End Fed Half Wave Antenna Calculator

The end fed half wave has become the default first HF antenna for one practical reason: it needs one support instead of two, and its feedpoint sits at ground level where a coax connector is easy to reach. On a suburban lot with a single tall tree, that is often the difference between an antenna and no antenna. The price is a transformer that loses a fraction of a decibel and a common-mode problem that has to be handled rather than ignored.

What is actually happening at the end of the wire

Current and voltage are not distributed evenly along a resonant antenna. On a half-wave wire, current is maximum at the centre and falls to nearly zero at the ends, while voltage does the opposite: minimum at the centre, maximum at the ends. Impedance is voltage divided by current, so it is roughly 73 ohms at the centre and several thousand ohms at the tip.

That is the entire problem the design solves. A dipole is fed at the low-impedance centre, which is why 50 ohm coax connects to it directly and the match is nearly perfect. An end fed half wave is fed at the high-impedance end, which requires a transformer to bring the impedance down to something coax can carry without enormous standing waves.

The transformer ratio comes out of arithmetic. Impedance transforms as the square of the turns ratio, so a 7 to 1 turns ratio gives a 49 to 1 impedance ratio. Fifty ohms times 49 is 2450 ohms, which sits in the middle of the range a real end fed wire presents. Some designs use 64:1 for wires that measure higher, and a few selectable transformers such as the GOOZEEZOO 150 W end fed antenna transformer$29.90 offer both.

Length by band, and the bands you get free

End fed half waves are resonant on even harmonics of the fundamental, which is the opposite of a centre-fed dipole and is the reason a single 40 metre wire covers four bands. The harmonic resonances are not exact, so most installations still want a tuner on at least one band, but the SWR is usually low enough to transmit without one.

The 80 metre wire is the one most people want and the one most lots cannot hold. At 130 feet it needs a genuinely long run, though it does not need to be straight: an end fed tolerates a dog-leg or a sloping run far better than its reputation suggests, because the high-current portion is near the transformer end and the far end can go wherever it fits.

The counterpoise question, settled

Marketing copy for these antennas frequently says no counterpoise or no radials required. That is true in the sense that the antenna will transmit without one and false in every sense that matters.

A transformer needs a return path. If you do not supply one, the outside of the coax shield becomes it, and the feedline is now part of the antenna. The symptoms are specific and recognisable: the SWR changes when you touch the radio or move the coax, you get RF burns from the microphone, computer equipment in the shack misbehaves on transmit, and signal reports change depending on where the coax runs. The antenna still works. Your shack has become part of it.

Two fixes, and you want both:

1. A short counterpoise wire at the transformer, typically about 0.05 wavelength on the lowest band, which the calculator above gives. It can lie on the ground or run along a fence. Its job is to give the transformer something local to work against.
2. A common-mode choke about a quarter wavelength down the coax from the transformer, which is the current maximum point on the shield and therefore the most effective place to put an impedance in its way. Twelve turns of coax on an FT-240-31 ferrite toroid core$11.67 gives several thousand ohms across 3.5 to 30 MHz, which is enough. A commercial choke such as a Palomar Engineers feedline common-mode chokenot on amazon does the same job in a sealed package.

The full diagnosis and cure is on common mode current and RF in the shack, and the choke turn counts by band are on the balun and choke selector.

Building one, or buying one

A homemade end fed half wave costs about $50 in parts and takes an evening. A commercial one costs $90 to $200 and arrives weather sealed with a known-good transformer, which is the part hardest to get right at home.

The build list

• The transformer. A Compact end fed half wave transformer module$15.24 is the cheapest working option for QRP and low power. For 100 watts, a GOOZEEZOO 150 W end fed antenna transformer$29.90 rated 150 W with selectable 49:1 and 64:1 ratios is the sensible choice.
• The wire. A Paladin 14 AWG stranded copper wire, 500 ft$74.50 spool builds an 80 metre end fed with enough left for a counterpoise, a second antenna and spares, which works out cheaper than buying two commercial antennas.
• An end insulator. Nylon dog bone antenna insulators, 10-pack$29.97 for ten nylon ones, which is four antennas' worth and does not shatter in cold the way ceramic can.
• Support rope. QNR 3/16 in polyester antenna rope, 500 ft$49.73 in UV-stable polyester rather than the cheap nylon that turns to dust in two seasons.
• A way to get the rope up. A Forester arborist throw line kit$21.99 puts a line over a branch fifty feet up from the ground, which is both safer and more accurate than a ladder. An EZ Hang antenna launchernot on amazon does the same job with a slingshot and a reel if you put up wires often.
• A choke. An FT-240-31 ferrite toroid core$11.67 and twelve turns of your coax.
• Weatherproofing. Self-amalgamating rubber splicing tape$9.99 as the first layer over the connector, then Hand-moldable sealant tape$9.69 in the gap, then vinyl over the top.

Ready-made options

The JYR8010 8-band end fed half wave, 150 W$89.99 covers 80 through 10 metres at 150 watts with a 1:64 transformer, which is the straightforward complete answer for a first HF antenna. The MFJ-1982LP 80 to 10 m end fed half wavenot on amazon is the long-standing reference at 300 watts, sold direct by its maker rather than through the usual retail channel. For portable work the PackTenna Mini 40 m end fed half wavenot on amazon weighs under four ounces on a winder and is built for carrying up a hill. All of them are compared on HF wire antennas.

Getting it up, and how the shape affects it

The two common configurations both work and behave slightly differently. A sloper runs from a transformer near ground level up to a single high support, and it favours the uphill direction slightly while keeping the whole install one-ended. An inverted L runs vertically up a mast for as much height as you have and then horizontally, which adds useful low-angle vertical radiation on the lowest band.

Height matters less on an end fed than on a dipole, because the high-current region is near the feedpoint end rather than the middle. That is genuinely useful: it means a wire whose far end sags to fifteen feet still radiates well if the transformer end and the first third of the wire are up. Do not take that as licence to put the whole thing at head height. Read the RF exposure section below first.

Trimming it so you only do it once

1. Cut the wire two to three percent long, which is about eighteen inches on 40 metres.
2. Install the transformer, counterpoise and choke as they will finally sit.
3. Raise the antenna to its final height and final shape. Trimming it at ground level wastes the afternoon.
4. Sweep with a NanoVNA H4 vector network analyzer and find the actual minimum on the fundamental band.
5. Trim from the far end only, an inch or two at a time on the higher bands and six inches at a time on 80 metres.
6. Re-sweep after every cut. Wire that has been cut short cannot be uncut.

Expect the harmonic bands to land slightly off resonance even when the fundamental is perfect, because the harmonic resonances of an end fed wire are not exact multiples. A reading of 2 to 1 or 2.5 to 1 on 15 metres with a perfect 40 metre dip is normal and entirely usable. If you want every band flat, that is what a tuner is for.