In , "Dave VanHorn"
wrote:


So I can feed my Ground plane or quad with a balun, then its a dipole?

no, it's not. be careful, you're making my case.

for a pair of quarter-wavelength wires to act as a dipole, several things
must be true.
physical arraingement, and feed are both important. your broken dipole is
somewhere between a real dipole, and the ground plane. basically, a
monopole, with a counterpoise.

Wrong. The only requirements for a dipole are that it has max current in the
center, and has two ends with max voltage but opposite polarity (hence the
origination of the term 'DI-POLE'). The only way to do this is to locate the
ends in polar opposition, and to use a frequency on which it is resonant (any
multiple of 1/2 wavelength). As I said in another post, a dipole can be nothing
more than a single wire fed at the center with coax and a gamma match. A dipole
is therefore not necessarily a doublet, nor does it necessarily need to be fed
from a balanced line or balun. In fact, a 1/4 wave vertical groundplane antenna
is really a dipole -- the groundplane (or 'counterpoise') functions as one pole
of a two pole (dipole) antenna. It meets all the criteria: the two ends are in
polar opposition, it is resonant on a specific frequency (the length of the 1/4
wave vertical being calculated as half the length of a dipole), has max current
at the center (the base of the vertical), and max voltage on the ends (the top
of the vertical).

And BTW, a 'doublet' only means the antenna has two elements. That's all.

if you significantly bend the wires, or re-arrainge them physically, then it
is no longer a dipole.

if we remove three radials from your ground plane, /i'm assuming it's fed
with unbalanced line directly, as it should be/ and straighten out the
remaining radial relative to the driven element, then we have your broken
dipole again.

there's another class of antenna, called a bicone, that has significantly
different charachteristics, but is conceptually very close to the dipole.

That's probably why it's also called a biconical dipole.

it's got broader bandwidth, and is commonly used in part 15 testing for that
reason. the discone is another very close relation, somewhere between the
bicone, ground plane, and a feedhorn.

The discone is a non-resonant antenna that works nothing like a dipole.

this stuff does matter. when you make changes, they have effects, even if
your particular arrangement is too sloppy to notice them. when you make a
change that should have an effect, and it dosent, this is telling you that
you have other problems.

do you not see the difference between a driven, and a passive element

You think the end thats fed with the shield of the coax is passive?
Come on over and grab the end of my dipole, (fed with coax and no
balun), when I have 1Kw running to it. You will change your mind
about it being passive.

no, i won't. where did you get the idea that passive elements wouldn't have
current flowing in them. and where pray tell, is that current coming from

grab the director on a beam, and see what you get. that's a simple wire
sitting in space, with no connection to the coax at all.. is it a passive
element, absolutely.. has it got rf current flowing in it, you'd better
believe it.

your two wires fed in the middle with coax, are not a dipole.
the best name i can give it, is a monopole with counterpoise.

It's the same thing.

throw it into mininec, and see if you get the same results as a properly
constructed dipole.

By that definition, any dipole that doesn't behave like an ideal dipole can't be
a dipole, including any dipole that doesn't exist in free space. Since there is
no place on earth that is equivalent to free space, it is therefore -impossible-
to build a "properly constructed" dipole!

that's freeware, a little limited, but it can do simple antennas like
dipoles with no problems.

you've also got a lot of rf current on your shield, which is making the
shield an active part of the antenna. you didn't think that this current on
your non-driven element magically stopped at the connector, did you.. why
would it stop there.. there is one possibility, that your feedline is an odd
number of quarter wavelengths long, so that this pont is high impedance. but
that only works at particular frequencies.

That doesn't work at all. The point where the coax shield meets the antenna is a
point of low impedance, and if the coax is an odd number of wavelengths long
with the radio end RF grounded, the result is an detuned mess. And if the radio
end -isn't- RF grounded, that's what puts RF in the shack and burns your lips.
But that doesn't mean the antenna isn't a dipole -- it's just a dipole that has
been poorly implemented.

in this case, it's still not a
dipole, /half the antenna isn't driven/ but it will work better than an
identical antenna with feedline an even number of quarter wavelengths long.

You will notice very little, or more likey no difference between a
dipole fed with or without a Balun.

like i said, errors in one area can obscure results in another area.

almost any damn thing will radiate and be tuneable.
a quick look at the antenna wall in the local truck stop will tell you that.

a proper dipole is resonant, given a balanced feed,

Wrong again. A dipole is resonant with or without the feed line.

and therefore does not
put significant signal onto the coax shield. /or it's fed with ladder line,
from a balun in your tuner/

Hit the books and look up "gamma match". Look up "dipole" while you are at it.





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Thanks everybody, after I sort out all this info I will have a PHD in Dipole
Antennas


Regards,
br549


"nocents" wrote in message
om...
Sorry if I am redundant, this is the fist time on this group, I am using
Central Florida Road Runner and their NG retention is about 48 hours, so I
can't search the old posts.

I bought a RS TRC-458 Navaho base station at a garage sale this weekend
for
ten dollars and I have a twenty foot mast and I would like to build my own
antenna, to save $$$.

The pattern needs to be at least 180 degrees as I live on the coast of
Florida. I plan to run barefoot (till I can buy some power) so I need some
efficiency.

Thanks,

BR549