Re: [TowerTalk] OCF dipole and "good" balun

[K4SAV <RadioIR@charter.net>]

The BL2 balun is intended to be inserted into a piece of equipment.  It 
will not be acceptable for outside use.  The balun you need for this 
antenna is a 4 to 1 current mode balun.  It MUST be a current mode 
balun, and it must be installed at the antenna feedpoint, or the antenna 
will not resonant on any of the frequencies shown in W8JI's plot.  W8JI 
also made some measurements of the performance of some baluns.  You can 
read it here:
http://www.w8ji.com/Baluns/balun_test.htm

It may not be obvious from the plot, but the bandwidth of this antenna 
is very narrow.  If you don't have a tuner, you may have trouble 
operating across the whole band.  For example when using the 4 to 1 
balun, the SWR at 3.5 MHz is 1.1 but at 4 MHz it is 11.7.  The frequency 
listed for 30 meters (10.7 MHz) was out of the band.  At 10.1 MHz the 
SWR is 6.1.   The frequency listed for 20 meters was 14.25 and SWR = 
1.9, but at 14 MHz it is 5.0.  The actual SWR numbers will vary a little 
with the installed height and ground quality.  The actual SWR measured 
at the shack end of the coax will be a little less than these numbers 
because of coax loss (and maybe balun loss).

If you use insulated wire you will need to modify the wire lengths. 

If you use this antenna a high power you may have some problems with RFI 
in the shack due to common mode currents on the feedline.  It is very 
difficult to remove the common mode currents from OCF antennas.  More 
chokes may be required.  You might not have any problem with low power.

The radiated pattern of this antenna will be completely different from 
that of a fan dipole on the higher bands.

Jerry, K4SAV


Samir Popaja, 7S7V (SM7VZX) wrote:

> Hello, 
>
> I would like to build OCF dipole, 137 feet long, with feedpoint located 80%
> from one end.  It's 27.4 ft from one end.
> (http://www.w8ji.com/windom_off_center_fed.htm). I would like to use it on
> 80, 40, 20, 15 och 10m, not interesting of WARC band.
>
> If we compare this antenna with FAN DIPOLE (for respective bands), is it any
> difference between them? 
>
> Can I use ELECRAFT BL2 balun? W8JI is talking about good and bad current
> balun. Is BL2 good balun for this antenna?
>
> It will be feed with 50ohm's coax from 100 watts transceiver without tuner.
>
> Best Regards
> Samir, SM7VZX
>  

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