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Old 6th Aug 2014, 3:46 pm   #1
stevehertz
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Default Balun works

As I've just acquired a couple of vintage sets that have VHF with 300 ohm inputs, and given that my VHF downlead and disty system are the now standard 75 ohms, I bought a balun for a couple of quid off the internet, soldered on a couple of short wires with banana plugs and it works great with my Grundig 3028. Much improved reception and sound quality over the internal aerial - as you'd expect of course. Just thought you may want to see it.
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Old 6th Aug 2014, 4:55 pm   #2
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Default Re: Balun works

Given the large signal these days I don't bother with baluns, at least the continental sets used easy to get 4mm plugs. It's good to hear a vintage VHF set giving the best it can, my VHF only plastic EKCO sounds very good, no hum and no hiss either, suprising for an AC/DC set.
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Old 6th Aug 2014, 5:31 pm   #3
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Default Re: Balun works

I'm not convinced that my reception or performance is any better for using the balun, I'm just doing what is technically correct to hopefully get the best performance.
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Old 6th Aug 2014, 8:35 pm   #4
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Default Re: Balun works

I bought my first colour TV back in 1970, a 22" NordMende direct import from Germany. It just had its sound IF twiddled, otherwise still German spec., 625 line only of course.
The aerial input was for 240ohms, and I was unsure how to connect it, so I wrote to BBC technical people. They suggested a simple coaxaxial transformer, and attached a sketch. For my channels (group c) a 6-1/4" length of coax was formed into a loop. The feed wire inner was soldered to the inner of the short piece, and the braid to that of the short length. the inner of each end of the loop made the connection into the set's 240 ohm ant. socket.
In the intervening years,with both TVs and Stereo radios, I have used the type shown in the first post, the coaxial loop, and direct connection without any form of balun, and to be honest, I don't think I have ever detected any differences.
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Old 6th Aug 2014, 8:41 pm   #5
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Default Re: Balun works

I have a few odd baluns and I just experiment. I generally find providing proper matching at each end of the cable improves the signal by a couple of dB, though this obviously won't be significant in a strong signal area.
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Old 6th Aug 2014, 10:41 pm   #6
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Default Re: Balun works

Quote:
Originally Posted by MotorBikeLes View Post
a 6-1/4" length of coal was formed into a loop.


I have one here too but find just sticking the co-ax cable into the terminals works fine in practice, the signal levels aren't all that low here for it to make much of a difference. The balun fitted looks proper though.
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Old 7th Aug 2014, 12:17 pm   #7
G8HQP Dave
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Default Re: Balun works

A balun may only boost signal strength by 6dB or so. Unlikely to be significant for normal broadcast reception.

However, a balun can greatly reduce interference pickup on the coax outer. How important this is will depend on how electrically noisy is your environment. For FM reception you may notice a small reduction in background noise on a decent transmission (e.g. Radio 3) - other stations may never have long enough silences to hear!
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Old 8th Aug 2014, 12:08 am   #8
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Default Re: Balun works

Quote:
Originally Posted by G8HQP Dave View Post
However, a balun can greatly reduce interference pickup on the coax outer. How important this is will depend on how electrically noisy is your environment. For FM reception you may notice a small reduction in background noise on a decent transmission (e.g. Radio 3) - other stations may never have long enough silences to hear!
I was under the impression that a balun at the aerial itself also helped reduce noise pickup on the coax outer. In the USA, TV and FM aerials were usually supplied with a balun of the kind pictured. But from the various texts on the topic, one has the impression that in the UK it was common practice to connect coax (unbalanced) direct to nominally balanced aerials. Now that it's Proms season, absolutely the best possible reception of R3 would seem to be desirable. (And here in NZ we are lucky as RNZ Concert FM will run 66 Proms programmes this year.)

