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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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25th Oct 2014, 9:29 am | #1 |
Dekatron
Join Date: May 2006
Location: Invercargill, New Zealand
Posts: 3,458
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FM aerial
This probably won't seem much to many here (especially the amateur radio guys) but I'm quite pleased with how it's looking so far!
Our analogue switch off was last April, and with the switch to digital our VHF aerial (for NZ channels 1, 3, 5, 7 and 11) became obsolete, as all digital TV is UHF only. I took the aerial down in May, and since then have been intending to modify it for band II FM. Reception of some stations in the house can be a little scratchy, and I've got enough RG6 around the place for FM feeds where required. A technician friend pointed me at this article, written by an old colleague of his when he was working at BCL (now Kordia). His aerial was in far better condition than mine - which after 11 years in an exposed position no far from the coast was missing half of one of the reflectors and had plenty of oxide around the joints. With a little effort I dismantled it completely, and found that some of the band III components were in pretty bad condition. Fortunately the band I bits (other than that reflector) weren't too bad. The article mentions DAB, but really it's a no-starter here. There were trials in Wellington by Kordia (and it sounds like they're still going), but other trials were of HD Radio which is favoured by commercial broadcasters. There's no real push to any kind of digital radio, so I'm not planning for it. So, to date I have cut down the folded dipole, director and reflector and mounted them on the original boom, which has been cut down to size. The original balun seems ok - I'd be interested in feedback here on any possible tests for it. The F connector seems to have kept the water out, even though the boot was perished and the coax has been out in the weather unterminated for over a year, the insulator and female portion of the connector on the balun appear in pristine condition. If the weather is fine tomorrow I'll see if I can track down a new U bolt and mount it on a spare barge mount I have lying around. I'm mounting it vertically, but will mount it from the end of the boom so hopefully it won't affect performance too much. I've attached three pics - the aerial in use and undamaged, in "storage" against the workshop (the bent reflector was from when I threw it down from the roof!), and the band II modification. I know that many here have quite an attachment to the "405" VHF aerials still around the place, but they're not particularly rare here - very few have been taken down so far - and after 3 years of crystal clear 1080i on three channels and 576i on the rest, no co-channel interference from Wellington or Obelisk, no static I don't really have much nostalgia for analogue TV! |
25th Oct 2014, 10:04 am | #2 |
Octode
Join Date: Jan 2013
Location: Barnsley, South Yorkshire, UK.
Posts: 1,517
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Re: FM aerial
A neat job and it should, hopefully, work well. I've got an old Ch 10 VHF array, slightly pruned, to try & get passable DAB reception which I've yet to put up. I just think there's some satisfaction in recycling and re-using; the modern way seems to be to throw any amount of money at a job and buy new - but anyone can do that. Another point is that many new aerials have elements so thin they're nearly foil, at least the older ones do actually have some metal in them.
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25th Oct 2014, 10:17 am | #3 |
Nonode
Join Date: Apr 2012
Location: Worcestershire, UK.
Posts: 2,052
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Re: FM aerial
I have detailed notes on the dimensions of the "Fuba UKA Stereo 8" (OEM = Hans Kolbe: Bad Salzdetfurth) if anyone is interested ...
Best wishes Guy
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"What a depressingly stupid machine." [Marvin: HHGTTG] |
25th Oct 2014, 10:37 am | #4 |
Heptode
Join Date: Apr 2008
Location: Eastbourne, East Sussex, UK.
Posts: 671
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Re: FM aerial
That should work well, looks like those aerials are well made, I am in NZ at the moment and those multi band arrays are still everywhere!
It's hard to get decent BII arrays in the UK now, I was going to order an Antiference 5 ele for the loft the other day only to find they have been discontinued! Might have to make something up myself. |
25th Oct 2014, 11:07 am | #5 |
Octode
Join Date: Jan 2013
Location: Barnsley, South Yorkshire, UK.
Posts: 1,517
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Re: FM aerial
Jon, there may be one in my mate's barn; I'll try and remember to have a look next time I'm up there. PM if it's of any interest otherwise I'll forget and I'll lose this thread,
Nick |
25th Oct 2014, 1:51 pm | #6 |
Guest
Posts: n/a
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Re: FM aerial
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25th Oct 2014, 1:55 pm | #7 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
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Re: FM aerial
Theres a big difference between loft and external aerials though. Loft aerials can be built from bits of wood and wire and will last indefinitely. Externals need to be highly weather (and bird) resistant.
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25th Oct 2014, 3:19 pm | #8 |
Guest
Posts: n/a
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Re: FM aerial
The (usually) larger diameter of the tube needed for an outside antenna does increase the bandwidth. I do remember seeing a UHF (400MHz or so) folded dipole with the element diameter of 40mm or so just for this reason.
One other difference is that is much harder to fall off a loft. |
26th Oct 2014, 7:37 am | #9 |
Dekatron
Join Date: May 2006
Location: Invercargill, New Zealand
Posts: 3,458
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Re: FM aerial
Here it is, in situ. It works, and it works very well - good strong/clear signal on all the Hedgehope and Forest Hill stations, clear signal lock on Life FM from Mid Dome, a handful of Invercargill LPFM stations and - this one surprised me - Life FM and Cave FM from Gore. The site there is tucked in on the other side of a hill, so shouldn't really hit here at all. That's all on a basic Samsung Bluray home theatre system with an FM tuner as an afterthought.
Next step is to sort out distribution to FM tuners, so I'll lose a bit of signal in splitters, but I'm quite happy with how it's going. |
26th Oct 2014, 11:22 am | #10 | |
Nonode
Join Date: Apr 2012
Location: Worcestershire, UK.
Posts: 2,052
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Re: FM aerial
Quote:
arjoll, that's a very good job - congratulations on getting such a worthwhile improvement for minimal outlay! Best wishes Guy
__________________
"What a depressingly stupid machine." [Marvin: HHGTTG] |
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1st Feb 2020, 11:50 pm | #11 | |
Nonode
Join Date: Apr 2012
Location: Worcestershire, UK.
Posts: 2,052
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Re: FM aerial
Quote:
There are several gaps in the detail, most notably the precise dimensions of the plastic saddle clamps which in most cases will need to be replaced by metal equivalents. Also, the construction of the dipole connection box with 300:75 ohm balun is something that I still haven't got finished! I hope it's of interest. Best wishes Guy (PS - thanks to Bill - AC/HL - for reopening the thread for me )
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"What a depressingly stupid machine." [Marvin: HHGTTG] |
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2nd Feb 2020, 1:58 pm | #12 |
Nonode
Join Date: Apr 2012
Location: Worcestershire, UK.
Posts: 2,052
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Re: FM aerial
Some pics of the home-made dipole I built for the intended 'clone' (which is obviously a long-term project!).
The connection box is a standard electrical item and the balun came out of a Maplin 'back of radio, 300 ohm balanced-to-75 ohm unbalanced' adaptor. When substituted for the Hans Kolbe-original dipole, the overall performance of the antenna was not as good in terms of overall gain as compared by reference to 'before and after' signal levels on a meter-equipped tuner (I have no way of measuring directivity/E&H patterns). Hope this is of interest/help. Best wishes Guy |