I was thinking (finally) about simple dipoles and such and have this
question:

I plan on constructing some simple dipoles. For fun, I have been modeling
them using EZNEC. It seems the "common" practice is to feed the antenna with
50 ohm cable. The antenna feed point impedance is usually ~70 Ohms. Would it
make more sense to feed a dipole with 75 ohm TV coax to keep the SWR low on
the cable and have the mismatch at the radio? If I plan on using an antenna
tuner, would this setup be preferred over 50 ohm feed cable? Does it really
make any difference?

Just Wondering,

Tom

"Tom Sedlack" wrote in message
...
I was thinking (finally) about simple dipoles and such and have this
question:

I plan on constructing some simple dipoles. For fun, I have been modeling
them using EZNEC. It seems the "common" practice is to feed the antenna
with
50 ohm cable. The antenna feed point impedance is usually ~70 Ohms. Would
it
make more sense to feed a dipole with 75 ohm TV coax to keep the SWR low
on
the cable and have the mismatch at the radio? If I plan on using an
antenna
tuner, would this setup be preferred over 50 ohm feed cable? Does it
really
make any difference?

Just Wondering,

Tom



unless these are uhf dipoles with very long feedlines the difference in swr
between a 50 and 75 ohm coax won't make any difference to you.

So Reg, would you advise using a (4 to 1) balun with the open wire on one
side, and a short run of coax, say 20 feet or so on the other to the antenna
tuner? Would this work well? Does one need to 'make' the open wire feeder,
or is the commercial stuff you buy ala ham coax suppliers good enough?

regards, Jerry

"Reg Edwards" wrote in message
...
I plan on constructing some simple dipoles. For fun, I have been
modeling
them using EZNEC. It seems the "common" practice is to feed the antenna
with
50 ohm cable. The antenna feed point impedance is usually ~70 Ohms.
Would
it
make more sense to feed a dipole with 75 ohm TV coax to keep the SWR low
on
the cable and have the mismatch at the radio? If I plan on using an
antenna
tuner, would this setup be preferred over 50 ohm feed cable? Does it
really
make any difference?
===============================

Tom you are quite correct. The lowest loss on a transmission line occurs
when the line impedance is matched to the antenna feedpoint impedance. So
a
standard 75-ohm line is a better match to the centre of a simple 1/2-wave
dipole which is around 70 ohms when at exact resonance.

Furthermore, the attenuation of a 75-ohm TV line with the same overall
diameter is less than a 50-ohm line. Especially if the 75-ohm line does
not
have solid polyethylene insulation but is partially air spaced. The two
effects, between them, may result in a noticeable increase in signal
strength on the 10m band if the feedline is very long, say 150 feet or
more.
For short feeders there will no practical difference between the two
impedances.

But coax of any impedance limits use of an antenna to frequencies at which
it is 1/2-wave resonant and 3/2-waves resonant.

To take advantage of a tuner you must use a balanced high impedance
feedler
such as 450-ohm ladder line or 600-ohm open-wire line. For the same weight
of line the higher the impedance the better. Line loss can then be
reduced
to a negligible amout. Less than either 50 or 75-ohm coax at any
frequency.

But the big advantage of the high impedance line is that its very low loss
applies to ALL HF bands from 160m to 10m. The antenna does not have to be
resonant at any frequency. It can be of random length and work all bands
if
it is long enough at the lower frequencies. The high-Zo feedline can also
be of random length. Best to bring it all the way from antenna into the
shack near the tuner.

You will never have a better, all-round, all band antenna than a high
dipole
fed via 600-ohm line, overall dipole length as long as you can fit into
your
back yard.
----
Reg, G4FGQ

Mike:

[snip]
"WB3FUP (Mike Hall)" wrote in message
...
If you want 600 ohm I think you will have to roll your own. I use
commercial 450 ohm line myself. There will be a balun in your tuner (if
it
will directly feed ladder line) so why not just run the wire to the tuner,
and not bother with coax at all.

--
73 es cul

wb3fup
a Salty Bear
[snip]

Many want to use a few feet of coax in tandem with parallel wire [open wire
600 Ohm or 450 Ohm window line] just to pass the transmission line "through
the wall".