Cheers,
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Old 8th Aug 2014, 9:05 am   #9
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Default Re: Balun works

That looks a very neat - inline - unit you show a photo of there. Do you have a source/link please? Nothing wrong with the one I'm using but it is not so elegant a solution as it hangs awkwardly due to the turn through 90 degrees of the unit itself. Physically, mine is meant for connecting 300 ohm feeder to a 75 ohm set, but I'm using it the other way round, hence the issue.
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Old 8th Aug 2014, 10:24 am   #10
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Default Re: Balun works

I've been restoring a Fuba UKA "Stereo 8" (one of two that I'd used for VHF DX back in the mid-'70s). The design incororates a 300/75 ohm balun - the photos show the antenna, the original assembly and my subsequent attempt at constructing a bifilar-wound balun of the same type.

I've always been a firm believer in maximising carrier-to-noise right at the start of the receiving chain - "once it's gone, it's gone"

Best wishes
Guy
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Old 8th Aug 2014, 10:53 am   #11
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Default Re: Balun works

I would like add to what G8HQP Dave said...

There are two different things going on here. The impedance match and the balun. Matching the impedance will improve the signal, the balan aspect may improve or reduce the signal.

The balun is about determining which bits of copper constitute the aerial. It isn't just interference that might be arriving on the coax outer, but multipath problems made worse (or better).

Of course the aerial itself must also balance correctly to the coax so it may need a balun too.

With a perfectly matched and balanced aerial system then you can orient the bit that looks like the aerial and that will be all that determines the results, making it nice and simple. Without the balun the receiver sees a mix of stuff from the proper aerial and a big vertical which is the feeder - the results are hard to predict.
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Old 8th Aug 2014, 11:41 am   #12
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Default Re: Balun works

Quote:
Originally Posted by GMB View Post
I would like add to what G8HQP Dave said...
Of course the aerial itself must also balance correctly to the coax so it may need a balun too.
The aerial spec terminates at 75 and the coax is 75, so should be a match already.
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Old 8th Aug 2014, 2:42 pm   #13
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Default Re: Balun works

Yes, that's the impedance match. But it may need a balan too.
I have seen aerials that skate over this issue and hope no-one will notice.
A crude half-wave dipole will match 75 ohm nicely, but needs a balun to match coax.

At VHF you can use a ferrite clamp or just make a few loops of feeder near the aerial to be the balun.
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Old 8th Aug 2014, 10:10 pm   #14
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Default Re: Balun works

Quote:
Originally Posted by stevehertz View Post
That looks a very neat - inline - unit you show a photo of there. Do you have a source/link please? Nothing wrong with the one I'm using but it is not so elegant a solution as it hangs awkwardly due to the turn through 90 degrees of the unit itself. Physically, mine is meant for connecting 300 ohm feeder to a 75 ohm set, but I'm using it the other way round, hence the issue.
That picture was taken from a Radio Shack site: http://www.radioshack.com/product/in...ductId=2103912.

It is a fairly standard type pf product, though, and I should imagine readily available at relatively low cost. E.g., see here: http://www.supremeantennas.co.nz/vie...un-wire-in.php. I'd be surprised if it were not a stock item in the UK.

Cheers,
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Old 9th Aug 2014, 12:25 am   #15
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Default Re: Balun works

Quote:
Originally Posted by GMB View Post
Yes, that's the impedance match. But it may need a balan too.
I have seen aerials that skate over this issue and hope no-one will notice.
A crude half-wave dipole will match 75 ohm nicely, but needs a balun to match coax.
At VHF you can use a ferrite clamp or just make a few loops of feeder near the aerial to be the balun.
I wonder if 75R-to-75R receiving baluns form domestic applications are available commercially. Back in the 1950s, when 75R twin feeder was used to some extent, Belling & Lee at least used to offer them, but I should imagine that they faded out as stock items from the major aerial manufacturers. On the other hand, I think it likely that FM aerials have used folded dipoles for many, many years, in which case the (readily available) 300R-to-75R balun might be more appropriate. Professional versions are available, e.g.: http://www.nhsignal.com/pdf/products...al-purpose.pdf.

The transformer type seems simple enough to make, though, as shown in the attached excerpt from King, Practical Aerial Handbook.

