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

[David Gilbert]

In my opinion, that would be counterproductive.

You don't get inductance out of a coil without current through it, and a 
coil will contribute the most shortening effect along a linear radiator 
wherever the current is highest.  For a dipole, a coil adds the most 
shortening effect at the middle and the least out at the ends.  You can 
easily see this in EZNEC as well.  The trouble is, for effective 
radiation you want the greatest amount of current spread across the 
greatest amount of physical length, which is why a good coil loaded 
dipole puts the coil out some distance from the center to achieve a kind 
of compromise between the two considerations.

The opposite is true for capacity loading.  Maximum charge displacement 
(current) occurs where the voltage is highest, which would be out at the 
ends of the antenna.

Putting a capacity hat inside the coil makes both of them less effective.

73,
Dave   AB7E



On 12/5/2010 7:51 AM, Jim Thomson wrote:
> 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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