> 50 feet of RG8X close wound (about 10" long) on a 4 inch diameter PVC tube
> (regular plumbing supply white tubing) works great for a 80/160 balun.
That similar to the answer I gave in private mail, 30-50 feet of cable in a
single layer 4" diameter. Such a balun actually works over a quite wide
frequency range.
Everyone should also be aware that in some cases adding a balun can actually
INCREASE common mode. For example, if the feeder is grounded near or over
1/4 wl away from the antenna and suspended in the air adding a balun can
hurt the common mode rejection. If I had a dipole fed with 1/4wl of line
that was grounded at the earth end and the line was suspended in air, I'd
never use a balun! If the line was taped to a tower, I would use one.
The point is baluns aren't always a good choice. There are cases where they
aren't needed and even cases where they hurt the system.
After making measurements I strongly disagree with advice 73 or 77 materials
are good in applications where significant common mode voltages excite the
balun.
You can test this yourself easily with current baluns. Just connect a balun
in the line to a dummy load, and reverse (or connect normally) the shield
and center conductor connection at the balun output. Be sure the dummy load
is grounded to the same ground as the rig with short leads.
You should see two things. First, the SWR should be perfect with the output
phase normal or inverted. Second, the balun should NOT overheat with the
phase inverted.
At 25% of 1500 watts the common mode voltage impressed across the beads is
EQUAL to voltage across the balun shield to ground with a perfect balanced
load and 1500 watts of power. In real life with some antennas the balun can
have considerably more voltage than this, so you might need considerably
more margin.
If you test bead baluns instead of relying on "recommendations", you'll
find many baluns are very disappointing. Some of the longer string bead
baluns marginally pass a pure 375 watt inverted output test, meaning they
will work IF the load is a perfectly balanced low common mode Z load (like a
symmetrical dipole) and the balun goes immediately to a low resistance
ground on the unbalanced side.
If you have a high common mode voltage, like an off-center-fed antenna or a
cable paralleling the dipole for a bit, a high common mode voltage
application like a balun on the input of a ground independent tuner, and
similar situations you can cook the balun with very low power.
I don't know how this stuff got past everyone for so long, but I'd certainly
never rely on high ui materials (materials with low Q) in high power
applications unless I knew for sure the balun didn't need to do much work. I
suspect some of it is the baluns are up away from the operator and no one
really notices unless something absolutely melts.
If a current balun overheats at 500 watts or less in an inversion
application, it sure isn't going to be spectacular at 1500 watts even when
feeding a dipole or Yagi.
73 Tom
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|