Cheers,
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Old 9th Aug 2014, 10:16 am   #16
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Default Re: Balun works

I believe my own six element VHF/FM aerial has a balun in the termination box.
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Old 9th Aug 2014, 5:36 pm   #17
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Default Re: Balun works

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Originally Posted by Synchrodyne View Post
I'd be surprised if it were not a stock item in the UK
I'd done a lot of searching before buying one. But I'd not seen a physically inline one like that. But bear in mind I'm in the UK, so before I spend a fortune on the cost of shipping and import VAT from the US or NZ for a straight one, I'm kinda happy to put up with the 90 degrees bend in mine for £1.95 delivered to my door. Just wondered if anyone knew of a UK source of an inline one. Thanks anyway, at least I know they exist!
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Old 11th Aug 2014, 4:09 pm   #18
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Default Re: Balun works

Quote:
Originally Posted by Synchrodyne
I was under the impression that a balun at the aerial itself also helped reduce noise pickup on the coax outer.
Yes, if the antenna is balanced and the cable is unbalanced.

Quote:
On the other hand, I think it likely that FM aerials have used folded dipoles for many, many years, in which case the (readily available) 300R-to-75R balun might be more appropriate.
Not necessarily. The main reason Yagis use a folded dipole is to raise the impedance, as a 'bare' Yagi often has a low feedpoint impedance. The folded dipole also increases the bandwidth a little, but this is mainly set by the lengths and spacings of the other elements. A folded dipole on its own will need a 300-75 balun; a folded dipole in a Yagi probably may need a 75-75 balun (if the maker has omitted it).
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Old 12th Aug 2014, 5:06 am   #19
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Default Re: Balun works

Ideally I suppose baluns would be aerial-specific, and would provide a match between the actual aerial impedance and 75R, as well as doing the balanced-to-unbalanced transformation. In the USA it seemed to be standard practice to use a regular 300R-to-75R outdoor balun whatever the aerial. Certainly the multi-element TV and FM aerials that I used all came with such baluns. It was as if the balanced-to-unbalanced transformation was more important than impedance matching. I suspect that might have been because historically 300R twin was used for domestic aerial feeders, with 75R coax being a later arrival (1970s maybe?). So 300R twin was attached to all TV and FM aerials, including the multi-element types whose actual impedance was in some cases well below 300R. Then when 75R coax arrived, it would have seemed the natural thing to do to connect “300R” to/from it using a standard balun.

It seems to me that a balun is primarily a balanced-to/from-unbalanced transformation device, one which secondarily can also be an impedance transformation device. However, its impedance transformation properties seem to have become its dominant feature, leading to the superficial definition that it is essentially a device for transforming 300R to/from 75R. In turn this may have led to the common misperception that a balun is not needed to go from say a 75R balanced aerial to 75R coaxial cable (unbalanced) because the impedances are the same.

Cheers,
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Old 12th Aug 2014, 5:26 am   #20
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Default Re: Balun works

Quote:
Originally Posted by stevehertz View Post
I'd done a lot of searching before buying one. But I'd not seen a physically inline one like that. But bear in mind I'm in the UK, so before I spend a fortune on the cost of shipping and import VAT from the US or NZ for a straight one, I'm kinda happy to put up with the 90 degrees bend in mine for £1.95 delivered to my door. Just wondered if anyone knew of a UK source of an inline one. Thanks anyway, at least I know they exist!
It would appear that at one time Antiference used to make an in-line balun (TRR/V/SP) for use at the receiver end. It is mentioned in the FM aerial section in Hi Fi Year Book (HFYB) 1974 and then in most issues through to and including 1980. A copy of the HFYB 1980 entry is attached, and I have found a picture from another book. It might be worth checking with Antiference (if they’re still around) to see if its balun remains available. It looks as if it was a response to the prevalence of imported FM receiving equipment fitted only with 300R balanced aerial inputs, rather than with 300R and 75R options.

Cheers,
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