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To my comprehension a BALUN/UNUN is an RF Transformer. If this is correct and since a BALUN/UNUN is part of the antenna system ... how much power does one expect to lose there?

Specifically, how much sense does it make to compromise ERP in lieu of band choice when a rig is capable of perhaps 15W at best?

VU2NHW
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2 Answers2

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A balun is any device for converting between balanced and unbalanced systems. Not all of them are transformers.

Losses depend on the kind of balun. On HF, it's common to use a common-mode choke as a balun. Losses on this type of balun are quite low: just the additional loss of the transmission line wound around the ferrite. It will depend on your transmission line and frequency, but this is likely less than 0.1 dB. If you want a more specific number, the spec sheet for your feedline will have a loss specification.

Phil Frost - W8II
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  • I'm looking at brewing a ladder line; no spec sheet. – VU2NHW May 25 '21 at 15:24
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    @VU2NHW I don't think you're going to wrap ladder line around a ferrite. In any case, at HF, most of the losses are resistive, and the losses will be mostly impacted by your selected wire gauge. So just don't use tiny wires. – Phil Frost - W8II May 25 '21 at 16:06
  • The balun will sit between the coax, and the ladder line.... – VU2NHW May 31 '21 at 23:22
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    @VU2NHW The loss is determined by the transmission line you use to wrap around the ferrite, which is not going to be the ladder line. – Phil Frost - W8II Jun 01 '21 at 02:22
  • Wrapping twisted pair around a ferrite is common, and that's very similar to ladder line. While a lot of loss can occur in the feed line, the ferrite also can have high loss, although usually you pick a ferrite designed to minimize loss at your target frequency. Loss in the ferrite comes from (in part) eddy currents and saturation, but also magnetic resistance. Hopefully your QRP can't saturate the ferrite! – user10489 Jun 02 '21 at 14:56
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    @user10489 For a common-mode choke, you often pick a ferrite to maximize loss. The ferrite plays no role in differential-mode power transfer because only the common-mode currents create magnetic flux in the core. Maximizing loss in the ferrite maximizes common-mode impedance, which is the design objective. Your concerns would be applicable to an RF transformer but not a common-mode choke. – Phil Frost - W8II Jun 02 '21 at 17:26
  • Phil -- you are partly right and partly wrong. If you wanted to maximize loss, you could just use a solid iron core instead of powdered ferrite. You definitely want to minimize eddy currents, but you also want to maximize magnetic resistance. AC power transformers typically use laminated sheets instead of powder, so that's doing a min/max on other effects as well, probably including maximizing magnetic conductance. – user10489 Jun 02 '21 at 17:30
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    @user10489 isn't maximizing magnetic resistance the same as minimizing permeability? What's a core material with low eddy currents and low permeability? Air perhaps? Is air a common core for common-mode chokes? – Phil Frost - W8II Jun 02 '21 at 19:50
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It depends. A choke and/or transformer between the feedline and the antenna changes the ratio of currents flowing on each, and thus where the EM radiation and thermal losses occurs. In some situations (SOTA QRP with a poor ground, short low antenna, low shock hazzards), additional EM radiation caused by unbalanced currents over a longer feedline might add up to send a better signal towards some destination. But with a better antenna (in terms of radiation resistance and directional gain) or a more lossy feedline, less likely so.

hotpaw2
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    A choke doesn't change current ratios. (A transformer does.) The choke prevents the feed line from becoming part of the antenna, which is why there is EM radiation coming from your feedline -- it's part of the antenna. – user10489 Jun 02 '21 at 14:51
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    A choke can reduce the amount of current flowing on the feedline in the near vicinity of the choke. That current would otherwise flow somewhere on the antenna. And a choke may or may not prevent the feed line from becoming part of the antenna. That depends on the routing of the feedline, any RF coupling to the antenna, and its length, etc. – hotpaw2 Jun 02 '21 at 20:01
  • True. I should say the choke reduces common mode current, and sufficient reduction would prevent the feed line from becoming part of the antenna. Unless it parasitically couples in some area not adequately blocked by the choke ...but that's starting to get long and pedantic. – user10489 Jun 02 '21 at 20:18
  • Does HF circuitry become more or less magical when you realize that random disconnected bits of properly sized and positioned metal can become part of the circuit? – user10489 Jun 02 '21 at 20:20