If passing the line through the wall is the problem then just use a foot or
so of electrical mains "zip cord" for that short passage. Simply solder a
short piece of zip cord into the 450 Ohm window line at the appropriate
place to allow the line to pass through a small 1/4 - 1/2 inch hole in the
wall. The "impedance bump" caused by the short hunk of zip cord will never
be noticed throughout the MF/HF bands.

--
Peter K1PO
Indialantic By-the-Sea, FL

Tom,
If you hadn't said 75 ohm TV coax, I'd say it would
work just fine either way. If you would change that
"TV coax" to something like RG-59 (or whatever the other
size of 75 ohm cable is) you would have much less trouble
making connections. The TV stuff uses aluminum braid, which
is a real P.I.T.A. ...
'Doc

On Mon, 11 Aug 2003 18:15:49 -0400, "Peter Brackett"
wrote:

[snip]

If passing the line through the wall is the problem then just use a foot or
so of electrical mains "zip cord" for that short passage. Simply solder a
short piece of zip cord into the 450 Ohm window line at the appropriate
place to allow the line to pass through a small 1/4 - 1/2 inch hole in the
wall. The "impedance bump" caused by the short hunk of zip cord will never
be noticed throughout the MF/HF bands.

Yes, the impedance "bump" isn't a problem, but I would be concerned
about voltage breakdown. Most zip-cord is rated at a maximum of 600
VAC and it is quite possible to have voltages higher than that on a
transmission line.

Danny, K6MHE

"Jerry" wrote
would you advise using a (4 to 1) balun with the open wire on one
side, and a short run of coax, say 20 feet or so on the other to the
antenna
tuner? Would this work well? Does one need to 'make' the open wire
feeder,
or is the commercial stuff you buy ala ham coax suppliers good enough?

=================================

To add a length of coax, balun or not, to the end of a long, beautiful,
exceedingly low-loss, open-wire transmission line should be regarded as a
criminal act worse than horse-stealing and be made a hanging matter.

The correct, most efficient arrangement is to connect the line direct to the
tuner via a 1-to-1 current (choke) balun. The length of coax between tuner
and transmitter can then be as long as is necessary to reach.

A great length of coax, a horrid lump of capacitance, on the antenna side of
the tuner will also restrict its impedance matching range and possibly
increase tuner losses.

A 1-to-1 choke balun is just a few (8 or 10) turns of twin speaker cable (18
or 20 gauge) wound on a ferrite ring, or more turns on a ferrite rod with a
length of the same order as the ring's diameter.

The actual impedance of an open-wire line is of no consequence. It so
happens when the wires are spaced apart 100 times wire diameter Zo is about
600 ohms. 18-gauge wires spaced 4 inches = 600 ohms.

14-gauge wire spaced 2 inches makes a nice line. At 30 MHz, loss = 0.15 dB
per 100 feet. With SWR = 6, loss = 0.48 dB. When used as a 1/4-wave matching
transformer, Q = 550.

The problem is flexing in the wind and metal fatigue. But amateurs don't
need a feedline to last a lifetime - projects don't last that long. Very
tedious to homebrew so most people seem to settle for 450-ohm ladder-line.
Choose the heavier wire gauges. Don't bother with flimsy 300-ohm ribbon
stuff.

Problems involved with extending lines into the shack, like Mark Twain's
death, are much exaggerated. Insulate the wires and bring them close
together through a single hole in a window frame or even a wall. A little
extra capacitance due to close spacing over a length of a few inches is
electrically entirely un-noticeable. You've only to look at it to realise
that. Ignore gobble-de-gook about impedance bumps on lines intended to
operate with high efficiency at high SWR.

If you like amusing yourself with numbers, to analyse performance of any HF
dipole with any arrangement of twin-line + coax line + balun + tuner,
download program DIPOLE3 from website below.
--
=======================
Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================

"Reg Edwards" wrote in message
...

Problems involved with extending lines into the shack, like Mark Twain's
death, are much exaggerated. Insulate the wires and bring them close
together through a single hole in a window frame or even a wall.
=======================



Why not drill two very small holes with the same spacing as the feedline?
Then splice on the inside.

Tam/WB2TT