It appears to me that "no-radial" verticals are also OCF dipoles,
vertically oriented. In my new QTH I will be limited to a vertical, and
I chose a Cushcraft (well, MFJ now) R8. It has a built-in 'balun' --
that is, a choke -- but I am taking no chances and have prepared another
choke that I will mount immediately under the antenna to ensure that it
will be decoupled from the feedline.
In addition, I am going to put yet another choke in the line where it
enters the shack. I made these according to GM3SEK's designs
<http://www.ifwtech.co.uk/g3sek/in-prac/>, so they won't cost a king's
ransom.
I hate noise!
On 5/1/2014 11:14 PM, K8JHR wrote:
K9YC raises an important issue. I love my OCF dipole, but only because
I was very careful in how it went up.
My friend in Germany is making a serious study of common mode noise on
OCF dipoles, and from his work I know you must bring the transmission
line away at a 90 degree angle, and use a good 4:1 balun with common
mode choke, or the OFC Dipole can be an inherently noisy thing - in
fact, causing its own noise!
I have a very quiet OCF dipole, but I was extremely particular in how I
assembled and hung it, with a good balun-common mode choke, and making
sure I did not have have a problematic length of transmission line, etc.
Great antenna, I use it as net control of a nationwide net, but you
must be careful in how you build it. If the coax comes near the
elements, even just a little, it can become problematic - affecting SWR
if not causing audible RFI.
Even if you put up resonant half wave dipoles, follow Jim's advice and
use a common mode choke. Some guys put chokes at both ends of the
transmission line - at feed point and near the shack - and I know of no
negative or down side from doing that.
In addition to what Jim refers you to, in general, see:
www.dj0ip.de
and in particular see:
http://www.dj0ip.de/off-center-fed-dipole/what-s-wrong-ocfd/
which is all in English, and the author makes the case that many times
the 4:1 balun is poorly constructed or improperly wound to minimize
noise and effectuate the proper transformation. And he uses 2 OCF
dipoles, himself, so it can be done with good results. It seems that
some things called a "balun" are transformers and other things called a
"balun" are common mode chokes, and sometimes you need both functions,
and how yours is made matters. So, read Jim's article, and the other
references herein, and maybe check out W8JI's personal web site, for
good information on this.
Just MY take.
=============== K8JHR ================================
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [email protected]
