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
and
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
answers!
73,
Gerald K5GW
In a message dated 12/5/2010 8:51:38 A.M. Central Standard Time,
jim.thom@telus.net 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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