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K9AY
works great on medium wave
By Al Merriman - K4GLU
July 4, 1998
In the September 1997 QST on page 43, with a modification
in the May 1998 QST on page 73, there is an article by Gary Breed,
K9AY on the Terminated Loop.
These looked like they held great promise as Medium Wave DX antennas.
In fact the termination resistance for these antennas is determined
by using a BCB station off the end of the loop and adjusting the
resistance for minimum signal.
In a word, these things work!
I have a single loop up at the moment oriented roughly N/S.
This antenna is parallel to a pair of slopers that are approximately
135 feet long, 50 feet at the high end and sloping down to 10 feet
at the bottom.
These antennas run parallel to each other but are sloped in opposite
directions. They are fed at the low end with coax through 4:1 matching
transformers.
This pair of slopers have a very good "front to back" ratio when
compared to each other. On many frequencies it is possible to swap
dominate stations just by switching from one sloper to the other.
Up until now I have considered slopers, and I have a total of 4
oriented roughly N/S/E/W, as being the best wire antennas a Medium
Wave DXer could use unless they were fortunate enough to have the
room to put up Beverages, rhombics, etc.
That was before I tried a terminated loop!
The loop I have up is a delta, 37 feet on a side for a total
of 117 feet.
It is hanging from a tree limb and the bottom is about 3 feet off
the ground.
The remote relay box uses a 12 volt relay from Radio Shack and the
termination at the moment is a 2K pot.
The transformer I used is 11 trifilar turns of #24 insulated solid
telephone wire on a Amidon FT82-75 core.
All performance comparisons are made to the N/S pair of slopers
which are parallel to the loop.
Signal levels are down about 8-10db across the entire MW
band compared to the slopers.
However, even with this signal loss I am still able to hear the
two JFK Airport TIS stations on 1630 and 1700 kHz during the day
on the loop. This is not bad for 200 miles and 50 watts or so max
even if part of the path is over water.
In addition the loss can easily be made up with a quiet amp.
Front to back is, in a word, impressive. It is no worse than
the pair of slopers over the band from 600 kHz up and on many frequencies
it is better.
Nulls vary from 5 to over 20 db in a couple of instances and this
is during the day. On many frequencies it is possible to swap dominate
stations just by reversing the loop feed.
At the moment my terminating resistor is a 2K pot. The ground
I am using is only a 3 foot Radio Shack ground rod. This gives a
very effective ground when used with one of my slopers but might
not be good enough for the loop.
Termination resistance for best nulls varies across the MW band
and unless this can be smoothed out with a better ground it is obvious
that to get maximum performance a variable resistance is going to
be necessary.
This of course brings to mind the Steve Byan system. Since I have
a couple of the VTL3A17 Vactrols that Steve uses in his system I
will be playing with this in the next few weeks. Stay tuned!
These terminated loops look real interesting. While much more playing
around is necessary they hold a lot of promise. |
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