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[TowerTalk] Multiband dipoles

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Subject: [TowerTalk] Multiband dipoles
From: (
Date: Sun, 28 Oct 2001 17:34:48 EST

  In a message dated 10/25/01 4:32:35 AM Pacific Daylight Time, 
    What makes you think an untuned COIL
    will make a better isolater than a 
    X(L)  =  2 pi F L
     Z(res) = Q X  =  Q x 2 x pi x F x L
    SO, for a loaded Q of 100, the resonant impedance
    will be 100 times the tuned coil inductance.
    You would need a coil with 100 times the resonant coil's
    inductance to have the same impedance magnitude,
    ASSUMING the self resonant (due to stray capacitance)
    frequency is above the operating frequency.
     Tom  N4KG
   I didn't say "IT WAS BETTER."  I very clearly pointed out that that some 
RF got by the "No-Xc Coil/Trap."  Adjustments of the 75m tip/ tail end on the 
lower band, affect the 40m higher band resonant frequency somewhat.  This is 
nothing to be concerned about.  

Now with the original coil/trap antenna of Lattin Labs, the 75m tip/tails 
were 4' long--just hanging vertically.   Why that way?--I thought you'd never 
ask.  On 40m they had SOME RF on them.  Each tip is .03 WL long and in the 
high voltage end.  You tell me just how much affect on the dipole pattern 
they had.  It is less than NIL, SLIM & NONE. I clearly pointed out that I got 
low SWR on 75&40m and very good bandwidth on 40m.  Since there are no other 
factors of importance I could control--what more do you want?  The coils 
didn't heat so there was no loss to get concerned about there. 
When I removed turns from the coil progressively as I stated in a previous 
Post, the coil/trap started to get warm on 40m doing their thing and I 
increased the diameter of the wire twice along with having to lengthen the 
40m wires slightly each time as well as the tip/tail wires to retain 
resonance on 75m.  Even with the slight extention of the tip/tails to about 
10' for 75m and lengthening each 40m wire to 40' to retain resonance 
(resistive load), a low SWR still existed on 40m now with less band width and 
a low SWR on 75m with more band width (less coil loading).  To increase the 
bandwidth on 40m again I made a tapered 3 wire flat cage 3' wide at the far 
end tapering back down in about 1' to a single wire to the coil/trap.  The 
larger diameter affect in particular in the high voltage area really 
increases bandwidth on 40m.  I had to shorten it 1' or so also.  I even made 
for a friend a 5 wire cage with crossed braces with a wire down the middle 
for even more bandwidth on 40m and he's still using it.  It had to be 
shortened a bit more.  There are simple ways to solve even bandwidth 

