|Subject:||Re: [TowerTalk] OCF dipole and "good" balun|
|Date:||Tue, 30 Oct 2007 22:55:31 -0500|
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:|
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.
Samir Popaja, 7S7V (SM7VZX) wrote:
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