Hey Starman,

Thanks for the answers. Following are replies interspersed.


"starman" wrote in message
...
Dave wrote:

Okay, I'll try to keep you posted. Here's what I asked on Yahoo!

OKAY, here are the new questions and clarifications...

Thanks for clearing up for me why it is called a random wire. I
thought that was just what they called it when it wasn't 1/4 or 1/2
wavelength.

I realized something today- I think I am actually talking about
installing a HALF-WAVE dipole, not a 1/4 wave. Since it is twin-lead
I am talking about, and since it is going to be joined at the far
end, each leg would be twice the 1/4 wavelength of the physical
cable. Am I right here?

It sounds like you're thinking about making a folded dipole. The total
length of the twinlead folded dipole should still be a half wavelength
but the impedance at the connection point (middle) for the downlead
changes from about 50-ohms for a simple dipole to 300-ohms for the
folded type. This means a 50 or 75-ohm coax lead would not be suitable
without a 6:1 impedance matching transformer. The higher impedance side
of the transformer connects to the middle of the dipole and the low side
goes to the coax.


Yes, I saw a diagram of a folded dipole today in an old Amateur Radio
Handbook, and it looks like what I am talking about, more or less. I also
found out that a half-wave dipole does not need to be grounded.

Correct me if I am wrong, but you do ground one side of a dipole,
right? So one leg would be grounded along with the shield of the
coax, right?

It's not necessary to ground the coax shield from a dipole because it's
a balanced antenna design. However you may want to ground the chassis of
the receiver itself. The coax should be connnected to a balanced antenna
input on the receiver. This means the coax shield is not directly
connected (grounded) to the receiver's chassis. You should be aware that
some receiver's don't have the proper input connection for a balanced
antenna like a dipole. In that case you would just connect the coax to
whatever external antenna input there is and not be concerned whether
the coax shield is grounded to the chassis or not. It's not the ideal
way to do it, but sometimes you don't have any choice. I forgot what
receiver you're using.


I am using a DX-402 (aka Sangean ATS 505). I would be surprised if it had a
balanced antenna input, and have been thinking about just clipping the
external antenna to my whip after all. I hooked 20 feet of 300 ohm twinlead
to a 300/75 ohm transformer and miniplug last night and plugged it in, but
the signal strength was about half of what my whip gives me. Therefore, the
change in plans.


What I am visualizing is two pieces of twin-lead, one called A (left)
and the other called B (right.) A would have one side of one end
soldered to the ground rod (call this the "near" end) with the other
side of that end hanging out in space. The rest of that piece of
twin-lead (side A) would go up the side of my house to the eaves and
over several meters. The far end of A would have the two conductors
soldered together. B would have one side of the "near" end attached
to the appropriate lead of a 300/75 ohm matching transformer, which
has it's other lead soldered to the grounding rod and goes to 75 ohm
coax. The other side of the "near" end of B hangs out in space. B
would then go up the side of the house to the eaves, just like A, but
would then stretch several meters in the other direction. The "far"
end of B would also have the two conductors soldered together. Each
leg is a little over 15 meters long (counting the part that goes up
as well as the part that goes over), which would make the whole
dipole a half-wavelength for some frequency slightly below 10 MHz.
(Have I got this right?)

The coax would travel approx. 4 meters to my window, where it would
go through another matching transformer and into the external antenna
input of the radio.

Wouldn't that be a half-wave dipole?

A folded dipole is made from twinlead by first cutting it to a half
wavelength long (total length) for the frequency you want. The two wires
in each end are connected together. Then you open (strip) an inch or so
of the insulation for one wire *only* (either wire), at the middle of
the twinlead. Cut the exposed wire and pull two short lengths of it
(pigtails) out of the twinlead. Connect each of those wires to the
matching transformer on it's high impedance side.


Gotcha. Saw a diagram like that today in that book, so I understand what
you are talking about. This would be easy.

Would it matter that the side of the house is covered in steel
siding, against which it would be lying?

Yes it would. You want to get the twinlead section out in the open, like
between two trees, and as high as possible. The coax would run from the
middle of the twinlead dipole (matching transformer) to the receiver.
The dipole should be orientated so it runs at a right angle to the two
directions you want to hear the best. If you wanted to hear signals from
the east and west, you would run the dipole (twinlead) north to south.


Gotcha. I was planning on running the vertical portion of twinlead that
went from ground level/grounding rod/coax up to the eaves over the steel
siding, but I could just as easily run the coax up the side of the house,
which would put all of the twinlead above the steel siding. Still not "out
in the open" but probably the best I can do. Wifey doesn't want anything
that obviously says "Hi there, I'm an antenna!" I am beginning to consider
running the coax around to the end of the house and up that side, and laying
the twinlead along the ridge of the roof from one end to the other. That
would be a up a lot higher and a lot more "out in the open."

Would this work, or would an inverted L still be better (or
quieter)? I could put the grounding rod immediately outside my
window, which might enable me to make the long leg of the L somewhat
longer than the combined 30 meters of the dipole. That would make
for a shorter run of coax, but the soil would be drier thereby making
for a poorer ground (where it is now, the grounding rod sits next to
the output of a downspout.)

It's not clear to me why you are thinking of making a folded dipole when
a simple dipole would be easier. The latter doesn't require any matching
transformer for the coax and uses just one wire (split in the middle)
for the dipole element. A dipole responds best to the frequency for
which it is designed and it's bi-directional. The inverted-L is a
broadband antenna and usually non-directional if it's not too long. I
suggest you build a low noise inverted-L. See the following website:

http://www.anarc.org/naswa/badx/ante...e_antenna.html


I will check this out. I was thinking of a folded dipole just because I
already have 50' of twinlead sitting around. Of course, I have plenty of
regular wire sitting around too.

If I went with the inverted L (with joined conducters on the far end
to make a loop) would I still ground one side of the twin-lead? Do
you ground one side of a loop?

If you join the ends, you no longer have an inverted-L design. It's
become a horizontal loop antenna. You wouldn't use twinlead to make an
inverted-L, just a single horizontal wire with either a single lead wire
from one end or coax. The website I gave you (above) shows the best way
to connect a coax lead to an inverted-L.


Gotcha. I'll look at it and sleep on it.


Okay, I've written War and Peace. Sorry for the great number of
questions, I'm just trying to understand the fundamentals of
installing an antenna. I hope I am not trying your patience.

Thanks,

Dave

No problem with the questions. It's nice to see an interesting on topic
thread for a change.

Thanks for the encouragement. I'm not yet sure what I will end up doing,
but I want to make the best decision possible.

Dave