Forget all that theory about traps you have heard.  There may have been some 
change in the figure 8 pattern on 40m as each dipole half was 40' long on 
each side plus whatever RF Spill Over over the coil/traps occured to the 
tips.  So what?  I couldn't care less. The major consideration is "I got 
Maximum RF in the dipole on 2 bands with low SWR with a "No External Xc Trap" 
and the coil didn't get warm."  Without a Xc in the trap it had the potential 
of being trouble free for as long as I want to use it which has been about 30 
years now.  I probably got a slight increase of gain on 40m (80' long before 
the coils/traps) and more gain on 75m also (fraction of a dB).  I improved it 
2 ways--if I wanted gain andnit was less than full length on 75m.  
  The original coil of 200 uh gives about 9000 ohm XL reactance and 80 uh 
gives about 3600 XL ohms on 40m.  By keeping my priorities in order I ended 
up with an improved Lattin Labs 2 band dipole except for less bandwidth on 
40m which I increased again with the 3/5 wire cage.  The new overall length 
was longer by reducing the XL reactance of the original coil/traps (fewer 
turns) from 8000 to about 3600 ohms and adjusting the wire lengths for 
resonance on both bands.   You can duplicate this antenna for a 160/80m by 
doubling everything or for 40/20m by 1/2 the original wire lengths and 
trap/coil values.  There are other variations I have on this antenna.  
You can duplicate this antenna in Eznec by inserting these XL reactances at 
the appropriate places, add the corresponding tip/tails and you can see the 
small amount of "Coil RF Spill Over" to the tips/tail wires.  I did it about 
10 years ago in Elnec.  Then look at the pattern, see why it should bother 
you and look at the Z values.  They compared to what I got.  I got "Max RF 
into the feedpoint" with a very low SWR and didn't care if the "Holly Grail 
Figure 8 Pattern" was altered slightly or not.  I made one up for a W6 DXer 
Loyd Colvin who used it on his expiditions and he loved it.  Anything I've 
ever described has been reasonably well tested I can assure you.  Now just 
who says "I have to have a certain impedance magnitude" for a trap?  Tell me 
who it is?  I say "you only need enough choke affect to do the job of 
creating a low SWR on 40m" and I shall illustrate it in another example.  The 
coil is just end loading on 75m.  There is plenty of RF in the tip/tail on 
75m beyond the coil/trap even before with 200 uh.  It cracked a couple of 
insulators above 250W.  The coil/trap is not a purely resistive RF Dam and it 
leaks slightly--so what?  There are simple ways to compensate for most 
anything--even for voltages on 75m high enough to crack insulators at 300W.  
I looked at it in Elnec and saw nothing to get concerned about and suggest 
you do so also.  The need for the perfect trap resonant in the band has been 
highly exaggerated and too many are wrapped up in theorectical trap design.  
I've used the coil/trap in another vertical application for 75/40m. 
A classic example of "totally ignoring the iron rules of trap design out of 
necessity and facing reality" is the traps used in the original HyGains 5 
band trap verticals.  The traps were all resonated much lower than the band 
also.  The 20m trap (with Xc) was resonant at 12.7 MHZ.  They used aluminum 
wire and this reduced the circulating currents that would have over heated 
the aluminum wire traps if resonant at 14.2 MHz--a design change out of 
necessity.  Since the trap was on the XL side at 14.2 MHz, the length of the 
electrical 1/4 wave tubing had to be shorted proportionally on each band.  
This shortened the overall length of the 80-10M trap vertical, allowed a 
smaller size of the tubing and lowered the cost and bandwidth.  What would 
you predict the Z of a 12.7 MHz trap with aluminum wire would be on 14.2 MHz? 
 HyGain and I didn't really care as long as I got a low SWR on each band.  
The practical aspects of what really happens with "Live RF" are more 
important than with "Theorectical RF" that so many preach.  Enough 
Theory--lets get down to "Practical and Live RF"--the stuff that will burn 
you if you get your fingers in it.  The 40m trap in the 80-10M HyGain 
vertical was a coil/trap with a 4' whip on top for 75M. 
I improved the traps by rewinding them with copper wire with 1-2 turns less 
on each side to get the end of the coil field away from the aluminum tubing 
support holding the coil form insulator.  Hy Gain screwed up as a "tubing 
end" is a "shorted turn" in the end area of the trap field, it raised the 
resonant frequency slightly and induced some loss on each end.  I also 
slitted the end of the tubing 3/4" on each end of the tubing to move the 
"shorted end" of the tubing back a ways that held the coil form and the 
resonant frequency went down 220 KHz.  I don't remember what the new resonant 
frequencies were now, I didn't measure or record them as I didn't care as 
long as it didn't heat.  I had to make the coupling sections longer in 
between the traps which I expected.  I still had a low SWR somewhere in the 
band and more bandwidth as the antenna was longer with less coil (now copper 
wire) in each trap.  I like to improve things.
I have another technique I used to actually broad band the trapped vertical 
and other antennas on each band with a "Triple Dipped SWR Curve."  Visualize 
the typical single dip SWR curve whatever it is centered in each band.  Where 
it starts to rise above about 1.5:1, visualize it dipping to 1:1 SWR on each 
side again of the center SWR dip that isn't necesarily 1:1 although it's 
either close to or 1:1 SWR on the 5 bands.  It's a very simple technique and 
the "how of this" will be publised elsewhere.  Anyone that follows 
instructions can do it.  The 50 ohm output rigs with no tuners really love 
this.  It works on all antennas. 

In the "Practical World of Antennas" there are times when it's useful and/or 
necessary to close the Theory Books and compensate when/where necessary--even 
without the typical fixed Xc across the trap coil--and still end up with a 
very effective and useable antenna.  I love hearing how others have done 
this.  K7GCO 

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