be hung vertically for best performance. Should you choose to move an antenna further away from the scanner, dump the RG/58 and get something lower loss. Plenty of threads on coax discussion here on RR.

 

[Rt169Radio]

Is a balun the same thing as a insulator? If not,what is the difference?

 

[LtDoc]

A center insulator is something that keeps the two halves of an antenna from making contact. It is usually also a convenient place for making contact between the feed line and the antenna's elements. They can be almost anything that doesn't conduct electricity.
A balun is a device for connecting a balanced antenna to an unbalanced feed line. A dipole antenna, two equal sides to it, is an example of a balanced antenna. Coaxial feed lines are a good example of an unbalanced feed line, one conductor 'inside' another so they aren't the same. A balun amounts to a transformer that handles impedances. Because it's a transformer it can have different 'ratios', it can convert one impedance to another impedance (either way, larger to smaller or smaller to larger).
So, while a balun may also incorporate the abilities of a center insulator, they are not the same things at all.
- 'Doc

 

[prcguy]

Don't you mean "a quarter-wave" on each leg of the dipole?

The OP mentioned listening to 2m and aircraft so I assume this is a VHF dipole? What is the actual size and orientation of the dipole?

If you move the coax around at the radio end and get any change at all that's a bad sign that the coax is a radiating part of the antenna and something has gone terribly wrong.
prcguy

Assuming it is a half-wave on each leg of the dipole, no balun should be necessary. I assume you have checked it for shorts between the center conductor and shield. The antenna probably should be hung vertically for best performance. Should you choose to move an antenna further away from the scanner, dump the RG/58 and get something lower loss. Plenty of threads on coax discussion here on RR.

 

[w2xq]

Yea, I misspoke when writing that late at night. At those frequencies, if one is going to use an external antenna to listen to 2m FM, a high-as-practical appropriate vertical with lower loss cable would be preferable over a 0-gain dipole.

 

[kb2vxa]

Something IS most certainly wrong, the coax is supposed to be decoupled from the antenna and when not it becomes part OF the antenna. The orientation of the feed-line should have no effect on the signal.

For 2M FM polarization is vertical and a half wave dipole is 39" end to end. The coax should be run horizontal away from it for at least a half wave so that in itself could be your problem. Then the length for aero band is 45" and a dipole isn't "broad" enough to cover them both effectively, so for optimum performance you need both and a VHF/UHF coaxial switch.

Oh you can compromise with one cut at 41" if you can abide by a slight degradation in performance.

As for the coax, because a 1/2 wave center fed dipole exhibits a 75 ohm impedance RG-6U quad shield is perfect and has reasonably low loss characteristics. Because a scanner (and likely you're using one for this) really doesn't care about impedance my dipole RG-6 setup works just fine. BTW, the coax being very popular for CATV use is cheaper than dirt at Wal Mart where I got a 500' spool for a smile and a credit card. Oh, being quad shield is an odd diameter you'll need F connectors made for it and an F female to BNC male adapter, they're cheap too.

BTW I'd love to grow aluminium in a roof garden but the landlord would kill me.

 

[N1BHH]

Good place to read up on Baluns is right here: BALUN Application & Definitions : BUXCOMM , Where Ham Radio Operators Come to Shop

 

[prcguy]

Why does everyone on RR seem to recommend RG-6 "quad shield"? It has no more or less loss than regular RG-6 single braid and the only real place for it is when tightly bundled together with other RG-6 cables carrying satellite L-band signals (950-2150MHz range) where signal leakage between cables could be a problem.

For DC-1000MHz SW/Scanner use the extra shielding doesn't buy you anything except more money and hassle installing connectors.
prcguy

Something IS most certainly wrong, the coax is supposed to be decoupled from the antenna and when not it becomes part OF the antenna. The orientation of the feed-line should have no effect on the signal.

For 2M FM polarization is vertical and a half wave dipole is 39" end to end. The coax should be run horizontal away from it for at least a half wave so that in itself could be your problem. Then the length for aero band is 45" and a dipole isn't "broad" enough to cover them both effectively, so for optimum performance you need both and a VHF/UHF coaxial switch.

Oh you can compromise with one cut at 41" if you can abide by a slight degradation in performance.

As for the coax, because a 1/2 wave center fed dipole exhibits a 75 ohm impedance RG-6U quad shield is perfect and has reasonably low loss characteristics. Because a scanner (and likely you're using one for this) really doesn't care about impedance my dipole RG-6 setup works just fine. BTW, the coax being very popular for CATV use is cheaper than dirt at Wal Mart where I got a 500' spool for a smile and a credit card. Oh, being quad shield is an odd diameter you'll need F connectors made for it and an F female to BNC male adapter, they're cheap too.

