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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 8th Mar 2006, 12:48 pm   #1
YC-156
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Default Fighting VHF interference with a DIY balun.

  • Another reason why you might want to use a balun with your vintage VHF set.
A few days ago, when I finally put my latest restoration project in service by connecting it to the cable network, I quickly discovered an unforseen snag: Interference from airborne FM stations on or near the frequency I was listening to.

I live in a tall building, quite literally within sight of some of the local FM transmitter sites. A few, quick tests quickly revealed that A) five of the unwanted stations could be picked up with no antenna connected at all, and B) the rest was picked up along the outside of the 10 metre antenna cable to the wall socket.

Even though the signal on the cable network is very strong, it still didn't prevent what sounded like distortion on some of the stations due to low level interference picked up by the long 'cable antenna'.

With the cable connected directly to the radio, cable shield to radio chassis and inner conductor simply to one of the balanced VHF inputs, I could identify at least 28 different stations between 87 and 101MHz. Unfortunately there are only supposed to be 18 channels on the network within that range...

Baluns have been discussed here in the forum before, and it is probably well known that one of the advantages of using a 75:300 ohm balun at the antenna socket is that proper impedance transformation will significantly increase the signal strength from the cable network.

However the other important property of a properly executed balun is that it will largely prevent any RF currents flowing on the outside of the cable shield, say those coming from unwanted pickup of strong local stations, from reaching the radio chassis and the balanced antenna input. This property is normally mostly of use when you couple a cable to a highly directional antenna, like an UHF TV beam, since the cable currents may severely distort the beam pattern.

In my situation a balun would both increase the signals of interest, and hopefully help in reducing the unwanted ones. While preassembled baluns are available from many sources, I decided to build my own, since I had the parts, I would be sure of its specifications and it probably would take less time to make one than shopping around for a factory made unit. Since a balun is basically a coil on a toroidal powdered iron or ferrite core plus a few odds and ends, there is not much to go wrong when making one.

Here is the finished unit, using a pair of banana plugs as connectors to the radio:
The most complicated thing about it is probably to find a suitable core and figure out the number of windings. The key to the last point is to know that the inductive reactance of each winding should be around five times the cable impedance at the lowest frequency of interest. 75 x 5 = 375 ohm. At 87MHz that is equivalent to 0.69 uH.

I had some Amidon T50-12 (50-200MHz) powdered iron toroidal cores, colourcoded white/lime green. A quick look at the datasheet and I knew that 20 windings would give an inductance of 0.72 uH. The three individual copper wires are wound lightly together before threading them onto the core as a single piece of wire. The number of windings is the number of times the 'wire' passes through the hole in the center. The toroid is affixed to the glassfibre base board with another scrap of bare PCB material and a nylon M3x15 bolt and nut.

With the balun in place the number of stations found dropped to 23, equivalent to those, which are supposed to be there, plus the set of five, which manages to be heard through direct radiation into the radio. Additionally at least so far I have not noticed any intereference problems, which was the main reason for building the balun in the first place.

Best regards

Frank N.
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Old 8th Mar 2006, 1:24 pm   #2
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Default Re: Fighting VHF interference with a DIY balun.

I have often wondered whether this would help so I am seriously interested in this as I have noticed that unwanted multi-paths are coming off our cable, and the 30 year old FM tuner must again work in spite of Heathrow since the DAB tuner has already died, aged 2.

But can you explain exactly how the aerial relates to the coax so that things are unbalanced at that end too? Do you need another balun at the dipole? If not then how does the "unbalanced" state work?

Also, at these frequencies, why do you need a core at all?
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Old 8th Mar 2006, 2:36 pm   #3
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Default Re: Fighting VHF interference with a DIY balun.

Quote:
Originally Posted by GMB
But can you explain exactly how the aerial relates to the coax so that things are unbalanced at that end too? Do you need another balun at the dipole?
Yes, indeed you do, a pair in total. If you didn't have a balun at the antenna, the signal difference between the cable screen and the antenna could travel down the inside of the cable, still causing interference.

Quote:
Originally Posted by GMB
Also, at these frequencies, why do you need a core at all?
You are trying to make a wideband transformer. Here the core helps ensuring a very strong, magnetic coupling between the different windings (the trifilar winding helps too). Also you need less copper wire for a given amount of inductance, around 50% less in this case, reducing the risk of unwanted, parasitic resonances. Finally a coil wound on a magnetic toroid core is pretty much self shielding. An airwound inductor/balun would need to be externally shielded if you didn't want to run the risk of the balun itself being a possible source for picking up interference.

Hope this helps a bit.

Frank N.
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Old 1st Jul 2006, 12:52 pm   #4
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Default Re: Fighting VHF interference with a DIY balun.

