Carolina Windom revisited: 4 to 1 balun does nothing to choke RF?
 

Cecil,
I sort of thought that this OCF antenna was typically used for a
multiband antenna, that's typically the 'idea' behind using them.
Keeping that 'thought' in mind, how can you 'optimize' this multiband
antenna for one particular band without 'de-optimizing' it for
others? That seems sort of going at it from the wrong direction, why
not use an antenna that works well for the particular band of
interest. Oh, I know that assumes that you can have more than one
antenna, which isn't always the case. So having an 'un-optimized'
antenna is what you'll end up with in most (if not all) bands except
for one. Which is the 'fault' for almost all multiband antennas.
Isn't it?
- 'Do

Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
 

wrote:
I sort of thought that this OCF antenna was typically used for a
multiband antenna, that's typically the 'idea' behind using them.
Keeping that 'thought' in mind, how can you 'optimize' this multiband
antenna for one particular band without 'de-optimizing' it for
others?

Given the SWRs that EZNEC predicts, feeding the OCF
at a current maximum point causes the XMTR (or tuner)
to see the following resistances looking into the
ladder-line.

3.8 MHz, 129 ohms; 7.2 MHz, 120 ohms; 10.125 MHz, 37.5 ohms;
14.2 MHz, 58 ohms; 18.14 MHz, 71 ohms; 21.3 MHz, 51 ohms;
24.95 MHz, 53 ohms; 28.4 MHz, 61 ohms

Let's say you wanted to optimize for 40m. You would
choose a 450 ohm ladder-line length of 67.5 feet and
expect 120 ohms looking into the ladder-line. Use a
tuner or transformer for a perfect match.

Oh, I know that assumes that you can have more than one
antenna, which isn't always the case. So having an 'un-optimized'
antenna is what you'll end up with in most (if not all) bands except
for one. Which is the 'fault' for almost all multiband antennas.
Isn't it?

Not for this one: http://www.w5dxp.com/notuner.htm
That antenna system is efficient on all HF bands.
It is fed with 1/2WL of ladder-line on 80m and 3/4WL
of ladder-line on 40m. Also appropriate lengths of
ladder-line on all other HF bands. The ladder-line
length averages around 100 feet.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
 

wrote in message
...
Cecil,
I sort of thought that this OCF antenna was typically used for a
multiband antenna, that's typically the 'idea' behind using them.
Keeping that 'thought' in mind, how can you 'optimize' this multiband
antenna for one particular band without 'de-optimizing' it for
others? That seems sort of going at it from the wrong direction, why
not use an antenna that works well for the particular band of
interest. Oh, I know that assumes that you can have more than one
antenna, which isn't always the case. So having an 'un-optimized'
antenna is what you'll end up with in most (if not all) bands except
for one. Which is the 'fault' for almost all multiband antennas.
Isn't it?
- 'Do

Seems to me it's not "optimized" for any band, but is just a fudged
radiator fed by ladder line, matched with a tuner or maybe a pi-net without
too much line loss. You can fiddle with it to your hearts content, but in
the end will it load up where you want it to and will you get out? It would
be nice to have an antenna analyzer to have some indication of YOUR
installation as you fiddle. Obviously you will have trouble using any of
the modern internal ATUs.

Years ago, I threw one up but fed it with coax. It didn't work so I rebuilt
it as parallel fed Dipoles for 80 and 40m.

Thanks for the NEC output, Cecil.

Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
 

Tam wrote:

Roy,
What's your opinion on the 4: or 6:1 balun between the 300 Ohm line and
the coax? I see no reason whatever to think that the impedance coming
off the 300 Ohm line is anywhere near 300 Ohms. Also, where is it
written that a 50 Ohm balun will work at, say, 2000 Ohms. The ferrites
as you suggest will clearly work if you use enough of them.

The reason for asking this is that a friend is in the process of putting
up a 75 m dipole, which he only plans to use on 75 m. Everybody is
telling him to feed it with ladder line going to coax through a balun.
Why in the world would you do that?

