Windom antennas - down to earth
 

Windom antennas - down to earth
 

On Mar 5, 9:18 am, John Passaneau wrote:

We have used OFC dipoles here for a few years at field day. They fill a
very specific set of needs for us. 1) They are simple to put up
2) The feed line coming from one end of the dipole is shorter than from
center feed dipoles in our setup. 3) OFC dipoles offer an impedance at
the end of the coax that is within the range of the tuners built into
our radios on the bands that are important to field day. This simplifies
our setup and operation. 4) They work as well as an antenna of that
physical length on any one frequency would no mater how it's feed.

The radiation pattern from an OFC is set by the length of the wire not
where RF is feed in/out of the antenna. In our setup open wire line and
tuners would be a pain in the butt, and an operational inconvenience
that gains us nothing. Fan dipoles or separate dipoles are hard to setup
and or tune and would perform no better for us. The antennas we use were
built by myself and use a 4:1 current balun which minimize feed line
radiation. On 80/40/20m we can easily match the antenna with the built
in tuners so the SWR must be under 3:1. OFC dipoles don't work well on
15m but with the current sun spot cycle not a problem. We see no
indication of common mode current problems, so we don't worry about it,
we just operate and have fun.

John W3JXP



Sounds fine... Just as long as I don't have to use it.. :) Myself, I
prefer either
separate, or fan dipoles on the low bands.. 20-10, a tribander.. "A4S"
I never use a tuner. All coax fed too... To each his own I say...
MK

Windom antennas - down to earth
 

Windom antennas - down to earth
 

On Wed, 07 Mar 2007 19:16:57 -0600, Mike Coslo
wrote:

OCF dipoles are obviously a compromise.

Hi Mike,

What happens to be the compromise?

As a multiband antenna it
better not outperform a specific band dipole.

Seems unlikely to perform any different than any equal length of wire
(the length of the fundamental band, that is). Only the feed Z
changes is all (or so modeling would lead me to believe).

73's
Richard Clark, KB7QHC

Windom antennas - down to earth
 

On Wed, 07 Mar 2007 19:16:57 -0600, in rec.radio.amateur.antenna you
wrote:

OCF dipoles are obviously a compromise. As a multiband antenna it
better not outperform a specific band dipole.

That's a general statement, but for a correctly fed OCF, your
statement is generally false.

If an OCF is shorter than 1/2 wave for the frequency of operation, it
will not perform as well as the 1/2 wave dipole. However, as the
length of the OCF increases over the 1/2 wave, it generates lobes of
gain in various directions. How many lobes and to where they point
changes according to the relative length to the band, but there is
actually gain.

You can find more information in the ARRL Antenna book or by modeling
the antenna on one of the antenna model ling programs.


--
73 for now
Buck, N4PGW
www.lumpuckeroo.com
N4PGW

Windom antennas - down to earth
 

On Thu, 08 Mar 2007 05:24:52 -0500, Buck
wrote:

If an OCF is shorter than 1/2 wave for the frequency of operation, it
will not perform as well as the 1/2 wave dipole.

Hi Buck,

Modeling would suggest otherwise.

The difference between a 1/4 wave center fed and off-center fed (at
10% from the end) is about a 12% increase in real resistance, and
..25dB gain improvement, both favoring the off-center fed.

Are you comparing short antennas to long antennas? If so, it wouldn't
be a surprise, would it?

Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

73's
Richard Clark, KB7QHC

Windom antennas - down to earth
 

On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:

On Thu, 08 Mar 2007 05:24:52 -0500, Buck
wrote:

If an OCF is shorter than 1/2 wave for the frequency of operation, it
will not perform as well as the 1/2 wave dipole.

Hi Buck,

Modeling would suggest otherwise.

The difference between a 1/4 wave center fed and off-center fed (at
10% from the end) is about a 12% increase in real resistance, and
.25dB gain improvement, both favoring the off-center fed.

Are you comparing short antennas to long antennas? If so, it wouldn't
be a surprise, would it?

Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

73's
Richard Clark, KB7QHC

The original comment was that a multiband OCF antenna is a
'compromise' antenna and wouldn't be as good as dedicated 1/2 wave
dipole cut to frequency (or so I understood it.)

If a 135 foot OCF were compared to a 1/2 wave 160 meter dipole, the
OCF would lose, but if it were compared to a 20 meter dipole, it would
have gain in the direction of various lobes.

Yes, the comparison is between different length antennas, and you are
right, generally speaking, the longer, the better. (no doubt someone
can find an exception to the rule, but that isn't the point of this
discussion.)

The reason for my statement of 'properly fed' is that I know that the
impedance changes radically from band to band at the feed point.
Buxcomm is selling a popular OCF with a 6:1 balun to coax. I don't
think that is good as the impedance will be pretty low at the coax at
times. However, to each his own.

Feeding an OCF directly (no balun) with coax probably isn't a great
idea either. I used one that way for a long time, and with great
results (75 ohm indoor cable-tv coax) and switched to Radio Shack
low-loss 300 ohm tv twin-lead which appeared to have better results.
The best results seemed to come from a friend's setup which used 600
ohm twin lead (the good wire-man stuff) all the way to the tuner.

My favorite OCF design is still the clothesline antenna... a loop of
wire to make a 40 meter dipole at 300 ohms, a 4:1 balun with coax
attached and a motor at the pulley on one end. Tune the antenna by
moving the balun towards or away from the center of the dipole. No
tuner needed and it worked adequately in the man's attic, according to
the article I read. It is an old design, but interesting still today.
;)


--
73 for now
Buck, N4PGW
www.lumpuckeroo.com
N4PGW

Windom antennas - down to earth
 

On Mar 7, 7:16 pm, Mike Coslo wrote:


If you have found such a *drastic* difference however, perhaps
there was something wrong with your particular antenna or setup?

