AM Radio Antenna

[Iain]

Arrays are directional and may not suit your purpose if you want Gippsland and Melbourne.
The link I provided shows a lot of passive units (park it next to your radio but no physical connection) which are portable.
Log periodics are nice but size would be a problem and ground plane could be a massive problem (we built a short wave log periodic in NW Australia in the 60's and dumped 20 tonnes of copper pellets in concrete to create an effective ground plane, the antenna was about 80' long and rotating), put one of those up and we would all know where you live.
Actually, if you know Bill Conolly in your part of town (Scooter) he might help you.
His house is covered in antennas which is a bit of a giveaway.

[Clinton1]

It might help if you told us what sort of radio you have. If its a under $150 Dick Smith "multi band" (AM, FM, SW1,2,3) or a all singing all dancing ametuer radio.
It makes a difference, due to the $ you will be willing to put into the antenna.
All theory aside if its a cheap radio (i.e. not sensitive), then just hang shedloads of wire about the place for the antenna, and see if you can earth the radio. If you have a tile roof, lay it out in the roof space.
If its a $500 WINRadio PC card, do the same or stick a brass welding rod in the back, its all good.

Otherwise you might need to obsess for a while and shell out some $, or build some all singing antenna you need a degree (or healthy obsession) to understand.

My cheapo MW capable radio has a 12 meter roll of wire I throw out over a tree when out bush, that ties off near the radio antenna. I also have some TV antenna ribbon cable I've made a diamond shape by splitting the ribbon and joining the ends, which hangs in the roof for when I'm at home. The ribbon extends to where I can sticky tape it to the extendable antenna on the radio. Cheapo, but so is the radio. Works though.

Its all horses for courses.

[scooter]

Interesting info, thanks for the input.

Clinton, the radio I use is nothing flash, either the tuner in our hifi receiver or a National Panasonic FM/MW/SW1/SW2 "ghetto blaster", or a portable tranny with MW/FM/TV1/TV2.

I am actually only intersted in mucking around with MW broadcast reception.

Simple is key for the antenna, prepared to do a little bit of constructing though if it will yield results. Either some sort of large loop in the roof space or a smaller one that wouldn't be too out of place near the actual radio would be the go I reckon.

Some useful & reasonably straightforward loop plans in one of the links supplied earlier sound promising, the use of multiwire ribbon cable looks good as that was fiddly with the plan I tried years ago, getting the wires spaced without touching.

Clinton, Interested in your mention of the split twin lead idea, connecting to the extendable antenna though - does that mean you use it for FM reception or SW, or does it make a difference to MW. Is it something like the rhombic idea Iain mentioned earlier?


Cheers...............Sean

[Iain]

The flat ribbon is 300 ohm antenna 'wire' used prior to coax, a pain to work with on TV but good for radio.
It has become available again after being off the market for a number of years, I think that Get Smart Hi Fi in Pakenham have it, it would retail for about $25 a 100 metre roll.
Probably those tunable coils would be the go for you, make it from hardwood and polish it, it can be almost attractive.

[Clinton1]

I just use it for MW/AM.

Your MW/AM antenna will use a coil around a ferrite rod as the antenna. I just use a folded Dipole from the ribbon (any convenient length, the largest that I can hang out of the way (mine is around 2 m end to end), and use this to simply get "a bit more juice going to the antenna". Yeah, lots of involved physics taken into consideration. Having the antenna hanging vertical helps.

There is a pic here: http://www.electronics-tutorials.com...nna-basics.htm

[Iain]

Having the antenna hanging vertical helps.

That would depend upon whether it is a horizontal or vertical transmission, I'm not even sure what is used in Melb.
TV is all horizontal (here anyway) although there are a few little pockets in the Dandenongs where vertical is used.
Actually I have no idea where the AM transmaitters are:confused:

[Clinton1]

I think Scooter is after MW stations, I'm AssuM(E)ing he is after ABC 774 Melbourne. Far as I know thats a 100 and something meter AM vertical transmitt antenna.
Could be wrong though, I often am, although I usually refuse to admit it.

[Iain]

I was not questioning your responses, merely raising a few points as I am more conversant with TV, digital rather than radio.
I have seen a lot of country stations suspend a tuned wire between two pylons...horizontal, I'm sure that there is no room for that in Melb as cow paddocks are somewhat lacking in their availability .
I would imagine that they come off a stick up on Mt Dandenong, but as I said, I have no idea.
In the back of a radio the ferrite rod is horizontal which would raise the polarity question, it is also possible that AM just doesn''t care.
Having said that I have seen some tuners with an external ferrite rod, an older model Pioneer comes to mind.
I know that with UHF TV if you are very close to a transmitter you can change polarity on the antenna and knock off about 10db, rather than use an attenuator if you don't happen to have one handy (of course I have quite a few, but I'm a bit different to most).
Now the big question is, where are the transmitters.

[Clinton1]

Iain, its all sweet, I was just pointing out that as I was making assumptions, I could be trying to answer the wrong question. i.e. I could be spouting irrelevant rubbish!

I just typed about 3000 words trying to answer your points, and we don't want that. I have to assume (that word again) that you do or don't know what fundamentals and so on: so answering is difficult.

