The first thing you should consider is why it's important to you to
achieve such a good impedance match. I'm all in favor of stimulating
intellectual exercises, but that's all it is. It won't help your signal
by any measurable amount.

And no, a dipole doesn't have a feedpoint impedance of 70 ohms. In free
space, if it's very, very thin, resonant, and perfectly straight, it
will have an impedance close to that. But you can't construct one in
free space. Over ground, expect anything from about 50 to 100 ohms
depending on height and diameter. And that's only at the single
frequency at which it's resonant. QSY a few kHz, and there goes all your
work down the drain. But have fun. It'll be educational. And if you're
like so many others, you'll be *positive* that all the work made your
signal get out better. Not because of low SWR though, but because of a
phenomenon well known to medicine -- the placebo effect.

Roy Lewallen, W7EL

Tom Sedlack wrote:
Thanks to all who have responded!

I have ordered the Antenna Manual and will take a peak "to see what they
know" ;-)

As a vast majority of dipoles have been successfully fed with 50 ohm cable,
I'm sure I'll do the same thing. I was looking at this more from the
theoretical side in that it just seems to make sense to try and change the
impedance. All I read states that the dipole will have a characteristic
impedance of 70 ohms, the coax will have a characteristic impedance of 50
ohms. The best possible VSWR is 1.5:1 which equates to about 14 dB return
loss (this is the unit of measure I am most familiar with). 14 dB is good,
but in my line of work, I usually see better then 25 dB (VSWR 1.12:1).
It's probably overkill on my part but the experimenter in me may give this a
whirl.

Tom



"Tarmo Tammaru" wrote in message
...

Tom,

There is a way of doing it, using two matching sections. It is described

in

either the ARRL Handbook, or Antenna Manual. It takes a length of 50 Ohm
coax (NOT 1/4 wave) + a length of 75 Ohm coax (NOT 1/4 wave) to match to

50

Ohms. If I run across it, I will post it.

Before you do all that, put up the antenna and feed it with 50 Ohm coax.
Measure the SWR. Often the dipole impedance is closer to 50 than 75 Ohms,
and it is not worth doing anything about it, especially an inverted V.
Certainly, don't do anything if your radio has a built in antenna tuner.

Tam/WB2TT
"Tom Sedlack" wrote in message
...

I asked this before and was told that for relatively short runs (under

50

ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get

a

VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm

coax.

I saw an equation someplace that, I believe, gave the length of 75 ohm

feed

required to transform the impedance to 50 ohms at a given frequency. I

would

cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be

a

perfectionist)? I don't want to use a tuner because; 1 - I don't own

one

and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom