Thread: AM Radio Antenna

Is this for a portable application? if not just string up the ongest piece of wire you can find, with insulators between two points and tap off at one end.
I have seen the type you describe which is a coil with a tuneable capacitor, not rocket science but I cannot recall the formula.
You could also try a Rhombic (sp) around the outside of your house, providing it only has four side, usually a diamond shape and big with a cap at the end and take off to your radio.
I had one of these in the roof of a house once and it was incredible.
Anyway try this for saome ideas...
http://www.mindspring.com/~loop_antenna/

Yup.. some electric fence insulators and the longest highest piece of galv fencing wire you can string up. Make sure you have a good earth connection on the radio's other terminal as well.
When I was a lad I remember Dad sending me up a tree with one end of the wire and he strung the other end off the house chimney 100 metres away. On a clear night it would pick up any AM station in NZ and a couple of Aussie ones

Ian

If you are going to use the long wire approach a couple of precautions.

Make sure that if the aerial is strained it will break rather than pull down your chimney/barge board when the tree sways in the wind or is hit by a truck etc.

Where the aerial comes into the house, run an earth to a terminal close to the aerial terminal so there is a small air gap for high voltage discharges. You can see/hear it arcing not only during thunder and lightning storms but at other times such as when the air is dry and moving. It is far better if the high voltage is conducted prior to the radio rather than in the radio.:eek:

The length of the aerial can be calculated for optimum performance using a variety of formulae based on the frequency, propogation speed and the end effects of the aerial. Basically the length can be say 1/4 of the wavelength of the radio signal or 1/8, 1/16 etc.

To calculate the wavelength, divide the speed of light by the frequency. The speed of light is 3 X 10^8 or 300,000,000.

So for ABC 3LO which transmits on 774 KHz the wavelength is 387.6m

Using ferrite rods and other tuning devices can optimise the reception on shorter aerials.

I think that's correct, it's been a little time since I studied that stuff.

Rob, for reception on AM, length is not that critical, on UHF it is a different story, and for transmitting yes.
I have always found that longer works well.
Having said that a cap tuned coil will elimintae a lot of local interference
But the original question was for a coil which hopefully I have addressed.
Back in the bad old days eg, when the Titanic went down the theory was that for transmitting the longer the antenna, the better the range.
How the theories have changed (for the better).
There was no knowledge of standing wave ratios then.

Iain,

It's not nice to SWR.

But I wasn't in your ohm when I did

That hertz.

if you SWR in my ohm you will get hertz beyond the capacitance that will make you volt a considerable farad

One night, when his charge was pretty high, Micro-Farad
decided to seek out a cute coil to let him discharge. He
picked up Milli-Amp and took her for a ride on his Megacycle.
They rode across the Wheatstone Bridge, around the Sine Waves,
and stopped in the Magnetic Field by a flowing current.
Micro-Farad, attracted by Milli-Amp's characteristic curves,
soon had her fully charged and excited her resistance to a
minimum. He laid her on the ground potential, raised her
frequency, and lowered her reluctance. He pulled out his
high voltage probe and inserted it in her socket, connecting
them in parallel and began short circuiting her resistance
shunt. Fully excited, Milli-Amp mumbled "OHM-OHM-OHM". With
his tube operating at maximum and her field vibrating with
his current flow, her shunt overheated, and Micro-Farad was
rapidly discharged and drained of every electron. They fluxed
all night trying various connections and sockets until his
magnet had a soft core and had lost all it's field strength.
Afterwards, Milli-Amp tried self-induction and damaged her
solenoids. And with his battery fully discharged, Micro-Farad
was unable to excite his field, so they spent the rest of the
night reversing polarity and blowing each other's fuses.

Have a look at Peter's website has some good stuff in it i must in the new year get my hf radio out to have a listen and see what i can hear round the world the good thing is i can talk back !!!! cheers all jules



http://www.alphalink.com.au/~parkerp/nojun98.htm

I'm late starting on this thread, earlier posters had an un farad vantage.

Thanks for the interest and links guys & gal.

I am most keen on either the portable loop antenna idea, or some sort of "array" (terminology? wire strung up) in the roof of the house.

On the farm when I was a kid we had a long wire strung between the hayshed and house which worked well, but I live in town now so don't have the room.

I have found plans before on the net and in Silicon Chip / Electronics Australia, I was more curious as to whether someone had actually built one and how it performed.


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

Mate, all BS aside it seems to me you are talking about a dipole antenna, probably half-wave. they do provide great reception but unless you want to transmit as well a much smaller antenna will suffice. have a look at the link jools oposted below, i havent looked at it but we are both amateur radio operators so i doubt she is giving you a bum steer

Jules & doug, will check it out, thanks.

Just to clarify, when I say AM I'm referring to ordinary MW broadcast reception, if that is relevant.

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

yep understand, send pmtomeor jools if u need further clarification