At 07:12 AM 2008-11-20, Michael Baker K7DD wrote:
>I have decided to put up a 43 foot vertical with 32 radials for the lower
>bands. (80,40,30,20M)
> >From what I have been reading on the various manufacturers web sites it
>seems the common feed method is a "4:1 Current Balun."
>Perhaps it is just my old school brain at work here but it seems to me that
>an UNUN is what is needed.
>Baluns to me were transformer type that went from BALANCED antenna to
>UNBALANCED feed. UNUN's were for UNBALANCED antennas and feeds. IE:Coax.
>Something to keep the RF off your coax feedline was a CHOKE or Open sleeve.
>Ferrite beads seem to be the item most used these days for that.
>So my simple question is "How are YOU feeding your 43ft vertical?"
>I have spent so much time running simulations with various types of
>materials, lengths and construction methods in EZNEC I have gone blurry but
>I am now down to the wire (pun intended ;>) at getting this thing put up for
>the CQWW and I need some collective advice.
>So, tell me what YOU use.
>IS there a design for a 4:1 UNUN someplace because if there is I can not
>find it anywhere.
Michael,
You've probably gotten lots of comments on this, but I'll throw in mine, too.
As far as the balun vs. unun issue, it probably doesn't matter too
much. You're right that both the antenna and coax are unbalanced, so
an unun should do the job. The reason that current baluns (AKA choke
baluns) are often recommended is that they tend to reduce the RF
current flowing back to the shack on the outside of the coax and the
noise current flowing from the house into the antenna ground
system. Whether this is an issue depends on how good the ground
system is (yours should be pretty good, but not "perfect"), the
length and routing of the coax, and the path to "ground" in the
shack. Some operators report lower noise on receive when using a
current balun. YMMV.
Note that a well designed current balun doesn't really care if the
impedance on the load side is perfectly balanced, totally unbalanced,
or something in between. That makes it somewhat foolproof (a
dangerous word, if there ever was one).
The design for common 4:1 VOLTAGE baluns, which amount to a bifilar
winding with a center tap, will work quite nicely as an unun if you
change the external connections. Ground one end, feed the center tap
from the 50 ohm system, and connect the antenna to the other end of
the winding. The 2-to-1 turns ratio will provide a 4:1 impedance
unun autotransformer.
With your EZNEC modeling, you may have already noted that the
impedance of the 43 foot vertical on a Smith chart circles very
nicely around a 200 ohm impedance, but this just means that the SWR
will be fairly consistently about 3:1 or 4:1 (referenced to 200 ohms)
over most of its useful bandwidth. Of course this also means that
the matching transformer will never be operating with a 200 ohm
resistive load and there may be modest heating (loss) in the transformer.
This antenna is a useful compromise and should work pretty well on
40-20m and will get by on 80m. If you think about it, it is similar
to the 88 foot dipole that some hams use on 80m and up, just cut in
half and worked against ground. People who claim it works really
well on 160m have obviously never had a good antenna on that band
(and never looked at the ground and feeder losses involved)!
BTW, I use a 26 foot flagpole vertical on 40m and up, with L-C
matching at the base and a 1:1 choke balun.
Good luck & 73,
Terry
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