ANATOMY
OF THE COMET CHA-250BXII
Despite the mixed reviews(1), about
4 or 5 months ago I decided to purchase a Comet CHA-250BXII antenna. These are sold as a “broadband HF vertical
that does not require radials”. Some
people think they’re the greatest thing since sliced bread; others rate them as
a glorified dummy load. Interestingly,
there don’t seem to be too many people with an intermediate view; they either
love it or hate it.
Physically, the antenna consists of a 7m aluminium
element fitted to the top of a 140mm tall “black canister”, the contents of
which seem to be the subject of much speculation.
Electrically, the vertical element has the
following lengths:
3.60 MHz 0.084?
7.10 MHz 0.166?
10.13 MHz 0.236? (~¼?)
14.20 MHz 0.331? (~??)
18.17 MHz 0.424?
21.20 MHz 0.495? (~½?)
24.95 MHz 0.582?
28.50 MHz 0.665? (~??)
A couple of weeks ago my antenna developed a
fault which, while it prevented me from transmitting into the antenna (SWR went
high), had no noticeable affect on its receiving performance. At this point I had two choices:
(a) Return it to the place of
purchase since it’s still under warranty or
(b) Attempt to find and fix the
fault.
Being the inquisitive sort that I am, I decided
to try and fix it. This entailed some
risk since it would obviously void the warranty and there was no guarantee of
success. The following is a description
of the disassembly procedure I used and what I found inside.
A word of warning!
This device is not designed to be
dismantled. While everything is screwed
together, every thread has been liberally coated with glue during manufacture
and only comes apart with great difficulty.
I had to saw through the plastic top cap to remove it. If you decide to dismantle yours at any time
then do so at your own risk.
The following pictures are actually in reverse
order because they were taken after I had dismantled the unit. This first image is of the antenna base
assembly. This was taken after I had put
everything back together. You can see
the silicone I applied to the joints to help seal out moisture (which turned
out to be the cause of the fault).
Image 1 – Antenna Base
Assembly
Image 2 shows the matching unit (a.k.a. black
canister, dummy load) removed from the antenna base.
Image 2 – The
mysterious “Black Canister”.
Image 3 shows how the top and bottom sections
are screwed into the central section (bit with the fins). Before the top can be removed, you must first
remove a small grub screw that locks the top aluminium piece in place. You can then start to unscrew the central
spigot at the top. Note that this has a
wire soldered into the centre of it.
Don’t even bother trying to undo this joint with your average soldering
iron; it requires too much heat. As you
undo this spigot you will be twisting the attached wire. This doesn’t matter since there’s plenty of
it inside. Once the screw has been
undone to the end of the thread, hold the end spigot with a pair of pliers and
heat with a gas blowtorch to melt the solder and pull the spigot off the wire.
Image 3 – Showing how
the top and bottom sections are screwed in.
Image 4 shows the parts in the top section
assemble. Once the spigot, washers, etc.
have been removed the black plastic cap can, in theory, be unscrewed. In practice, the torque required to break the
glue on the threads is colossal. What I
did next (after trying various things involving hammers and screwdrivers) was
to carefully saw into the plastic cap (with a hacksaw) right next to the
fins. Bear in mind that, at this stage,
I didn’t know the cap was screwed in; I assumed it was just glued in. I had to cut to a depth of 10-12mm before the
cap came off. This left the part with
the threads still in the top of the central section. Using a hacksaw blade I made two cuts through
this piece 180o apart. After
this a light tap with a hammer was enough to break the glue and the two pieces
came out. With hindsight you could
perhaps drill some holes in the cap so that a specially fabricated lever can be
attached to it with screws. You might then be able to apply sufficient
torque to unscrew it in one piece, but I still think it unlikely. At this point I was left with the plastic cap
in three pieces; the top cap plus the threaded section in two halves. These were put back together with superglue
and some small self-tapping screws (the cap in images 3 and 4 is after I put it
back together).
Image 4 – Top section
parts.
Images 5 and 6 show the matching unit with the
top cap removed. There are two wires
going to the SO-239 socket in the base.
These are just visible through the central assembly. In my case, the ground wire to the socket was
already broken due to corrosion. After
much poking and prodding with a screwdriver I managed to break off the second
wire at the centre pin of the coax socket.
The central assembly was then pulled out of the canister.
Image 5 Image
6
Next I used two of the mounting holes in the
base to attach the canister to a piece of 4mm thick steel strip which was in
turn held in a vice. A vehicle
oil-filter removal tool was then used to unscrew the central section from the
base. Even with this arrangement, I
still had to heat the join between the two sections with a gas blowtorch to
soften the glue before I could undo it (hence the need to break wires and
remove the insides first). This is the
cause of the discolouration visible on the lower part of the fins in images 2 and
3. Image 7 shows the aluminium sleeve
after removal from the base.
Image 7 – Threaded
aluminium sleeve.
The Bits Inside
Images 8 to 12 show the matching unit
assembly. The small fibreglass cylinder
in image 8 is used as packing to make sure the parts can’t move around. The matching unit appears to be a transformer
with a ratio of about 12:1. The ring at
the end of the wire with clear insulation goes under the nut on the SO-239
socket while the green/yellow wire goes to the centre pin. The black wire on the right goes to the
antenna element.
Image 8 – Matching
unit side ‘A’
Image 9 – Matching
unit side ‘B’
The brass tubes through the toroids are the
first “turn”. The two tubes are joined
at the top and this point is connected to ground. The bottom end of one tube is connected to
the coax centre while the bottom of the other end connects to the start of the
coil that goes through the tubes. The
end of the last turn goes through one of the tubes then loops back to the top
(left hand tube in image 10).
Image 10 Image 11 Image
12
Images 13, 14 and 15 show the parts in the
bottom section assembly. Note that the
base section is threaded to take the SO-239 socket. Image 16 shows the matching unit refitted to
the base section prior to reassembly.
Image 13 Image
14 Image
15
Image 16 – Matching
unit fitted to base section.
Image 17 – Test setup
Image 18 – Test results
Image 18 shows the loss over the range 3 – 30
MHz. The upper trace is the 0 db
line. The markers are at 10 MHz
intervals. The vertical scale is 2.5 db
per division. This is not an entirely
accurate test because the output impedance of the matching unit is nowhere near
50O.
The sketch above gives a clearer view of how
the matching unit is wired up. I will
try to answer any questions you may have regarding the construction of this
device. Please send them to cha250 at vk5zdb.com (replace ‘ at ‘ with @).
Cheers
Iain Crawford
VK5ZDB
References:
(1) eHam.net - http://www.eham.net/reviews/detail/5175