Towertalk
[Top] [All Lists]

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

 To: jim.thom@telus.net, towertalk@contesting.com Re: [TowerTalk] Can capacity hats be installed inboard of a coil or LL ?? TexasRF@aol.com Sun, 5 Dec 2010 10:49:57 EST mailto:towertalk@contesting.com>
 ```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 _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread Re: [TowerTalk] Can capacity hats be installed inboard of a coil or LL ??, TexasRF <=