我們什麼時候想在火腿收音機中使用電壓不平衡變壓器?


8

在一個共模電流為敵的世界中,我們一直在竭盡所能避免在棚屋中出現射頻,為什麼我們會選擇電壓型不平衡變壓器而不是電流不平衡變壓器?

我了解到,通常,人們會使用電壓不平衡變壓器來保持電壓相等和相反。但是在火腿收音機中在現實世界中應用的例子是什麼?

即使DX Engineering網站說,當前的巴倫幾乎總是更好的選擇。那麼,為什麼他們甚至出售電壓不平衡變壓器呢?電壓不平衡變壓器是最佳選擇,這種罕見的情況是什麼?

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You'll find voltage baluns in plenty of circuits. For example,

the QRP Labs receiver module:

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HPSDR Pennywhistle (twice, T1 and T3):

enter image description here

The Elecraft KX3:

enter image description here

A voltage balun can do some things a current balun can not.

Firstly, the turns ratio can be varied to provide impedance transformation. Some of these circuits use something other than a 1:1 turns ratio to optimally load the transistors.

Also, a voltage balun provides galvanic isolation. This can be useful for example to avoid ground loops which might be problematic with audio interfaces. It also allows a DC bias to be added: all three of these designs make use of that feature.

One advantage of a common-mode choke as a balun is the core needs to be sized only to handle the common-mode current, not the full current of the signal. This is a significant advantage when designing a balun for a 1 kW antenna, but in a receiver or at the output of a 20 W amplifier, this particular advantage isn't of much value.

Furthermore, in these applications the load is a circuit with equal impedances, and not an antenna which could be mounted asymmetrically or even deliberately unbalanced such as an off-center fed dipole. Thus another advantage of current baluns is again not especially needed in this application.


As for why companies continue to sell voltage baluns for antennas, I suspect they do so because people continue to buy them. A solution does not need to be technically the best to be economically viable.

I haven't done a rigorous cost analysis, but I'd suspect 4:1 voltage baluns may be cheaper to manufacture:

  • they require only 1 core where a 4:1 current balun requires 2
  • for powers of 100W or less, the necessary core may be smaller than what's required to get a useful choking impedance from a common-mode choke

A voltage balun works fine when the load really is balanced. This isn't always the case so a voltage balun may not be a robust solution, but there will always be buyers that value low cost over robustness, or may not even be aware of the issues. So I'm not surprised manufacturers continue to make voltage baluns.


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I attended a seminar on baluns --as applied to antennas-- at a local hamfest, given by no less a guru than Glenn Shultz, W9IQ.

He nicely explained that the only place we need to use a voltage balun in an antenna system is for end-fed antennas.

Every place else, use a current balun. And the simpler, the better.