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Old 29th Nov 2021, 6:44 pm   #1
stevehertz
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Default Long wire AM aerial - directional?

Regarding a long wire AM aerial for MW and LW reception, is it directional? If so in what way, eg is it better for it to be at 90 degrees to the transmitter? or what? Please keep it simple, I don't need a tutorial on long wire theory etc!

Thanks.
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Old 29th Nov 2021, 6:47 pm   #2
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Default Re: Long wire AM aerial - directional?

I'm no expert, but I think they are only marginally directional.
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Old 29th Nov 2021, 7:01 pm   #3
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Default Re: Long wire AM aerial - directional?

A theoretical radiation pattern for a dipole would be a figure of eight, major lobes at right angles to the wire.

For a long wire, I would think that the major lobe would be away from the end of the wire.
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Old 29th Nov 2021, 7:05 pm   #4
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Default Re: Long wire AM aerial - directional?

To be significantly-directional it needs to be a significant number of half-wavelengths long.

Most domestic-gardens do not have the space for "long wire" antennas. Google "Beveridge" and "Rhombic" antennas to see what the broadcast-guys used in times-past for significantly-directional MW/LW antennas.
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Old 29th Nov 2021, 7:17 pm   #5
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Default Re: Long wire AM aerial - directional?

But if it is long enough to be directional, then which direction - length of the wire "pointing to" the transmitter. or "broadside-on"? I've always assumed the former.

Mike
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Old 29th Nov 2021, 7:23 pm   #6
stevehertz
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Default Re: Long wire AM aerial - directional?

Let me put it another way. I am 'here', and Sutton Coldfield transmitter is 'there'. Ideally, should my long wire aerial be in a straight line stretched out between us, or 'across' the signal path, at right angles to it?
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Old 29th Nov 2021, 7:26 pm   #7
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Default Re: Long wire AM aerial - directional?

Far-field electromagnetic waves tend to have the electric field at right angles to the direction of propagation. So if you're using the electric field to drive AC in the wire then presumably you want the wire broadside on to the direction between you and the transmitter. Conversely if the wire happens, unluckily, to be pointing directly towards the transmitter won't it struggle to have any current at all induced in it ?

Cheers,

GJ
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Old 29th Nov 2021, 7:46 pm   #8
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Default Re: Long wire AM aerial - directional?

So far as I know it's the vertical part of the antenna that does the picking up because the broadcast transmissions on MW and LW are vertically polarized (E field) The horizontal part increases the capacitance of the antenna and increases its effective height, which means more Volts (or part thereof) per Metre.

Lawrence.
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Old 29th Nov 2021, 7:52 pm   #9
Ian - G4JQT
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Default Re: Long wire AM aerial - directional?

Any random wire - be it in a straight line or not - is very unlikely to be significantly directional on MW or LW, unless it's screened by buildings or very dense trees. (The exception being a Beverage antenna at multiple wavelengths long and a termination!)

Just get it as high as you can. If possible you're better off trying to locate as much as possible away from local interference. Walking about with a portable radio will be useful here.

Ian
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Old 29th Nov 2021, 7:56 pm   #10
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Default Re: Long wire AM aerial - directional?

I don’t think Steve is talking about a tuned broadcast grade aerial, just a random length of wire, in time honoured 1930s fashion.
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Old 29th Nov 2021, 7:58 pm   #11
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Default Re: Long wire AM aerial - directional?

Quote:
Originally Posted by Ian - G4JQT View Post
Any random wire - be it in a straight line or not - is very unlikely to be significantly directional on MW or LW, unless it's screened by buildings or very dense trees. (The exception being a Beverage antenna at multiple wavelengths long and a termination!)

Just get it as high as you can. If possible you're better off trying to locate as much as possible away from local interference. Walking about with a portable radio will be useful here.

Ian
Thanks Ian, understood, appreciated.
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Old 29th Nov 2021, 9:10 pm   #12
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Default Re: Long wire AM aerial - directional?

If a long wire is longer and longer in terms of wavelengths, you get more and more lobes and more and more nulls. The positions become more influenced by nearby obstacles, ground conductivity and the rate of crime.

Terminated long wires can be quite different to unterminated ones.

David
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Old 29th Nov 2021, 10:33 pm   #13
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Default Re: Long wire AM aerial - directional?

In my experience these days in my suburban location, the key task for an AM aerial is little to do with maximising wanted broadcast signal and practically everything to do with minimising local interference signals from switched mode power supplies, LED lighting etc. A long wire is of little help because, as has been pointed out, it’s typically short compared with a wavelength of the signal and hence pretty well omnidirectional. It’s probably beneficial to erect it as far as possible from housing and mains wiring and spend time experimenting, but unless you’re in an isolated location, results are likely to be disappointing.

