Aerials and masts

[Refugee]

I have had a look for aerials and masts. There is no thread about spotting them here.

This one looked like a giant yagi but I am having second thoughts

   

It may well be on a rotater as the Google photo shows it side on and my photo 3 years later rotated slightly north with a few trailing cables in the foreground strung between the buildings.
I avoided the sign and pointed the camera up from under it, Google did not do this.
The Google photo does show a medium sized van to give a bit more idea of the size of it.

[Radio Fixer]

Not that hot on aerials, know the basics of course and can recognize some. Guess you know more if you have been or are in the ham world.

I always study them when out and about and try to guess purpose ... still see really old 405 line TV aerials and later 625 line ones. I have two on my stack that I may get removed as I'm now on cable ... but should I keep them in case the cable ever gets cut thorugh?

Gary

[pwdrive]

Possibly a trapped multiband.

Lawrence.

[pwdrive]

3 of the elements appear to be trapped and 1 element doesn't appear to be.

Lawrence.

[Radio Fixer]

What does "trapped" mean?

ta Gary

[pwdrive]

What does "trapped" mean?

ta Gary

Hi Gary, In the case of a trapped dipole: Two tuned circuits place at a point equidistant from the feed point along the dipole allowing the dipole to operate at two seperate frequencies.
The above is a very simplistic explanation.

Lawrence.

[pwdrive]

I knew I had seen that antenna before, it looks like a Cushcraft Multi (click on link below)

http://www.mydarc.de/dk4qt/basteln/atb34/atb34-beam.htm

Lawrence.

[pwdrive]

The book of words for the Cushcraft ATB 34.

http://www.qsl.net/kn5l/atb34/index.html

Mentions mounting a VHF antenna above it, maybe the vertical pole in OP's photo was for that, having said that it looks more ike a trapped vertical?
The Cushcraft ATB 34 might have been fed via a choke balun?

Lawrence.

[Yorkie]

It is a multi-band rotary beam antenna. Such antennas consist of a driven element to which the transmitted power is sent via a coax cable, a director element and a reflector element, neither of which have any connections to them. Their job in life is to make the antenna omni-directional so that most of the transmitted power goes in the direction of whichever country one wishes to be in contact with.

The driven element is a ‘trapped dipole’. At the highest frequency only the shortest part of the dipole between the traps is resonant, then on the next highest frequency the centre section and the traps are resonant, then on the lowest frequency, the whole of the dipole including the traps is resonant.

Such antennas, being directional are fitted with a rotator. You have a control panel with points of the compass marked on it. You turn the control knob to the bearing you want the antenna to point to, and it slowly rotates till it gets to that heading then stops. Basically, the rotator works on the Wheatstone Bridge principle – there is a potentiometer in the rotator which moves round till it balances with the bridge in the control panel, then cuts the power to the rotator.

Any type of trapped antenna has the dual benefit of being multi-band and shorter than would be a conventional dipole.

For about 12 years up to 1991, when I lived on the outskirts of Sheffield, I used a homebrew copy of a ‘G4MH mini-beam, which isn’t quite so obtrusive. I fitted it with a rotator, and it was quite directional. Like this one:

http://www.zx-yagi.com/mini.htm

I had excellent contacts with stations on all continents and many countires, including Indonesia, Australia, Brazil, North America, Canada, Zambia, South Africa, Russia - all over Europe – about sixty countries. I enjoyed receiving 'QSL Cards' confirming those contacts, which enabled me to gain the 'Worked All Contienents' Award'. But really, it’s such a sedentary and fogeyish armchair hobby that once the novelty had worn off, I felt that there are so many things to do in life that it was just time-wasting and making me old before my time.

I also found it truly bizarre that when amateurs are speaking on air, they commonly use Morse abbreviations which make sense in Morse (73 = best wishes, WX = weather, QTH = location, QSO = contact, etc), but to my mind sound just plain silly in speech and often use more syllables - 'the WX here today is Sunny' - 'the QTH is Hull' rather than 'I'm ln Hull' - '73' instead of 'best wishes'. I guess that some will say it's 'tradition'. Not really, just a quirky habit. They also use expressions such as 'I'll call you on the twisted pair' (phone), and they don't laugh on air, instead, they say 'Aitch Eye' (HI). Why I wonder - don't they have functional 'chuckle muscles'? ('HI' is what's sent in Morse for laughter 'cos you can't make a Morse key laugh. Well I can't). If my kids heared me talking like that, they'd laugh me off the face of the planet.

Having lost interest in going on air, I recently had a clearout of five amateur radio antennas and sold my telescopic mast too. I sold the lot to one local amateur, and recently drove past his council house and noticed that the telescopic mast and all five antennas were erected, so he must have very accommodating neighbours! One of the antennas I sold was the one at the link below, which I’d had for thirty years, having bought it at a car boot sale, refurbished it, but due to a house move never erected it and doubtless never would. The boom is 12 feet long, and the longest element – the reflector, is 26’ 6” long:

http://www.mosley-electronics.com/pdf/manuals/ta33m.pdf

Such antennas are really not suitable for an urban environment, which is why so few are seen. The one that I had on top of my telescopic mast was called a ‘Cobwebb’, so named after its inventor, Steve Webb, and looking like a cobweb or rotary clothes line. The mast could be retracted and lowered by a hand winch. It worked very well, but I hadn’t used it for years and it was something of an eyesore so it had to go. See pics of it extended and lowered.

Antenna design and construction is an aspect of the hobby with lots of scope for experimentation. Rather a pity that more amateurs don't do it - the net abounds with many excellent designs.

Pedantically postulated Yorkie, rambling right of topic again.

[AlanBeckett]

"Their job in life is to make the antenna omni-directional ......"
Not 'omni' surely as that means 'all'.

Alan