Dear TopBanders
Long intro to take the bull by the horns in the end.
To finally get a better DX performance on 160, I converted my superb 80m CW
antenna to "an elevated inverted L with a bunch of elevated radials of
random length".
The 80m antenna was well matched by means of 2 elevated radials. I
sacrificed 80m CW monoband performance to get on 160m and make the antenna
operational on 80m SSB. In order to get this thing matched over such a broad
range, I placed an MFJ-998 automatic tuner in a remote position, about
10-15m away from the antenna's elevated feed point. Please don't start about
the MFJ, it was the easiest and fastest solution to my problem. I have a
'field day' setup for the low bands so each contest makes for different
impedances, making fixed matching almost impossible.
To make a long story short: this specific antenna was almost impossible to
match on 160m for the MFJ. Matching is done with about 30-35W. I could run
QRO on 80m but the tuner NEVER settled on 160m. I tried matching an 80m
dipole on 160m which didn't work either. I added coax ferrite bead chokes at
the tuner's output (feed point hard to reach). This improved the situation
yet did not yield satisfaction.
Desperate measures were then taken. I bought a waterproof enclosure, stacked
5 big ferrite cores on top of each other, and made a choke using copper
wire. I put this right at the feed point. You can see the result here
(scroll down for image):
http://on5zo.spaces.live.com/blog/cns!2C9EE99D7E3D2933!412.entry
This added choke with the big ferrite cores, solved the MFJ's problem. Its
circuitry is very sensitive to currents on the shield. I worked numerous new
ones on 160m and this antenna piloted me to #8 SOAB(A) in this year's ARRL
CW contest, thanks to the multipliers on Top Band which I lacked the
previous years. The MFJ did its thing. Top Band opened up a bit for me.
Now we've come to the bull's horns. I wanted to replace an M4 bolt by a
bigger type for the radial terminals. So last week I opened the choke's
enclosure. I used pieces of melting gun sticks as padding to keep the cores
in place. Some of these sticks were molten. The copper wire is held tightly
to the cores with white cable 'zip' ties. They are turning brownish.
Clearly there is a lot of heat going around in the copper wires. I hope
someone here can help me to understand what's going on:
- Is a ferrite core choke better than the ferrite beads over the coax? Given
that the core material is suited for the application.
- Is it better to use small Teflon coax to wind around the cores, in stead
of the tight copper wire pair?
- Is the heat in the copper wire due to I²R losses? If so, the currents must
be high? Making a piece of wire make the glue melt...
- I placed the MFJ tuner closer to the antenna so it's only 5m coax i.s.o.
about 12m. Could this improve the situation? I haven't tested this so far
because I
took down all radials to manage a big construction project in the garden.
- I have an AEA antenna analyzer. How do I use this to measure the choking
impedance of a choke on a given frequency?
I'd rather settle for resonant monoband antennas but given the situation,
this is the best I can do for Low Band Fun.
Hoping to learn something,
73 de Franki ON5ZO / OQ5M
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|