It's well-known that under certain conditions verticals and dipoles can gather static charge, which is a common reason of transceiver damage. There are several ways to solve this issue. Some hams suggest to place a 100 kΩ high-power resistor to the feed point (not an ideal solution, because the charge dissipates slowly). Others suggest to use a coil with inductance about 200 µH instead of the resistor. The inductance works as a short circuit for DC and has little effect (high resistance) on HF.

Here is what surprises me. I don't believe I ever encountered an advice to use a regular 1:1 transformer, nor could I find such an advice online. It isolates the feed line from the antenna and works as a short circuit for DC. This is probably the simplest solution I can think of.

Assuming that power losses in the transformer are acceptable, and that it's placed after 1:1 current balun (which eliminates common-mode current), is there any reason why it might be a poor solution?

  • $\begingroup$ Why a transformer? What did you have in mind? A lower value resistor in parallel with your feedpoint as a static drain is what most hams use. $\endgroup$ – Mike Waters Feb 20 at 22:40
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    $\begingroup$ Please include a schematic of your design. $\endgroup$ – Mike Waters Feb 20 at 23:11
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    $\begingroup$ I suspect the 1:1 transformer is redundant with the current balun. Depending on how the balun is wound, it might actually bleed static all by itself. $\endgroup$ – user10489 Feb 21 at 1:13
  • $\begingroup$ @user10489 I'm not aware of any current baluns that bleed the static. If you know such a balun, please give a little more details. From what I know only voltage baluns do this. $\endgroup$ – Aleksander Alekseev - R2AUK Feb 21 at 6:22
  • $\begingroup$ This is fort enough to not warrant a full answer, but such transformers are called isolation transformers: en.wikipedia.org/wiki/Isolation_transformer $\endgroup$ – Liam Clink Feb 24 at 22:08

Assuming that power losses in the transformer are acceptable, and that it's placed after 1:1 current balun (which eliminates common-mode current), is there any reason why it might be a poor solution?

Well, the assumption about loss may not be a great assumption to make. It takes a substantial core to avoid saturation and overheating even at modest transmit powers, and then you've necessarily introduced some amount of loss.

Of course designing a suitable transformer isn't impossible, some might not even consider it difficult. It's just costly compared to other solutions, which I suspect is why it's not often recommended.

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  • $\begingroup$ What about receivers? For me, I have been using an active mixer with isolated baluns for a while, since baluns were already needed, I got protection for "free", and haven't seen ESD damages to this day (but ESD did damage my radio one day when I wasn't using my mixer...). BTW, one MiniCircuits note said radios are vulnerable if RF/LO ports are swapped to allow DC. Thus, IMHO, signs indicate a XFMR can be protective, but my guess is there's no vendor guarantee - most XFMR aren't designed as protective devices, thus not recommended. I doubt those tiny SMD transformers can survive a large ESD. $\endgroup$ – 比尔盖子 Feb 21 at 2:46

A transformer can help and could also make things worse. I have some bad experience with this.

The way that a radio is damaged by static is simple:
Something charges up to a high voltage, and then there's a sudden breakdown resulting in a high voltage on the first transistor of the receiver.

This can happen in a few ways:

  1. A long-wire or monopole type antenna is not grounded, and the radio has a decoupling capacitor before the front-end amplifier. When the antenna charges up to a few hundred volts, the capacitor breaks down and the charge surges into the transistor.

  2. A dipole type antenna is connected with a transformer, where the antenna side of the transformer, and the antenna itself, are not grounded.

  3. Someone already charged, walks up to the antenna and touches it.


simulate this circuit – Schematic created using CircuitLab

In both cases, even if the receiver has an inductor or resistor for DC grounding, it will be damaged because the impulse caused by the breakdown or spark is so fast.

It isolates the feed line from the antenna...

You can see that there are cases that adding a transformer could actually make things worse, by isolating a the antenna and allowing it to charge up. Eventually it will jump across the transformer insulation...

... and works as a short circuit for DC.

If there is something isolated that can be charged up, then the DC short on the radio side will mean nothing. The breakdown is an impulse and its initial peak will go right past the DC short.

The solution in cases 1 and 2 is to prevent anything in or near the antenna from becoming charged up. This could be done with a resistor or inductor to ground, or by shorting the centre tap of the transformer. Transmission-line transformers are generally DC short throughout, so that solves it completely.

In case 3, someone touching the antenna, the grounding is of limited use. You can add fast limiter diodes to the input - these are widely available, have low capacitance (0.2 pF maybe) and can handle the full current of a human body discharge. They clamp at just a few volts though, so aren't appropriate for transmitting antennas. A bandpass or highpass filter will also help to reduce the energy that reaches the receiver.

None of this is advice about lightning protection, though some of it may help a bit in the case of distant lightning.

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The answer is no.

The best way to protect against static build-up would be to have both antenna elements DC-connected to ground.

This would be automatically ensured with a 1:1 voltage balun.

enter image description here

With a 1:1 current balun, only one element would be DC-connected to ground. Hence a resistor would be required to be connected between the two elements.

enter image description here

With the 1:1 transformer suggested, both the elements would not be DC-connected to ground.

enter image description here

Hence it would offer no protection.

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