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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