BTW I'd love to grow aluminium in a roof garden but the landlord would kill me.

 

[Rt169Radio]

A center insulator is something that keeps the two halves of an antenna from making contact. It is usually also a convenient place for making contact between the feed line and the antenna's elements. They can be almost anything that doesn't conduct electricity.
A balun is a device for connecting a balanced antenna to an unbalanced feed line. A dipole antenna, two equal sides to it, is an example of a balanced antenna. Coaxial feed lines are a good example of an unbalanced feed line, one conductor 'inside' another so they aren't the same. A balun amounts to a transformer that handles impedances. Because it's a transformer it can have different 'ratios', it can convert one impedance to another impedance (either way, larger to smaller or smaller to larger).
So, while a balun may also incorporate the abilities of a center insulator, they are not the same things at all.
- 'Doc

Okay,thanks for replying.

 

[kb2vxa]

"Why does everyone on RR seem to recommend RG-6 "quad shield"?"

Because it's like I said, cheaper than dirt and you can get it just about anywhere. I got mine at China Mart because the store is local and it costs too much to ship a heavy spool.

"For DC-1000MHz SW/Scanner use the extra shielding doesn't buy you anything except more money and hassle installing connectors."

The extra cost is nominal and I have no trouble at all with the connectors. The crimping die for for standard F connectors works just fine. Oh, a stripping tool helps, a knife and pliers tend to make a mess of things. (;->)

I should add that I used a chassis mount female F for the center insulator, that eliminates factoring in lead length when measuring wire for the antenna.

 

[majoco]

Yes, you do need a balun. A simple quarter-wave dipole is balanced, coax cable is not. Your problem now is that the outer of the coaxial cable is acting like part of the antenna, adding or subtracting from the signal as you move it around.

Go to your local friendly hardware shop and buy a 75ohm TV antenna balun and fit it up on your antenna. Cheap types have screw connections for the cable and a couple of mounting lugs for the antenna elements. Reckon on $5!

 

[prcguy]

Do you mean a 1/2 wave dipole? If so adding a 75 to 300ohm TV balun would create more problems than not because of the impedance ratio.
prcguy

Yes, you do need a balun. A simple quarter-wave dipole is balanced, coax cable is not. Your problem now is that the outer of the coaxial cable is acting like part of the antenna, adding or subtracting from the signal as you move it around.

Go to your local friendly hardware shop and buy a 75ohm TV antenna balun and fit it up on your antenna. Cheap types have screw connections for the cable and a couple of mounting lugs for the antenna elements. Reckon on $5!

 

[prcguy]

Crimping die? I thought most of the world moved on to compression type F connectors and precision strip tools. I think I saw an old hex pattern crimper in a museum recently though.

If you don't mind the extra cost and a little more hassle if using compression connectors there is nothing wrong with quad shield, I just see some people touting it as lower loss than regular RG-6 which it is not.
prcguy

"Why does everyone on RR seem to recommend RG-6 "quad shield"?"

Because it's like I said, cheaper than dirt and you can get it just about anywhere. I got mine at China Mart because the store is local and it costs too much to ship a heavy spool.

"For DC-1000MHz SW/Scanner use the extra shielding doesn't buy you anything except more money and hassle installing connectors."

The extra cost is nominal and I have no trouble at all with the connectors. The crimping die for for standard F connectors works just fine. Oh, a stripping tool helps, a knife and pliers tend to make a mess of things. (;->)

I should add that I used a chassis mount female F for the center insulator, that eliminates factoring in lead length when measuring wire for the antenna.

 

[majoco]

A simple quarter wave dipole - a quarter wave each side - not folded - is strictly speaking 72 ohms. That's why I said a 75ohm balun.

 

[prcguy]

That would generally be called a half wave dipole which is a quarter wavelength per side.

In the US the common TV balun is 75 to 300ohms and I can't say I've ever seen a consumer type 1:1 or 75ohm unbalanced to 75ohm balanced balun.

I have some that are for commercial/lab use but their not available at the local hardware shop in the US.

If I'm ever make it way down south to your area I'll need to check out your hardware stores.
prcguy

A simple quarter wave dipole - a quarter wave each side - not folded - is strictly speaking 72 ohms. That's why I said a 75ohm balun.

 

[ffhelmet]

Sorry for the long delay, thread.. Yes it is a vertical half wave dipole, on my roof. The rcvr is a scanner.

So I made a current balun based on replies here, and info here (VK5AJL - Why buy baluns - make your own.). I also shortened the feedline a bit. Things are much better. Moving the feedline around has almost no effect.

There are still some weak stations that I bet I could pick up if my antenna were better. So you think a plain vertical will work better?

thanks for all replies