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View Full Version : Need some info to build an AM Radio Antenna.



CP/M User
April 30th, 2006, 06:04 PM
I was hopeing someone here might know something about this -
cause it's all practally theory for me at the moment. What I
want to do is build an AM Radio Antenna which is powerful
enough to pickup a Radio Signal some approx 200k away - which
would be inland.

I know people have done this with FM - cause where I'm from
they used to do this all the time using FM antenna's pluged
into their Stereos, but is it all possible to do this with AM
stations? I thought if I made something big enough & high
enough it would do it easily. The station I'm after I can only
just barely get but it's distorted with noise & I have to pump
up the volume - static isn't particular good - would a large
antenna fix this?

Also if it is possible - I just wanted to know what sort of
wire could I use to make this Antenna? Is 300ohm wire okay -
or do I need something a bit better than that? I think my
Stereo uses 300ohm stuff - so if it's something else - it
might have to be compatable.

Anyway, if anyone has thoughts, ideas or even suggestions -
anything that'll be great.

Thanks.

DimensionDude
April 30th, 2006, 06:38 PM
Oh my, there is a LOT of info on antenna theory. :)

What it comes down to, though, is that an AM antenna doesn't have to be high, just long; the waves tend to follow the Earth's curvature. Best reception is by a single straight wire horizontal to the ground and perpendicular to the direction the wave is traveling. You don't want the antenna to be "end on" or pointing toward the transmitter. The best length is one-quarter wavelength but multiples of that (one-eighth, one-sixteenth) will work, too. A good earth ground for the receiver is also necessary.

I'm sure that others will point out the things I've missed.

Oh yeah, amplified antennas are ok, but they need a fairly decent signal to work with.

One more thing, I assume that you mean the standard broadcast band (medium wave).

(edit) The antenna should be a single wire (not 300 ohm twinlead). Any size is ok, 22-gauge stranded will work fine.
Kent

CP/M User
April 30th, 2006, 07:49 PM
DimensionDude wrote:

> Oh my, there is a LOT of info on antenna theory. :)

Well it's kinda been a work in progress for a number of years!

> What it comes down to, though, is that an AM antenna
> doesn't have to be high, just long; the waves tend to
> follow the Earth's curvature. Best reception is by a
> single straight wire horizontal to the ground and
> perpendicular to the direction the wave is traveling.
> You don't want the antenna to be "end on" or pointing
> toward the transmitter. The best length is one-
> quarter wavelength but multiples of that (one-eighth,
> one-sixteenth) will work, too. A good earth ground
> for the receiver is also necessary.

I follow you on some bits - having an long antenna. Obviously
I won't know which way the reception is travelling - I just
thought about having this thing running horizontally. But then
I guess the only way to find out is to get the wire & see
which way the signal runs best (if you mean perpendicular to
the direction the wave is travelling - do you mean Horizontal
to the ground?). Transmitter - as in stereo do you mean?
Is one-quarter wavelength of a standard length - I'm just not
sure! I'm also not sure what you mean't by a good earth ground
reciever.

Just to let you know, my stereo has a 2 plug setup my current
antenna is squarish with 2 connectors which run into the
stereo (if one is for the earth then I guess that's fine),
unfortunately my knowledge is lacking a little bit with this
stuff, I guess if anyone knows of a good website which
demonstrates - I'll have a look, otherwise I'll have a look
around myself. But yeah, I didn't know about the length of the
wave thingy - so it's good you pointed that out.

> I'm sure that others will point out the things I've
> missed.

Yeah, hopefully cause I'm just not sure about this. I
certainally would like to do something about this though.

> Oh yeah, amplified antennas are ok, but they need a
> fairly decent signal to work with.

No well, I do have an amplified antenna, though I think if I
make up my own design to get a good reception - it would be
better.

> One more thing, I assume that you mean the standard
> broadcast band (medium wave).

Yeah, just want to pick up some radio stations up in the bush.
Nothing fancy.

> (edit) The antenna should be a single wire (not 300
> ohm twinlead). Any size is ok, 22-gauge stranded will
> work fine.

Okay, I can see some 22-gauge stranded wire in my book for my
local electronics store - but I don't think it's insulated, is
it possible to get this stuff insulated (I was thinking of
having it outside in the weather)?

Cheers,
CP/M User.

tgunner
May 1st, 2006, 04:21 PM
Alright, I've tried this before with Shortwave. First, what make and model are your reciever? Are their any large land masses in between you and the station? I would think that non-insulated wire would be the way to go, it should still last a good 10 years or more in even harsh enviroments. Earth Ground is basically a copper wire running from the ground connection on your reciever ( if present at all!) to a long spike inserted into the ground. The antenna (from what I understand should point like this:



Antenna: [ Looking down ]
#
| (Broadcast Wave)
|
| | ------- | ----- | --- | Station
|
|
|
#

DimensionDude
May 1st, 2006, 06:00 PM
tgunner has it right, if you could float in the air directly above your antenna, your setup should match his diagram. So, you have to know in advance where the radio station is transmitting from. Perhaps searching for "longwire antenna" will show better pictures.

Also, there are ways of forming a longwire antenna so that reception is omnidirectional (receives from all directions).

The antenna should be connected to your radio receiver at one end, the other end is free (not grounded). The cable used between the receiver and antenna should match the characteristic impedance of the receiver (usually 50 ohms).

I'll have to get back to you on wavelength, my memory ain't what it used to be :)

Kent