After about a dozen years of using the same Beverage matching transformers
(at two QTHs), I decided to try some changes and additions to the Beverage
farm. I purchased some of the MN8X cores a year or so ago and finally got
around to winding new transformers last week.
My existing transformers are surplus from an old BC carrier-current system,
discovered at the local electronic junkyard. They had a 50-turn to 50-turn
coil on a 2-inch toroid of unknown specs, with a switch selecting taps on
the primary. I simply selected the tap for best match (strongest
noise/signal). There was plenty of room in the box for relays that I use in
switching of single wires to use in two directions, or multiple beverages
off the same feedline.
For the new cores, I epoxied two of the MN8X cores together as suggested for
lower loss. I decided to try both the trifilar winding as suggested by
ON4UN's book (4t for 75 ohm feed), and also a simple 4-turn to 10-turn
transformer. The latter was so I could isolate the grounds on the feed and
antenna side. Some have suggested this will help eliminate common-node noise
and BC signal pickup.
I built several of the new toroids and then put a pair with the secondaries
back-to-back so I could put them in-line in the shack to test them out. I
was surprised to find that my OLD toroids had about 4-5 db insertion loss
(total 8-10db for back-to-back pair), while the MN8X transformers had no
measurable insertion loss (using the S-meter on my TS940S and listening to
loud BC stations, as well as signals in the 160 meter band).
So I built up several boxes with the new coils and installed them. In terms
of local ambient noise pickup, I didn't see much if any difference between
the Beverages using the new cores and the old. Nor any difference between
the transformers using the separated grounds and those with the common grounds.
However, there was ONE big exception -- my 835-foot Beverage, which runs NW
and SE and has hardline going to either end with relays to make it a
single-wire beverage in either direction. I had put one of the 4t-10t MN8X
transformers on the feed for the NW direction -- which aims directly at the
local BC station about 2 miles away -- and kept the grounds separated at the
feedpoint. I had MAJOR BC overload on just this antenna! Loud
spurs/harmonics all across the band that were not at all evident on any of
my 10 other Beverages! Apparently the MN8X core was saturating, as KM1H
recently suggested it might here on the reflector.
So I went out after work last night (fortunately the moon was full and high
in the sky at 2 a.m.!), crawled through the brush and the briars and swapped
out the NW box. I put one of my "old" boxes with the 2-inch cores at the NW
feed, and put the one with the MN8X that had been "saturated" in its place
on the 110-degree Beverage.
The NW Beverage is now "clean" -- no BC overload. And the MN8X works fine on
the 110-degree Beverage -- which is roughly broadside to the BC station.
All 10 Beverages now working fine -- some with the old and some with the new
boxes. That includes two (long and short) for Heard Island!
Still have plans for a couple of additional Beverages but first I have to
rebuild my switching box to allow for more positions!
73/Jon AA1K
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Sponsored by Akorn Access, Inc & N4VJ / K4AAA
|