Here goes (again).
You know that the Radio Frequency portion of the ElectoMagnetic Spectrum is made up of an Electric and Magnetic wave? When a wire is made to carry a Current, a Magnetic field forms around the wire. This is the Right Hand rule. Grasp the wire with your right hand, with the thumb pointng in the direction the current is going. Wrap your fingers around the wire and they "represent" the magnetic field.
Well a radio wave is electro-magnetic energy, having a electric field and a magnetic field.
Your wire or rod-style (TV) antennas have to be 'held' up so that the wire in in line with the electric field. Polarity. Vertical and Horizontal polarity means the antenna has to be vertical or horizontal to be in line with the radio wave, in order for the material of the antenna to be exposed to the most amount of the Electrical field part of the Electrical and Magnetic field of the RF wave.
Bluk.
Can I use a mental image to explain this?
I usually explain this using two broom sticks, you be the radio wave, vertically polarised. I'll be the vertically polarised antenna. See, when the sticks touch, they have lots of surface area touching. Now I'll put the broomstick horizontal. See the mental image? Easier with a mental picture.

Sorry if I'm getting confusing.

The ferrite rod antennas are at right angles to the radio wave, if you take the rod as the 'reference point'. Consider it from the wire windings, if the rod is horizontal, then the windings will be vertical, and in line with the vertically polarised radio wave. ?? see it? Pretend to grab the rod with the right hand, curl the fingers - the fingers are 'vertical' although they are wound around the wire, right?
Yes, AM cares about polarity. Tune your radio to a weak AM station, tilt your trannie vertical, and hear the station face out.

The ferrite rod is a part of the circuit of the antenna as well, but I'll leave that as it will confuse me.

[Termite]

Why did I read this.... I was lost after the word radio.

[Clinton1]

The horizontal wire antennas you saw could be one of several different types. If they were for HF radios, then they could have been a HF Dipole, or a Long Wire, amongst others.

As I have assumed that scooter wants to listen to ABC 774, or 774 kHz (a Medium Frequency transmitter (referred to as Medium Wave or Short Wave) - then the radio that transmitts this will be connected to a Marconi style antenna. This will be around 1/4 of a wavelength, or 96 meters of wire (actually a tower) standing vertically. This will be surrounded by lengths of wire 96 meters long that are laid out on the ground radiating out from the base of the mast in all different directions. This is your Marconi or 1/4 wavelength ground plane antenna. Actually the antenna will be a longer or shorter tower, and will be for all the AM stations, but thats what engineers are for.
The ground plane is the radiating wires, but I'll assume you know all this from the post you made about the 20 tonnes of copper pellets for the log periodic.

Long post hey?

Anyway the ABC transmitter towers are those 100 meter masts that are in Templestowe, near Greensborough. I think more are around Frankston?

Mount Dandy has the FM antennas (VHF) and the TV and mobile phone and telecom trunks (the drum and small dish antenna's) on it, from what I thought when I had a look at the antennas hanging off the tower.

Anyway, last point. MF (AM stations) can also produce a skywave signal, which is most often done accidentally at night. The best ones for this are the ones out bush that have to transmitt at very much higher power to overcome the poor conductivity of the soil. The costal stations don't use as much juice, as generally the conductivity of the soil is bette, and wetter.
So if only 1% of the groundwave energy is accidentally lost as skywave, and the transmitter is at 50kW instead of 10kW, its a dramatic increase. If ionospheric conditions are right, then the stations can be picked up all over the world on your little AM trannie, even more so if you hook up some extra wire to the antenna and throw it over a tree branch.

Sorry for the enormous post, which I intended to be a lot smaller.

Your stations that had a tuned wire antenna could have been transmitters for ABC Radio Australia Short Wave http://www.abc.net.au/ra/pdf/frequency.pdf or if for houses, then for receiving on a SW or HF radio. 9.710 MHz is a good one for Southern Vic, comes off the side of the Mackay transmitter. That antenna is a Dipole that has 10 kWatts pumped into it, and points to North America. Vic gets the 'wasted energy' that leaks off the side.

[Iain]

Thankyou Clinton, I understood all that, I am an ex Rad Tech RAAF, albeit a long timeago and we never had much exposure (me anyway) to MW, I spent a little time working with radar and a lesser amount of time on long wave in the NW, the Omega project, the pellets were to launch the RF to the ocean which was not far away but still dry close by.
Of course thats all gone now, and I think the copper is still there if you can be bothered trying to get it out of the concrete.
My final exposure to this was in 1971 when I got out of the RAAF and pursued a different path, I often thought about ham radio but failed to appreciate sitting down talking all night to someone unknown.
Now I know where the towers are for MW in Melb, never knew there were any near Greensborough.

[scooter]

By hell, lads, the threads gone full tech..

Will pursue the tunable coil idea methinks when time permits.

Just a comment on the polarity discussion, I seem to recall reading somewhere about some tunable coil designs incorporating a "alt - azimuth" (hope I got that right?) adjustment, ie. some means to easily tilt the coil, which the idea was (I think) to give better nulling of local interference or strong signals.

Aw, bugger, I've played into your tech trap now....

And I wanted to keep this simple.


Cheers........................Sean, simpleton

[Harry72]

Whats AM radio?

[Iain]

AM=Absolutely Meaningless
FM=F******g Marvellous
CB=Childrens Band

I remember in the UK LW was all the go, MW was unheard of commercially, never really happened here.