Jim, I'm not advocating bead chokes; I fully appreciate the benefits of "N^2" on a toroid core and the ability to put the SRF somewhere useful because of the increased stray capacitance. Rather, I am
Jim, Thank your for the explanation. As you say, we see things differently - I would never view 160m, 80m and 40m as comprising more than half our HF spectrum allocation! 73, Steve G3TXQ ____________
Jim, Sorry to disagree again, but the CM choke dissipation DOES also depend on the SWR! Say we had a perfectly balanced 50 Ohm antenna being fed 1kW - the differential voltage at the feedpoint would
Jim, Let me try to explain it more simply. Picture Roy Lewallen's "classic" diagram showing how the differential-mode current flowing on the inside of the coax braid splits two ways at the feedpoint
Gerald, Exactly so - simple Ohm's Law, and easy to confirm with modelling! 73, Steve G3TXQ _______________________________________________ _______________________________________________ TowerTalk ma
I was contacted privately on this issue by someone whose engineering ability I well respect. He confirmed what I have been saying, but suggested that the link between choke dissipation and SWR is "be
Gerald, That sounds like a very similar analysis to this one from Jim: "What if the choke is on the output side of an antenna tuner (for example, up in the air at the feedpoint, or somewhere along th
Dave, Too true! But fortunately on this issue the theory, the modelling, and the reported practical observations all agree - changing the differential load impedance affects the choke dissipation. St
If the system is perfectly balanced there is no common-mode voltage and you don't need a choke. Once the system is unbalanced, the common-mode voltage depends directly on both the degree of imbalance
Gerald, If the addition of the choke reduces the CM current to a negligible amount, you have restored balance to the system. Did that put the chicken first, or the egg first? I don't know! Steve G3TX
Tod, Your reasoning is correct - you can have zero CM current (perfect balance) and high SWR, and you can also have high CM current in a perfectly matched system (SWR=1:1). However, in an unbalanced
Tod, Typical SWR meters are affected by any CM current that is present, and because the CM signal is a standing wave it can cause the meter to read differently for different cable lengths. In fact if
David, For a fixed feedline Zo, a change in SWR *always* indicates a changed load impedance; and a change in load impedance always causes a change in choke dissipation unless the system is perfectly
Jim, I may be wrong that it is a *direct* effect of the presence of CM current. Another explanation would be that changing the coax length alters the CM path impedance, that changes the net impedance
Tod, Picture a dipole with no feedline; say the feedpoint impedance is 75 Ohms (SWR=1.5:1). Suppose we now connect a length of coax a multiple of a half-wave long, which is well earthed at the shack
Gerald, You analysis is correct! Of course, the CM impedance "looking into" the coax braid at the feedpoint varies depending on the coax length: if the coax is well grounded at the shack end and is a
On my web site I explain how a predominantly reactive common-mode choke has the potential to increase CM current if the reactance of the CM path is of similar magnitude and opposite sign to the choke
Joe, If the coax in the shack sees a low impedance path to ground, and it is an odd multiple of a quarter-wave up to the feedpoint, very little CM current flows; and for any that does flow the shack
Frank, The centre conductor of the coax and the inside surface of the braid flow currents that are balanced - equal in amplitude but with a phase difference of 180 degrees; they wouldn't produce any
Tod, Good points! I originally used a Log scale for the Y axis but it couldn't handle the change in sign of the reactance, which of course is critical to the discussion. In the end I opted for Linear