Quote:
Originally Posted by YC-156
  • Another reason why you might want to use a balun with your vintage VHF set.
A few days ago, when I finally put my latest restoration project in service by connecting it to the cable network, I quickly discovered an unforseen snag: Interference from airborne FM stations on or near the frequency I was listening to.

I live in a tall building, quite literally within sight of some of the local FM transmitter sites. A few, quick tests quickly revealed that A) five of the unwanted stations could be picked up with no antenna connected at all, and B) the rest was picked up along the outside of the 10 metre antenna cable to the wall socket.

Even though the signal on the cable network is very strong, it still didn't prevent what sounded like distortion on some of the stations due to low level interference picked up by the long 'cable antenna'.

With the cable connected directly to the radio, cable shield to radio chassis and inner conductor simply to one of the balanced VHF inputs, I could identify at least 28 different stations between 87 and 101MHz. Unfortunately there are only supposed to be 18 channels on the network within that range...

Baluns have been discussed here in the forum before, and it is probably well known that one of the advantages of using a 75:300 ohm balun at the antenna socket is that proper impedance transformation will significantly increase the signal strength from the cable network.

However the other important property of a properly executed balun is that it will largely prevent any RF currents flowing on the outside of the cable shield, say those coming from unwanted pickup of strong local stations, from reaching the radio chassis and the balanced antenna input. This property is normally mostly of use when you couple a cable to a highly directional antenna, like an UHF TV beam, since the cable currents may severely distort the beam pattern.

In my situation a balun would both increase the signals of interest, and hopefully help in reducing the unwanted ones. While preassembled baluns are available from many sources, I decided to build my own, since I had the parts, I would be sure of its specifications and it probably would take less time to make one than shopping around for a factory made unit. Since a balun is basically a coil on a toroidal powdered iron or ferrite core plus a few odds and ends, there is not much to go wrong when making one.

Here is the finished unit, using a pair of banana plugs as connectors to the radio:
The most complicated thing about it is probably to find a suitable core and figure out the number of windings. The key to the last point is to know that the inductive reactance of each winding should be around five times the cable impedance at the lowest frequency of interest. 75 x 5 = 375 ohm. At 87MHz that is equivalent to 0.69 uH.

I had some Amidon T50-12 (50-200MHz) powdered iron toroidal cores, colourcoded white/lime green. A quick look at the datasheet and I knew that 20 windings would give an inductance of 0.72 uH. The three individual copper wires are wound lightly together before threading them onto the core as a single piece of wire. The number of windings is the number of times the 'wire' passes through the hole in the center. The toroid is affixed to the glassfibre base board with another scrap of bare PCB material and a nylon M3x15 bolt and nut.

With the balun in place the number of stations found dropped to 23, equivalent to those, which are supposed to be there, plus the set of five, which manages to be heard through direct radiation into the radio. Additionally at least so far I have not noticed any intereference problems, which was the main reason for building the balun in the first place.

Best regards

Frank N.
Do you mean a 2-1 balun? this gives a ratio of 2-1 or 4-1 impedance ratio.
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Old 1st Jul 2006, 1:13 pm   #5
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Default Re: Fighting VHF interference with a DIY balun.

Quote:
Originally Posted by zak
Do you mean a 2-1 balun? this gives a ratio of 2-1 or 4-1 impedance ratio.
The circuit shows a 4:1 balun (300 ohm balanced <=> 75 unbalanced (coax)).
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Old 30th Jul 2006, 8:43 pm   #6
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Default Re: Fighting VHF interference with a DIY balun.

Maybe a passive bandpass filter or trap could be fitted to the aerial input? Is the set earthed? It could be the signals are being picked up by the chassis as your in line of sight?
Only a guess though...
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Old 1st Aug 2006, 7:13 am   #7
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Default Re: Fighting VHF interference with a DIY balun.

Quote:
Originally Posted by Superhet Si
Maybe a passive bandpass filter or trap could be fitted to the aerial input?
This will not make any difference I'm afraid. The interfering signals are legal, on frequency and on the air. The balun helps sort out signals from two distinct physical sources, both on the same frequency.

Quote:
Originally Posted by Superhet Si
Is the set earthed?
No. It is fairly hard to make a proper ground connection at VHF. A tuned quarterwave radial might help a bit with signals picked up directly by the chassis. It migth also make the problem worse, and so far I haven't been annoyed to a point, where I feel like experimenting with VHF ground currents.


Quote:
Originally Posted by Superhet Si
It could be the signals are being picked up by the chassis as your in line of sight?
As mentioned above then I did test for this, and a few of them are. Not the majority, though, which is where the balun helps.

Frank N.
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