I must confess that in my own location, I’ve practically abandoned long wire aerials and prefer a loop aerial which has two advantages:

1. It receives the magnetic component of the electromagnetic signal, which has a much lower level of nearby electrical interference than the electric component.

2. It’s directional and can be rotated to reject interference and unwanted nearby strong stations.

I use a Wellbrook loop, but others are available and you can of course also build your own. Search the forum for more information on this.

I know that this is a very indirect answer to your original question, but you’re asking about aerial directionality and a loop is the only way that you’re going to get it.

Martin
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Old 30th Nov 2021, 3:15 am   #14
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Default Re: Long wire AM aerial - directional?

Quote:
Originally Posted by Hartley118 View Post
I must confess that in my own location, Iíve practically abandoned long wire aerials and prefer a loop aerial which has two advantages:

1. It receives the magnetic component of the electromagnetic signal, which has a much lower level of nearby electrical interference than the electric component.

2. Itís directional and can be rotated to reject interference and unwanted nearby strong stations.

I use a Wellbrook loop, but others are available and you can of course also build your own. Search the forum for more information on this.

Martin
Yes indeed, to all those comments. You can buy a ready-made PCB to build a Wellbrook clone (i.e. the "Wellgood Loop") and I would recommend that (much, much cheaper)! Various threads on the forum on the subject.

B
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Old 30th Nov 2021, 9:33 am   #15
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Default Re: Long wire AM aerial - directional?

My dad had a 1930's type long wire aerial at various houses we had. It was about 60 ft in length and about 20 ft above the ground. With a Marconi 564 and an Hmv 650 also, we seemed to receive stuff from all over the uk and the world.
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Old 30th Nov 2021, 11:19 am   #16
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Default Re: Long wire AM aerial - directional?

A directional, long-wire receiving antenna for LF and MF would be the Beverage.

It consists of a horizontal wire, one-half to several wavelengths long, 10 to 20 feet above the ground and pointing in the direction of the station.

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At the far end end, it is terminated to ground using a 450 Ω resistor to match the characteristic impedance of the antenna.

At the other end it is connected to the receiver using a transmission line and a a 9:1 transformer to match the antenna to line.

Nandu.
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Old 30th Nov 2021, 1:34 pm   #17
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Default Re: Long wire AM aerial - directional?

Quote:
Originally Posted by rontech View Post
My dad had a 1930's type long wire aerial at various houses we had. It was about 60 ft in length and about 20 ft above the ground. With a Marconi 564 and an Hmv 650 also, we seemed to receive stuff from all over the uk and the world.
The problem now is electronic noise from numerous sources; in many locations it will be S8 across a range of frequencies and listening through that it pretty limiting. The mag loops are relatively immune to such interference. You cannot compare conditions now with the 1960's.

B
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Old 30th Nov 2021, 6:50 pm   #18
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Default Re: Long wire AM aerial - directional?

Many people have mentioned about the length of an aerial and comparing it to the wavelength of the signal you are trying to receive. But it's not been pointed out exactly what wavelengths we are talking about. Probaly the most extreme is Radio 4 LW at 200kHz. The wavelength is 1.5km, so almost a mile. Even the transmitting aerial is nothing like that. Also, it shows that since the transmitting antenna is essentially a long wire (although centre fed as I understand, so technically a dipole) there isn't a significant directionality to it.
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Old 1st Dec 2021, 5:16 pm   #19
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Default Re: Long wire AM aerial - directional?

The R4LW transmit antenna - readily visible when driving along M5 - is a "T" - there's a horizontal section supported by the two masts, and a vertical section connects to the middle of the horizontal section. RF excitation is applied at the bottom of the vertical section.

The horizontal section is best viewed as a 'capacity hat' used to encourage the RF amps to flow in the vertical section.

Similar, though less-tall, T-antennas are used by many of the remanent MW broadcasters.

The old advice to get your MF/LF antenna as high as possible is not necessarily the way we do things these days: antennas optimised for NVIS - Near Vertical Incidence Skywave - can give impressive results.

See https://en.wikipedia.org/wiki/Near_v...idence_skywave

You only need a few lengths of plastic pipe stuck in the ground, and the antenna can be only a few feet above ground.
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Old 1st Dec 2021, 7:43 pm   #20
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Default Re: Long wire AM aerial - directional?

Quote:
Originally Posted by G6Tanuki View Post
The R4LW transmit antenna - readily visible when driving along M5 - is a "T" - there's a horizontal section supported by the two masts, and a vertical section connects to the middle of the horizontal section. RF excitation is applied at the bottom of the vertical section.
Sorry, had ignored the vertical bit and its significance. It's even more visible from Webbs of Wychbold (a garden centre). We have family nearby!
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