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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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6th Aug 2014, 3:46 pm | #1 |
Dekatron
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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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6th Aug 2014, 4:55 pm | #2 |
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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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6th Aug 2014, 5:31 pm | #3 |
Dekatron
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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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6th Aug 2014, 8:35 pm | #4 |
Nonode
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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. Les. |
6th Aug 2014, 8:41 pm | #5 |
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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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6th Aug 2014, 10:41 pm | #6 |
Octode
Join Date: Jul 2006
Location: Willand, Devon, UK.
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Re: Balun works
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. |
7th Aug 2014, 12:17 pm | #7 |
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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! |
8th Aug 2014, 12:08 am | #8 | |
Nonode
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Re: Balun works
Quote:
Cheers, |
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8th Aug 2014, 9:05 am | #9 |
Dekatron
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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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8th Aug 2014, 10:24 am | #10 |
Nonode
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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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8th Aug 2014, 10:53 am | #11 |
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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. |
8th Aug 2014, 11:41 am | #12 |
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Re: Balun works
The aerial spec terminates at 75 and the coax is 75, so should be a match already.
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8th Aug 2014, 2:42 pm | #13 |
Dekatron
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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. |
8th Aug 2014, 10:10 pm | #14 | |
Nonode
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Re: Balun works
Quote:
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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9th Aug 2014, 12:25 am | #15 | |
Nonode
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Re: Balun works
Quote:
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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9th Aug 2014, 10:16 am | #16 |
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Re: Balun works
I believe my own six element VHF/FM aerial has a balun in the termination box.
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9th Aug 2014, 5:36 pm | #17 |
Dekatron
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Re: Balun works
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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11th Aug 2014, 4:09 pm | #18 | ||
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Re: Balun works
Quote:
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12th Aug 2014, 5:06 am | #19 |
Nonode
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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, |
12th Aug 2014, 5:26 am | #20 | |
Nonode
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Re: Balun works
Quote:
Cheers, |
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