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Re: [TowerTalk] Can capacity hats be installed inboard of a coil or LL ?

Subject: Re: [TowerTalk] Can capacity hats be installed inboard of a coil or LL ??
Date: Sun, 5 Dec 2010 10:49:57 EST
List-post: <">>
Jim, these are the same issues that come up with mobile HF antennas and  
various loading methods.
All of the techniques you mentioned will make the element electrically  
longer and that is what you are basically wanting to do.
Capacity hats beyond the loading coil add a C component across the loading  
coil (as do the element tips).
When you add a -j across a +j, the net j increases and the effect is the  
same as a larger inductor. The numbers add like resistors in parallel, 
observing  the + and - values. For example, taking a coil having 100 ohms +j 
shunting  it with a capacitance of -200j results in (100 X -200) / (100 + 
-200) = -20000 /  -100 which equals +200. You have multiplied the inductance by 
2X and the  frequency will go down accordingly.
Or, in the case of a mobile antenna, maybe you can use a smaller inductor  
to achieve resonance and have less loss.
If this added capacitance is in the form of a hat below the coil, this  
inductance multiplication can't happen. It does add a shunt capacitance across  
the part of the antenna below the coil and that will lower the frequency; 
just  not as much as if placed above the coil.
There are never any free lunches (or GW sez some are less expensive) and in 
 this case, the added capacitance across the inductor causes an increased 
voltage  across the inductor which in turn causes an increase in the current. 
Losses  being related to current squared, means higher coil loss and the 
need to pay  more attention to the coil Q.
As always, I am just the concept guy, you can work out the details and  
Gerald K5GW
In a message dated 12/5/2010 8:51:38 A.M. Central Standard Time, writes:

I have a  an ongoing project with my F-12 EF-180B.    [68' long 80m   
rotary dipole]. 

I scrapped the stock LL wires.   I added a  17.5' long ....'T-bar' 
[capacity hat]  15'  out on either side of  the feed-point.
This is similar to the newer F-12  sigma 180-S   loading scheme.
The stock 1" solid fiberglass  insulator's    are about  2.5'   past [out 
board]  of the  the   new T bars.
The stock 1" insulator's  will  have to  have a  shorting strap  placed 
across each of them.

Then it occurred to me  that either an AL coil  could be placed across 
those 1" solid  insulator's...OR
the LL scheme as used in the F-12 40M  'N'   style  eles  could also be 
used at those points. 

I have a  F-12  3-el  40m yagi already  [340N]..and that loading scheme  
F-12 uses, would be real easy to
add to the 80m rotary dipole.    That would effectively add another 8'  to 
9'   to each  end.

The pair of T bar  capacity hats should add another  8.75'  to each end.  
Total effective length should
be aprx  101.5'     OK, the trick is, the T bar capacity hats   would be 
INBOARD of the LL scheme. 
Even if large diam AL tubing coils were  used instead of the N style 
loading scheme, the T bar
capacity hats would  still  be inboard of the coils. [ by 30"]

Now will my proposed  scheme work?   The F-12  40m 'N'  loading scheme  
uses  3/16"  solid AL rod.
The rod starts  on the inboard  side of the insulator..and goes OUT, 
towards the ele tips,   for  about
5-6', then does a 90 deg angle..for about 10-12", then comes back on  
itself, terminating on the outboard
side of the same insulator.    IE: the LL  heads towards the ele tips..and 
NOT towards the   feedpoint, like 
the original 12 ga alumoweld wires  did. 

Part  of the plan is to use the Tornado drive [made by seco systems] at the 
 feedpoint to do the balance of the
loading.   The tornado drive  is just a pair of motor driven  compressible  
coils inside a  5"  diam pvc tube. They
use  1/4" cu  or al  tubing  coils, and they can be made in any size.

The new F-12 sigma   180-S  80m rotary dipole is  54'  long...and also uses 
a pair  of 18'  long T bar  capacity hats,
[albeit, they are 18' on  either side of the feedpoint, vs my 15'  on 
either side of the  feedpoint].   When the sigma
180-S  is used in conjunction  with the model 80-D tornado drive, the swr 
is flat from 3190 khz   to  4090 khz  [ 900 khz wide]

The idea with my conversion  proposal is  to minimize the required uh  for 
the pair of coils  inside the tornado drive. IE:
do as much loading out on the ele, and the  least at the feedpoint.   That 
should  increase the feedpoint  Z.
With less coil being used at the feedpoint, the peak V  drop across  each 
coil at the feedpoint  would be way less. 
Also, with a higher  feedpoint Z, the RF current through the pair of coils  
inside the tornado  drive  would be way less. 

The stock F-12  180-B  80m  rotary dipole used aprx 44'  of  12 ga  AL wire 
 on each  ele  half....and  feedpoint Z  was 18 ohms. 
On paper, after  the conversion, the feedpoint Z  should be aprx 30 ohms. 

I was  going to just add the 17.5'  T bars, one per side... and add the  
compressible motor driven coils..and call it a day. But
after looking at it  closer, extra loading, 18'  on either side of the 
feedpoint, could easily  be added, [hb al coils  made of  3003
alloy, or N syle LL   loading].   However,  I see no reference made to 
using capacity  hats  inboard of either LL  or  a coil.
On paper, I see no  reason why it shouldn't work.    It  would sorta amount 
to  a  coil / LL loaded stinger on each end...past the
capacity hat   T bars. 

In any event, the peak V  across the compressible coils  at the feed point 
is just wicked. [ longitudaly]  On either side of the  center
insulator, the insulation will have to be beefed up.  I will  place a disc 
dead center in the stock insulator,  to increase
the peak  v  breakdown.  I'll also  wrap the inner ends of the 2" al  
tuning with teflon sheeting, before sliding the grey
pvc slotted   tubes over the 2"  al  tubing.    When the  compressible  
coils are added, the balun/hairpin/coax then goes to
the  center of the pair of compressible coils.  The stock, single  original 
 insulator is no longer used as the feedpoint, however, it  still
has to remain.   

Does anybody have experience with  placing capacity hat bars  INboard of 
coils / LL ?? 

Tnx...  Jim    VE7RF       

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