"Lionel Carter" wrote in message ...
I would also be interested in the answer.
My impression is that particular balums are used purely on a custom and
practice basis and 'suck it and see'. I have not seen any guide to measuring
the rf resistance/impedance of a throw out or long wire antenna.
If someone doesn't answer your question the chances are they don't know
either.

Lionel Carter

It can be modeled. Or you can use a antenna analyser, etc. Random wire
antennas feedpoint will vary radically with freq changes. So for the
most part, it is "suck it up and see". Not much you can do about it
except try a different ratio transformer. Most of the antennas I use
are not random element designs, and have a fixed pre-known feedpoint
for the bands they are designed for. IE: most coax fed dipoles will
run from appx 50-75 ohms depending on height above ground, etc. So
naturally a 1:1 is the best choice. And you will still have enough
signal on most any other band for a usable s/n ratio. The only
exception might be with short coax fed dipoles used on very low
freq's, and in that case all you need to do is just unhook the ground
shield connection from the radio and let the center pin make the only
connection. EZNEC will spit out a SWR graph of any freq range you want
to punch in. You see a green "ball" on the top of the graph line. Say
if I scan from 1 to 30 mhz. I can place the green ball on 15 mhz and
see the feedpoint data. You can do this with the EZNEC demo.

I ran a swr scan on a 65 ft random wire, end fed. 1-30 mhz, every 500
cycles.
The "ball" is on 15 mhz. The feedpoint Z is anywhere from very low on
1 mhz to high on many frequencies. In the program, you can click on
any "500 cycle" portion and see the feedpoint specs. This can be used
with any antenna you want to punch in, and you don't even have to
leave your puter. You can d/l the eznec demo on the web. MK
http://web.wt.net/~nm5k/swr.jpg

That's right, only for the resonant (band) frequency. However it also
depends on the installation conditions, particularly the height above
ground. In the real world there are many variables that determine how a
particular antenna performs. Here's a webpage with more info' about
dipole impedance.

http://216.239.51.104/search?q=cache...hl=en&ie=UTF-8


RHF wrote:

Starman,

This is only true for the specific frequency (band) that the Dipole is 'cut' to.
- - - Or Am I wrong on that point too?

iane ~ RHF
I Ain't No Elmer !
.
.
= = = starman
= = = wrote in message ...
Michalkun wrote:

Is it true that if you feed a dipole in the middle you don't need a balun?

Yes, providing the dipole is a half wavelength (not folded) type and
your receiver will accept a 50-ohm antenna feed line (coax) input.


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baluns are transformers and the impedance ratio is related to the turns
ratio.

If you have a dipole antenna the centre feed point is about 75 ohms and if
you have 75 ohm coax the balun needs to be 1 : 1 .

If you have an end fed antenna the feed point can be anything from 600 -
1000 ohms so a 4:1 to 8:1 ratio matches the coax.

Bit of experimentation.

use the one that works best.

Feed point impedance depends on the frequency, so measurements, while
possible, tell you an exact answer to a question where close enough is an OK
answer.

Robert

"Robert Jeffares" wrote in
:

baluns are transformers and the impedance ratio is related to the
turns ratio.

If you have a dipole antenna the centre feed point is about 75 ohms
and if you have 75 ohm coax the balun needs to be 1 : 1 .

If you have an end fed antenna the feed point can be anything from 600
- 1000 ohms so a 4:1 to 8:1 ratio matches the coax.

Bit of experimentation.

use the one that works best.

Feed point impedance depends on the frequency, so measurements, while
possible, tell you an exact answer to a question where close enough is
an OK answer.



I have Alpha Delta DX Ultra and it's a dipole. There is an output for the
coax, so I was wondering if such antenna needs a balun or this is
neglegable and connecting 50 ohm coax will do the job.