In a message dated 10/23/01 5:42:09 PM Pacific Daylight Time,
firstname.lastname@example.org writes: <<
Hi all , I am wondering if anyone could tell me where I could find
information on choke decouplers, I would like to build a multiband dipole
and want to try using coils instead of the traps with capacitors .Also
looking for information on linear decoupling traps. I have looked around
but have found no information on these.
Thanks in advance
Kevin: Here is a short course (there is a lot more). About 30 years ago
Lattin Labs in QST advertised a multiband dipole. One version had a 40m no
Xc coil trap. It had about 200uh inductance of about #22 wire about 1' long
and 1" in diameter. It was placed 35' each side of center (not 33') with
about a 4' tail or tip wire for resonance on 75m. It was only 4' long due to
the large amount of end loading on 75m. It had narrow bandwidth on 75m but a
good match and reasonable bandwidth on 40m. Total length was about 80'.
For reference purposes a typical 40m tank circuit trap would take only 10 uh
of coil (20 times less) and 50 uufd Xc and about a 22' tail for 75m on each
side. I decided to shorten the Lattin coil which would require lengthening
the 4' tip/tail. I expected more bandwidth on 75m and less on 40m which I
got. There was a surprise. I used 2 coils I made and loaded it up on 40m
with no problem with fair bandwidth and lucked out on the tail/tip length I
first used with resonance at 7.2 & 3.8 MHz. I kept increasing the power with
my KW-1 AM rig and at about 250W I noticed the SWR jumping indicating a lose
connection. I increased the power more and all of a sudden the SWR increased
and I heard a "Thunk." I went out and looked and one side was down. I had a
broken insulator. I thought it was just mechanically defective so I just
replaced it. I loaded it up again and I heard the "Thunk" again. I examined
the broken insulator and there was a black carbon path. The voltage is so
high on 75m at the insulator with 200 uh XL loading and a 4' tail--in cracks
or actually breaks the insulator. I look at his ad again and it shows the 4'
wire tail hanging straight down and a rope connected to the end of the coil.
He had the same problem and solved it with the rope and the 4' tip/tail
hanging down free.
I remembered a story the Chief Engineer of that Marine SW Station South of
San Francisco told me. He was the Chief Engineer at a BC SW station back
East where they had just installed a vertical antenna and loaded it up. The
maintenance man came in and told them "fire was dancing off the top of the
new antenna." It wasn't top loaded but they ran big power. The diameter of
the vertical tip was too small. So they connected 2 wires to the top of the
vertical which ran to opposite sides of a loop about 1' in diameter. It
cooled the vertical tip and it took full power. The Chief told me he had a
"Standard RF Tip Test." If "you can play The Stars and Stripes Forever with
no fire off the end of the antenna, the loop is large enough." The problem
with the fire off the ends of the yagi used by the SW Station in SA could
have been solved with loops. Had they used it the quad wouldn't have been
Since I didn't want any hanging tip or tail wires, I extended 2-8" wires to
a 6" loop from one end of the insulator where the 4' tip/tail wire from the
coil connected. The loop was beyond the end of the insulator which connected
to a wire for support. It could be a rope but didn't have to. This cooled
the tip and the insulator would take a KW of AM on 75m. One late evening I
even played the Stars and Stripes Forever while watching outside in the dark.
It passed the "Patriot RF Test."
So with this new design I started removing about 20% of the turns from each
coil and lengthening the tail for 75m resonance again. However I had to
lengthen the 40m section to about 1' each side to retain resonance--each time
I took off 20% of wire from the 40m no Xc trap coil. The bandwidth increased
on 75m and decreased on 40m also progressively. After about the 3rd
shortening I noticed the coil was warm from 40m RF. I figured the
circulating currents on 40m were now higher. I also noticed that any
adjustments on the 75m tail had some affect on the 40m resonance. The no Xc
trap didn't fully isolate the 40m section from the 75m tail. With these
traps you can have a lower Z than a resonant L/C trap usually set in the
middle of the band but as long as a low SWR was obtained on 40m--this is
So I rewound the coil with heavier wire twice before I finished. I think I
ended up with #14 wire and about 60-80 uh of XL. The tip/tail lengths were
about 10' to the loops. The surprise was that I had to make the 40m sections
to the choke coil 40' on each side. I concluded that the shortened 40m no Xc
traps were highly inductive and being 1/4 wave away from the feedpoint, it
inverted the reactance to Xc which required the antenna be lengthened to 40'
instead of the usual 33'. I have the exact figures packed away somewhere.
The total length was about 101' now. When the 4' tip/tail lengths were
adjusted even a few inches, it had a fair affect on the 75m resonant
frequency. The longer they got the less affect each length change had. The
longer the tips/tail wires the lower the voltage at the insulator and need
for the loop. I never tried it without the loop when I got finished with the
reduction of coil inductance.
It's possible to make a multiband dipole with these no Xc choke coils but the
much larger needed wire no Xc coils even with less than 20 times the XL of
the original design, will end up with very short wires between coil/traps
compared to L/C traps and 30,17&12m couldn't be added. 10&20m would be about
all you could add. I never even tried it with coil traps for 10&20m as I
could see the handwriting headache on the coils. With all the coils the
antenna would be a fairly short and with limited band width. It's bad enough
with L/C traps on 10,15,20&40m. This assures a figure 8 pattern on each band
but with slightly less gain with all the coil loading. I used another
virtually no loss technique for 20&10m similar to what Lattin Labs used only
with open wire line instead of 300 ohm ribbon which wouldn't take a KW.
Lattin had an article on this in QST but I don't remember the date. If
someone knows what it is, they could mention it on TT.
Using a Johnson Match Box and 135' open wire line to the center of a 132'
dipole gives the highest efficiency with progressive gain toward the end of
the dipole on each higher frequency without SWR limitations over the whole of
each band. The rig sees 50 ohms all the time. I've always used 5 wire cage
dipoles of about 1' spacing which lowers the Z a fair amount when voltage
feeding the dipole on the higher frequencies. It improves the band width
substantually, required about a 5% shortening, lowers the losses a bit and
requires no maintenance if you build it right. The picture in my 8/60 QST
article on the inverted vee shows a 5 wire cage dipole with a 6" spacing fed
with open wire line. That's the last article on "Cage Construction for
Dipoles" I've seen since. Cage dipoles have far more wind drag but I sure
like the way they work. The next one I make for 160/80m will have a 3'
spacing and use insulated wire. They work great for quad/delta loops also
and have never seen anyone utilize this design. Try it--you will like it.
Cebik in AntenneX found that single wire loops on the LF have lower gain. No
it's not a Hoax. You will love the bandwidth of a 5 wire cage dipole or loop
and absolutely swear it works better. Don't be intimidated by the fact that
cage dipole construction is from the Stone Age from the last Century. Just
do it, don't tell anyone about it and avoid harrassment on TT. There are
still a few ways you can improve your antennas in the year 2001. Try
something different. When towers are used for verticals, 2 or 4 wires can be
easily added with stand off insulators to "fatten the vertical," improve
bandwidth and lower it's resonant frequency. I've done this using aluminum
wire on 3' irrigation pipe verticals.
I use a single coil/trap in a 75/40m vertical. Adding another coil in a self
supporting vertical presents mechanical problems. The biggest coil is on
top. I use another technique for 160m I'll finish this winter.
Trap coils have some applications but require a lot of adjustments and
balancing of the compromises. A 40m dipole requiring 80' to be resonant
between 2 coil/traps can throw you for a loop if you don't know why. K7GCO
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