Hi Frank, I just don´t see a reason to make chokes self resonant. If one really needs that a parallel resonant LC circuit can be formed with a normal choke and a capacitor. On my 160m dipole the choke at the feed point is wound with RG 141 on a stack of two large ferrite toroids which gives me a Z of about 10k. In addition the outer conductor is connected to the tower after a quarter wave length and the coax run inside the tower and grounded to one leg. The same tower is direct fed as an elevated GP at the 25m level against two sloping radials with the outer connector bonded to the tower. I never saw any common mode current when the common mode chokes XL was at least 100 times the coax Z.
73 Peter -----Original Message----- From: [email protected] [mailto:[email protected]] Sent: Freitag, 30. März 2018 19:30 To: [email protected] Subject: Re: Topband: 160M Balun Hi Peter, What you need in a common mode choke is adequate resistive component to absorb common mode signals as heat. The choke doesn't need to be at resonance to achieve adequate choking resistance, but it doesn't hurt either. It takes some thought and technical understanding to determine how much common suppression is enough. And common mode chokes aren't the only way to adequately suppress common mode signals. A low impedance radial system provides good common mode suppression. Grounding the coax feed line to a center-fed dipole 1/4 wavelength or 3/4 wavelengths from the dipole feed point also provides good common mode suppression, certainly more than you ever need for a dipole. This assumes that the feed line runs at approximately 90 degrees from the dipole. Pulling the feed line significantly away from 90 degrees significantly increases common mode. An off-center-fed dipole is much worse because its very difficult to build a choke with adequate power handling capability and good choking performance. You can't understand your needed common mode signal suppression without understanding your signal environment and the directive performance you want to achieve from your antenna. For example: A multi-transmitter or SO2R station needs much more attention to common mode suppression than a single operator low power station A highly directive multi-element Yagi needs much more common mode suppression than a dipole or a vertical. Coax feed lines properly installed and bonded to a tower has less common mode signals than a coax feed line suspended well above the ground. Perhaps the worst case need for common mode suppression is directive receiving antennas with very low signal levels such a small terminated loop (K9AY, Flag, etc) or a small "magnetic" loop. Both require extreme attention to common mode signal suppression, including a strong preference for buried feed lines, control lines and power cables. 73 Frank W3LPL ________________________________ From: "Peter Voelpel" <[email protected]> To: "Jim Thomson" <[email protected]>, [email protected] Sent: Friday, March 30, 2018 4:47:44 PM Subject: Re: Topband: 160M Balun Why do you want it resonant? 73 Peter -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of Jim Thomson The 160m CM choke needs to resonate lower in freq. It requires more cores to shift the res freq down. _________________ Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