- 73 de Mike KB3EIA -

Wasn't mine..It was one they were using at a field day. As far as I
could
tell, it was a regular ole carolina windom, fed with their feedline. I
can't
remember if he had a tuner inline.
It was a dud though compared to a standard coax fed dipole.
A good 2 s units down on *everything*. "40m" Noise, desired signals,
the
whole ball of wax. Obvious feeder loss... Sure, you can make contacts
with such a device, but it's not for me.. Two S units difference is
about the equal of adding an average amplifier to a 100 watt radio.
I'm used to coax fed dipoles where the appx system efficiency is in
the
mid/upper 90's % range.. So almost any other compromise antenna is
going
to be inferior as far as total system loss. The main problem with the
carolina
windom I tried was the goofy feeder system with coax, choke, twin
lead,
etc... What a cluster%$#@ of engineering that is... :
If I'm going to use a compromise one wire/all band antenna, it's going
to be
fed with ladder line the whole way to a tuner which will be carefully
tuned using the least inductance possible. It will also be center fed.
Even that will be inferior to my usual coax fed... I've compared..
But usually my preferred multi band antenna will be paralled dipoles,
with the
legs spread apart as far as possible.. Fed with a single coax feed.
Thats what I use here at home. No loss in system efficiency compared
to many other multi band designs.
MK

Windom antennas - down to earth
 

In rec.radio.amateur.antenna you write:
Wasn't mine..It was one they were using at a field day. As far as I could
....[snip]....
But usually my preferred multi band antenna will be paralled dipoles,
with the legs spread apart as far as possible.. Fed with a single coax
feed. Thats what I use here at home. No loss in system efficiency
compared to many other multi band designs.

Maybe you can suggest why our formula-cut paralleled-dipole didn't
work all that well at our last "Kids Day at the Mall" effort:

Using some 4-foot-high decorative pillars about 15 feet outside the
mall doors as base mounts, we were able to erect and guy two 25-foot-high
metal masts about 40 feet apart. We then pulleyed-up a 10/15/20 meter
coax-fed parallel-dipole (20m on top, 15m on bottom, 15m between) with
the ropes to each dipole spread about 2 feet at the masts.

VSWR was something like 6:1, although a "tuner" brought it down to
where the radio worked OK. We made lots of contacts, but still
haven't determined why the "raw" VSWR was so high. Any ideas?

Sure. Coupling between the elements being they are so close
together, and inline with each other.
Thats why I spread mine apart as far as possible. The wider apart,
the less coupling in general.
If two dipoles are at right angles, they is almost zero coupling.
In such a case, I've had a leg of one of the dipoles fall down,
with little change in SWR for the other band being used.
Also the wider apart, the less skewing of the pattern from a
normal dipole pattern.
You can use the type antenna you describe, but tuning is tricky.
You have to start with the lowest band, get it tuned, and then
add the next higher band, get it tuned, and so on. Thats about
the only way you will get one tuned for each band.
And I've seen weird things happen with close coupling. I've
had cases where I would have to increase the length of the
legs on a higher band to go *up* the band. Pretty weird..
With them spaced as far apart as possible, you don't have
to go through all that tuning torture, and the antenna will
act more like a normal dipole for each band used.
I've used that system here at home for years. I change it
up quite a bit depending on season, etc..
Right now, it's a turnstile on 80m, and a dipole on 40.
At times, I'll have it 160,80,40 , and others 80/40/20..
Just depends what I'm doing at the time.
MK

Windom antennas - down to earth
 

Buck wrote:
On Thu, 08 Mar 2007 08:03:56 -0800, Richard Clark
wrote:

On Thu, 08 Mar 2007 05:24:52 -0500, Buck
wrote:

If an OCF is shorter than 1/2 wave for the frequency of operation, it
will not perform as well as the 1/2 wave dipole.
Hi Buck,

Modeling would suggest otherwise.

The difference between a 1/4 wave center fed and off-center fed (at
10% from the end) is about a 12% increase in real resistance, and
.25dB gain improvement, both favoring the off-center fed.

Are you comparing short antennas to long antennas? If so, it wouldn't
be a surprise, would it?

Coming in late, did I miss someone's extravagant claim that a OCF
could do better?

73's
Richard Clark, KB7QHC


Sorry, I haven't been able to find replies until now- musta been a news
server burp or something..


The original comment was that a multiband OCF antenna is a
'compromise' antenna and wouldn't be as good as dedicated 1/2 wave
dipole cut to frequency (or so I understood it.)

If a 135 foot OCF were compared to a 1/2 wave 160 meter dipole, the
OCF would lose, but if it were compared to a 20 meter dipole, it would
have gain in the direction of various lobes.

And I'm not sure I would define those lobes as something other than a
compromise. If the lobe is in a good place for you, fine. If not, not so
fine.


Yes, the comparison is between different length antennas, and you are
right, generally speaking, the longer, the better. (no doubt someone
can find an exception to the rule, but that isn't the point of this
discussion.)

I'm a little dense here. 8^) Is a antenna cut for a half wavelength at
80 meters a better antenna at 10 meters than an antenna specifically cut
for 10 meters?

I'd also have to go back and look, but isn't the SWR on some bands on
the Capacitive reactance end, even though it may be 50 ohms?

I know my old Icom did not like capacitive reactance very much.

The idea that an OCF is superior to a dipole, and certainly Richard's
statements would indicate that; makes me wonder why everyone isn't using
them!

- 73 de Mike KB3EIA -