"Insert66" wrote in message
...
I have a few questions for the gurus

when we say 468 \ mhz = feet
and 234 / mhz = feet

The 234 / frequency formula gives you the quarter-wave length. A dipole
consists of two quarter-wave length elements, one tied to the antenna input
and the other to the antenna ground. The total length, therefore, is twice
that number (also given by the 468 / frequency formula)

would a full wave dipole be better?

No, because the feed point impedance would not be a good match.
The half-wave dipole is what should be used for best results. To get
better performance, beam antennas are made using a half-wave dipole along
with properly placed reflector and director elements giving the antenna
directionality and gain.

what if the entire antenna was twice full wave long?
would i get better reception?

I'm not sure if you're using a dipole configuration, but there are other
antenna types
based on full wave elements that would work better for certain
applications...that's a bit of overkill for your application however. On
the other hand,
at this wavelength, it's not too hard to experiment...all you need is wire.

i am only interested in receiving, and only intertested
at this point, in 108 to 136 mhz AM. I am surrounded by buildings
and a transmission towers of every type, in every direction

Then altitude is your friend. Another possibility is to use a beam antenna
(Yagi, for example) if you're monitoring a particular station for more gain
at the cost of directionality. Otherwise, an omnidirectional antenna with
no gain placed as high as possible should suffice. Since you're looking at
a fairly narrow range of frequencies, it will be easier to optimize (tune)
the antenna.

the question of matching the antenna for he frequency you are after
and the idea of matching the antenna ohms to the ohms of the wire
used to feed it

Ideally, your antenna should present a 50-ohm impedance at the receiver
antenna input. For receive only, matching is less critical. You can use a
75-ohm impedance about as successfully. If I recall correctly, a dipole has
an intrinsic impedance of around 72 ohms at resonance; that is, at the
frequency it is cut for. That should be fine for your purposes. If the run
to your antenna is long, you should use a quality 50-ohm coax between the
radio and the dipole. For best results, the dipole can be tuned using a
balun or a gamma match (among other things...you'll have to Google these
things since my theory is a bit weak in that area), but for your
application, I would say that matching is not critical as long as the
antenna is cut and built properly.

after thinking about the idea that the lower ground
element and the top pointing driven element should be equal in
impedance, and that the lead should be the same, if one made a dipole
out of coaxial, and split the cable 23 inches from the end, and
dangled the mesh down and the center wirte up, the splitting
of the coaxial would change the ohms of the driven element and the
dangling mesh gound element

Yes. That works, too, and is super easy to make. You can us a single piece
of coax, then extend the braid opposite the direction of the center
conductor to form a dipole. The impedance is determined by the combination
of the driven element and the ground; thinking of them as having separate
impedances is not really correct.

- Doug

thanks for the advice

I have had the most sucess with the dipole
made of 75 ohm coaxial, and the run made of 75 ohm as well
though the radio specifications seem to suggest 50 ohm
Should i bother trying with 50 ohm, or as you say the dipole
is 75 ohm on its own?

I made the start of a yagi, more like a directional boom
just a reflector and a director, it almost beat my coaxial
dipole, maybe more directors will add signal and reduce hiss?

as i have said im surrounded by buildings, the directional seems
to get the best signal when pointed 90 degrees off the direction
of the signal, I assume the building have soemthing to do with this,
and my head seems to improive the signal?
can anything be learned from that? other than just keep you
head near the antenna and all will be well? heh

its like a Mr Bean episode

Im only into 108 to 136 AM and am not interested in wide band ..yet..
nor can i run a line to the roof, so if there are any interesting
antennas i can experiment wtih, Im all about it also the source of
signal is largely all in the same direction away form me

--------------------------------

1 the yagi elements are 1/4 wave dipole adding to 1/2 wave in total?

2 with the reflector 10% larger and the director 10% smaller?

3 are there ideal dimensions, and idear number od directors
i should start off with? - is there a formula to use or should i
assume trial and error?

4 and just to confirm is is worth driving this yagi with 50 ohm wire?

5 thin element or thick?

6 is grounding this stuff to a water pipe the way to go?

7 its hard to see from diagrams available, but are the
top and bottom elements of the directors and reflectors
one piece, or are they isolated? are the bottom elements
all connected to one another, or are they isolated,
and the boom itself should be non conducting?

lots of questions






On Mon, 3 Jan 2005 08:17:25 -0500, "DougSlug"
wrote:

"Insert66" wrote in message
.. .
I have a few questions for the gurus

when we say 468 \ mhz = feet
and 234 / mhz = feet

The 234 / frequency formula gives you the quarter-wave length. A dipole
consists of two quarter-wave length elements, one tied to the antenna input
and the other to the antenna ground. The total length, therefore, is twice
that number (also given by the 468 / frequency formula)

would a full wave dipole be better?

No, because the feed point impedance would not be a good match.
The half-wave dipole is what should be used for best results. To get
better performance, beam antennas are made using a half-wave dipole along
with properly placed reflector and director elements giving the antenna
directionality and gain.

what if the entire antenna was twice full wave long?
would i get better reception?

I'm not sure if you're using a dipole configuration, but there are other
antenna types
based on full wave elements that would work better for certain
applications...that's a bit of overkill for your application however. On
the other hand,
at this wavelength, it's not too hard to experiment...all you need is wire.

i am only interested in receiving, and only intertested
at this point, in 108 to 136 mhz AM. I am surrounded by buildings
and a transmission towers of every type, in every direction

Then altitude is your friend. Another possibility is to use a beam antenna
(Yagi, for example) if you're monitoring a particular station for more gain
at the cost of directionality. Otherwise, an omnidirectional antenna with
no gain placed as high as possible should suffice. Since you're looking at
a fairly narrow range of frequencies, it will be easier to optimize (tune)
the antenna.

the question of matching the antenna for he frequency you are after
and the idea of matching the antenna ohms to the ohms of the wire
used to feed it

Ideally, your antenna should present a 50-ohm impedance at the receiver
antenna input. For receive only, matching is less critical. You can use a
75-ohm impedance about as successfully. If I recall correctly, a dipole has
an intrinsic impedance of around 72 ohms at resonance; that is, at the
frequency it is cut for. That should be fine for your purposes. If the run
to your antenna is long, you should use a quality 50-ohm coax between the
radio and the dipole. For best results, the dipole can be tuned using a
balun or a gamma match (among other things...you'll have to Google these
things since my theory is a bit weak in that area), but for your
application, I would say that matching is not critical as long as the
antenna is cut and built properly.

after thinking about the idea that the lower ground
element and the top pointing driven element should be equal in
impedance, and that the lead should be the same, if one made a dipole
out of coaxial, and split the cable 23 inches from the end, and
dangled the mesh down and the center wirte up, the splitting
of the coaxial would change the ohms of the driven element and the
dangling mesh gound element

Yes. That works, too, and is super easy to make. You can us a single piece
of coax, then extend the braid opposite the direction of the center
conductor to form a dipole. The impedance is determined by the combination
of the driven element and the ground; thinking of them as having separate
impedances is not really correct.

- Doug