|
Baluns?
|
Baluns?
In message , John Smith
writes JB wrote: ... There ya go. One of the problems of breaking up the path to ground on the shield is that now static can build up. If you can provide a way to bleed off the charges that build up on both halves, while breaking up the shield currents, now you have made it worthwhile. But when you guys start discussing off center fed dipoles, I step aside. I have computers in the shack. I have better luck with fan dipoles. Also running a Butternut vertical. Works a lot better than a 4btv, but a pain to get it right on all bands. Thanks Also liked the other pdfs presented. I never have time to read them all because of constant interruption around here. Back to work. OCF antennas were not fully appreciated by me, a few of Cecils' helpful insights and encouragement and I built one and was rather surprised ... I do not have one right now but that is only due to neighbors/property constraints. Here is a URL for design/implementation of "non-standard" baluns/transformers, but of a highly useable and desirable nature--or, Dr. Sevick strikes again!: http://www.highfrequencyelectronics....104_Sevick.pdf Fig. 6(A) is very interesting. A 5-winding, 1:1.56 bootstrap transformer which provides 50/75 ohm connections/substitutions. Perfect for allowing one to use 75 ohm "junk" (or found in dumpsters) tv coax in place of more expensive 50 ohm coax. I have made good use of this since I have thousands of feet of NEW 75 ohm coax I purchased from a scrap dealer for next-to-nothing! A lot of large dia coax and hard-line mixed in! Regards, JS What about the loss in the balun? You need to decide if the loss in the balun is less than that you would get if you simply used the 75 ohm coax direct. And ask yourself 'In my application, why would 75 ohm be more lossy?' Also, with similar physical dimensions, construction etc, 75 ohm coax is slightly less lossy than 50 ohm (current is less, so 'I squared R' loss is less). -- Ian |
Baluns?
Ian Jackson wrote:
... What about the loss in the balun? You need to decide if the loss in the balun is less than that you would get if you simply used the 75 ohm coax direct. And ask yourself 'In my application, why would 75 ohm be more lossy?' Also, with similar physical dimensions, construction etc, 75 ohm coax is slightly less lossy than 50 ohm (current is less, so 'I squared R' loss is less). Simple solution, I trust my S-meter! Regards, JS -- It is like a nightmare where the public servants are the people which the police are supposed to protect us from! |
Baluns?
Richard Clark wrote:
... How true - not even fundamental terminology. 73's Richard Clark, KB7QHC When you are right, you are right; and, YOU GOT THAT ONE RIGHT! :-) Regards, JS -- It is like a nightmare where the public servants are the people which the police are supposed to protect us from! |
Baluns?
wrote:
I disagree. There is no choking, there is merely a polarity and phase shift between the "primary" and "secondary" of the 1:1 balun "transformer". In a w2du balun, zero flux in the beads is introduced by the transmission line currents. All of the flux in the beads is introduced by common-mode currents. Thus the impedance of the beads *IS* a choking impedance aimed only at the common-mode currents. My choke-balun is ten turns of RG-400 on an FT-240-77 toroid. It's only purpose is to provide a choking impedance to common-mode current. That choke-balun doesn't know if it is being used as a UNUN or as a BALUN. Most baluns are designed such that near-zero flux is introduced in the ferrite toroid by the differential mode currents. Essentially, the only flux introduced in the toroid is due to common-mode. The resulting impedance *IS* a choking impedance. -- 73, Cecil http://www.w5dxp.com |
Baluns?
wrote:
By the way, I have seen so many articles about baluns written by other hams and they tend to repeat the same mistakes and assumptions. Most hams do not understand how a balun works. Your following statement puts you in that category. Inserting a balun does not "choke" the current in the shield, it merely shifts the output phases so that the current (voltage) is directed towards the dipole at all times (see my other post in this thread). This applies to a W2DU balun. Why would you want differential current fields to be introduced into the #77 ferrite beads? Their entire purpose is to provide a large choking impedance and dissipate common-mode power. -- 73, Cecil http://www.w5dxp.com |
Baluns?
wrote:
Many US amateurs do not understand how a balun works. Quoting "The ARRL Antenna Book" re a w2du balun: "Maxwell made a test balun by slipping 300 #73 beads over a piece of RG-303 coaxial cable. The impedance of the outer conductor of the cable measured 4500+j3800 at 4.0 MHz." The differential current emerging from the inside of the coax braid encounters, e.g. 30 ohms, looking toward the antenna. It encounters 4500+j3800 = 5890 ohms looking back down the outside of the coax. Ohm's law does the rest. Without the beads, that differential current might see an impedance lower than 30 ohms looking back down the outside of the coax for certain unfortunate lengths of coax. The common-mode choking impedance forces ~equal currents in each dipole leg at the BALanced antenna to UNbalanced coax junction. That common-mode choking impedance causes the balun function to occur. -- 73, Cecil http://www.w5dxp.com |
Baluns?
Cecil Moore wrote:
Most baluns are designed such that near-zero flux is introduced in the ferrite toroid by the differential mode currents. That should have been, "Most 1:1 current baluns ..." -- 73, Cecil http://www.w5dxp.com |
Baluns?
Richard Clark wrote:
wrote: Many US amateurs do not understand how a balun works. How true - not even fundamental terminology. http://en.wikipedia.org/wiki/Balun (see 1st photo) "This is a simple RF *choke* which works as a *balun* by preventing signals passing along the outside of the braid." From "The IEEE Dictionary": "balun (1) A network for the transformation from an unbalanced line or system to a balanced line or system, or vice versa." From an Unabridged Webster's: "balun - a device for converting a balanced line into an unbalanced line and vice versa." 10 PRINT "A W2DU balun is both a choke and a balun!" 20 Goto 10 -- 73, Cecil http://www.w5dxp.com |
| All times are GMT +1. The time now is 05:11 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com