As you suspect, the impedances encountered by the transformer on some
bands are wildly different than its nominal design impedances. In the
one which I carefully measured, the result was no surprise. When the
antenna impedance was substantially different from 300 + j0, the
transformation ratio wasn't 6:1, and the transformer added series and/or
shunt reactance, sometimes a pretty large amount. And this was the case
on most bands.

This isn't to say that an OCF dipole can't be fiddled until, radiating
feedline and all, it manages to present an acceptable SWR on several
bands. But when it does, it's not working at all like predicted by a
simplified analysis which ignores the strong feedline coupling and very
non-ideal transformer effects.

Roy Lewallen, W7EL

Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
 

Roy Lewallen wrote:

...
This isn't to say that an OCF dipole can't be fiddled until, radiating
feedline and all, it manages to present an acceptable SWR on several
bands. But when it does, it's not working at all like predicted by a
simplified analysis which ignores the strong feedline coupling and very
non-ideal transformer effects.

Roy Lewallen, W7EL

I have switched my balun designs to the one in this URL, Figure 4 -
Improved 4:1 Current Balun, page 3:

http://home.earthlink.net/~christras...k4to1Balun.pdf

Can be used balanced, or forced to unun fashion ... bandwidth becomes
increased (and, it "just works better for me!")

But, what works for you is the most important, always ...

Regards,
JS

Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
 

JB wrote:
Thanks for the NEC output, Cecil.

I showed how to optimize an OCF for a single band, e.g.
40m. If the 50/120 ohm transformer is link coupled, it will
cause a common-mode current node at the transformer on 40m.
Any antenna system can be optimized if one knows what they
are doing.

For instance, a shunt 1000 pf cap at the twinlead to coax
junction will optimize a G5RV for 75m.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
 

Roy Lewallen wrote:
This isn't to say that an OCF dipole can't be fiddled until, radiating
feedline and all, it manages to present an acceptable SWR on several
bands.

I bought the 300 ohm feedpoint myth when I was at Texas A&M
in 1958 when I didn't know any better. I had an OCF fed with
300 ohm twinlead fed through a 6:1 air-core Heathkit balun
driven by a DX-40. The results were amazing to me at the
time. Now I know the pi-net output of the DX-40 would achieve
a match to almost anything and I should have been using a 1:1
balun. The Heathkit balun didn't have much loss and the 300
ohm twinlead didn't have much loss. I have no idea what the
actual impedances were, but losses were minimized and the
system successfully worked the world at the height of the
most active sunspot cycle in recorded history.
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
 

"Cecil Moore" wrote in message
...
JB wrote:
Thanks for the NEC output, Cecil.

I showed how to optimize an OCF for a single band, e.g.
40m. If the 50/120 ohm transformer is link coupled, it will
cause a common-mode current node at the transformer on 40m.
Any antenna system can be optimized if one knows what they
are doing.

For instance, a shunt 1000 pf cap at the twinlead to coax
junction will optimize a G5RV for 75m.


Of course but I'm too lazy to run the numbers if it isn't my project. My
favorite single band wire antenna is the Dipole. How is OCF an improvement?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference, considering
coax losses.

Carolina Windom revisited: 4 to 1 balun does nothing to chokeRF ?
 

JB wrote:

...

Of course but I'm too lazy to run the numbers if it isn't my project. My
favorite single band wire antenna is the Dipole. How is OCF an improvement?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference, considering
coax losses.


A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my
humble opinion (or, IMHO ...)

Regards,
JS

Carolina Windom revisited: 4 to 1 balun does nothing to choke RF ?
 

"John Smith" wrote in message
...
JB wrote:

...

Of course but I'm too lazy to run the numbers if it isn't my project. My
favorite single band wire antenna is the Dipole. How is OCF an
improvement?
I also liked my Inverted L made out of a homebrew 9' bug catcher mobile
antenna with 25' horizontal clip on extension for 40/80 and clip on
shunt
caps at the mobile mount. Works well in the campground.

Keeping the antenna matched to the coax DOES make a difference,
considering
coax losses.


A "one size fits all" antenna is still VERY MUCH a pipe dream ... in my
humble opinion (or, IMHO ...)

Regards,
JS

At least the "one size" will be very large. I once saw a truly monumental
commercial or military log periodic.

Tam/WB2TT