Even an Inverted-V Needs a Choke

A coax-fed monoband inverted-V often looks like the perfect “simple” antenna: two equal legs, center fed, tuned until the SWR looks great, and many times it lands somewhere near 50 Ω.

It will usually radiate and you’ll make contacts. But without a choke at the feedpoint, you often end up building something slightly different than you think: an antenna system that includes part of the coax feedline.

That’s why even an inverted-V benefits from a 1:1 current balun (common-mode choke) at the feedpoint.

What the choke is actually doing (in plain language)

Inside coax, the wanted transmit/receive current flows as a normal transmission-line mode:

• current on the center conductor
• equal and opposite return current on the inside of the shield

But coax also has an unwanted path: RF can flow on the outside of the shield. That outside current is what hams usually call “common-mode current.”

A common-mode choke is simply a “high resistance gate” for that outside-of-shield current. It lets the wanted signal inside the coax behave normally, while strongly discouraging the unwanted current on the coax exterior.

“But my inverted-V is symmetrical!” (why it never really is)

A key idea: geometric symmetry is not the same as electrical symmetry.

Your inverted-V may look perfectly balanced (same wire, same length, same angle), but the environment makes each half “see” something slightly different:

• one leg is over wetter ground than the other
• one leg is closer to a tree, gutter, fence, mast, roof edge, or power line
• wind changes the angle/height of one side more than the other
• rain, fog, and humidity change the dielectric environment unevenly
• ground conductivity is not uniform ... there is no perfect “average ground” under a real antenna

Result: the two legs don’t carry perfectly equal and opposite currents. The coax exterior becomes the “easy extra path” the system uses ... and that’s when the feedline starts participating.

The “X = 0” point: what it means ... and what it doesn’t

Antenna impedance is commonly written as:

Z = R + jX

• R = resistive part (radiated power + losses)
• X = reactive part (stored energy in electric/magnetic fields)

When people say “the antenna is resonant,” they usually mean X = 0 at that frequency.

Here’s the trap: X = 0 does not mean “perfectly balanced.” You can have a beautiful resonance (X crosses zero) and still have:

• unequal currents on the two legs (imbalance)
• RF current on the coax exterior
• pattern distortion

Resonance is about reactance cancellation ... not about guaranteeing symmetry. And the X=0 frequency moves with weather, moisture, vegetation, and nearby objects.

“Then there is Z ...” why it’s rarely exactly 50 Ω

Many people talk as if a dipole is “50 Ω when cut right.” Reality is messier.

A straight half-wave dipole in free space is typically around the 70–75 Ω range, and real installations shift that number depending on height, ground, and surroundings.

An inverted-V often lands closer to 50 Ω because bending the legs downward changes coupling and typically lowers feedpoint impedance ... but “close” is doing a lot of work:

• height above ground changes impedance
• nearby objects change impedance
• moisture and weather change impedance
• feedpoint hardware and coax routing change impedance

So even on a single band, Z is rarely exactly 50 + j0 Ω at the feedpoint all the time. That’s another reason the feedline can get “invited into the party” unless you block that outside current.

Why coax behavior matters (and why the feedline can start radiating)

Coax is happiest when it behaves like a contained transmission line: fields inside the cable, predictable currents, and no RF on the outside.

In the real world, once the antenna system is not perfectly symmetric and not a perfect 50 Ω situation, the coax exterior can become a convenient return path for “unwanted current.” That’s when your inverted-V starts acting less like “a clean dipole” and more like “a dipole plus whatever shape your feedline happens to take.”

If you want the deeper version of that concept (in practical ham language), this ties directly into: 50 Ω coax: balanced at its design impedance ... unbalanced when it’s not.

What goes wrong if you skip the choke

These are the real-world symptoms that make people finally add a feedpoint choke:

• Your feedline becomes part of the antenna ... and now the “antenna shape” changes depending on coax routing.
• The radiation pattern becomes lopsided ... nulls and lobes aren’t where you expect them to be.
• SWR becomes touchy ... moving the coax (or coiling excess coax) changes what the rig sees.
• RF in the shack ... hot mic, USB glitches, audio distortion, RF bites, “mystery” issues.
• More receive noise ... the coax exterior can also act like a receiving antenna for local noise sources.

So ... where should the choke go?

Best place: at the feedpoint.

That’s where you stop outside-of-shield current right as it tries to start. It’s the single most effective location because it prevents the feedline from becoming an extra radiating element.

Often helpful: a second choke at the shack entrance.

This can reduce any remaining shield current and can also help with receive noise picked up along the feedline on its way back to the station.

How “strong” does the choke need to be?

You don’t need to memorize ferrite mixes to understand the goal: the choke should look like a very high impedance to current on the outside of the coax shield.

(Practical rule of thumb) ... aim for “many hundreds of ohms” of choking impedance on the band of interest, and more is generally better. Exact needs depend on power, band, installation, and how “noisy” the environment is.

If you want a practical “how much is enough” guide for both RX and TX, this is the matching deep-dive: How much choking do you really need for RX and TX.

Builder’s appendix: simple choke options that work

For a monoband inverted-V, you generally have two common approaches:

• Ferrite choke (recommended) ... a proper 1:1 current balun built for the band and power level. It’s mechanically compact, repeatable, and doesn’t rely on “the right coil diameter” to land on the right frequency.
• Coiled-coax “ugly balun” ... can work on a single band, but performance is more frequency-dependent, and coax type/coil diameter/turn spacing can shift results. It’s better than nothing, but ferrite tends to be more predictable.

Regardless of method, the installation details matter:

• Put the choke at the feedpoint (or as close as physically possible).
• Add strain relief ... don’t let the coax pull on the feed terminals.
• Weatherproof smartly ... keep water out of connectors and feedpoint hardware (drip loops help).
• Route coax away cleanly ... don’t run it tight along one leg for meters.

Still unsure if you “need” one or “have enough” choking in your station? This checklist is a good sanity check: Do I have enough baluns.

Bottom line

An inverted-V is popular because it’s forgiving, compact, and often lands near a coax-friendly impedance on a single band. But outdoors, perfect symmetry doesn’t exist, resonance (X=0) does not mean balance, and Z is not a fixed “50 + j0” forever.

A 1:1 current balun at the feedpoint is cheap insurance that makes the antenna behave like the dipole you intended ... not a dipole plus “whatever shape your coax happens to take today.”

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