Why does QRPGuys EFHW use less than a half wave?

[WB5KHB]

Hello all, thanks for allowing me here, this is my first post. I operate a good bit of QRP from Meridian, MS and various POTA locations. I have a question...I recently purchased a QRPGuys EFHW kit and assembled it. When testing with my antenna analyzer, I had to trim the radiator even shorter than the suggested length on the kit pc board, by maybe a foot or more. Firstly, why do they recommend a length shorter than a halfwave to begin with, and secondly, why would I have to trim even more off? It seems to work ok, with good swr on 20 meters and less than 3:1 on a couple of other bands. Any ideas? is their unun wound differently than a standard 49:1, or capacitor in the circuit affecting it? Thanks!

[N8TGQ]

Did you get your kit directly from QRPGuys? If not, its a cheap Chinese ripoff. Toroid could be anything.

Double-check your windings count. Its easy to be off, especially if its your first time winding one.

What are you using to measure SWR? Did you cut your wire for 20m?

GL de Rick N8TGQ

[SkipRD]

Rick N8TGQ is right. There are lots of boards printed with "QRPguys," including those sold on amazon, that are not made by QRPguys. It's really shameful. Also, that style of board (with a copper ground plate incorporated into the winder) seems to have some quirky performance. There was a thread about these on a qrp message board and the consensus seemed to be that boards without that ground plate seem to have more consistent performance. 73 Skip K4EAK

[Brian - K3ES]

Hello all, thanks for allowing me here, this is my first post. I operate a good bit of QRP from Meridian, MS and various POTA locations. I have a question...I recently purchased a QRPGuys EFHW kit and assembled it. When testing with my antenna analyzer, I had to trim the radiator even shorter than the suggested length on the kit pc board, by maybe a foot or more. Firstly, why do they recommend a length shorter than a halfwave to begin with, and secondly, why would I have to trim even more off? It seems to work ok, with good swr on 20 meters and less than 3:1 on a couple of other bands. Any ideas? is their unun wound differently than a standard 49:1, or capacitor in the circuit affecting it? Thanks!

A few more thoughts:

1. The speed of electricity in a wire will be less than the speed of light in a vacuum, so the length of wire under ideal conditions will be shorter than theoretical.

2. Antenna height and configuration will affect feedpoint impedance (and SWR) leading to the need to adjust length for your set up.

3. Ground conductivity varies by location, also requiring adjustment.

4. CW and digital operators work at the low end of the band, requiring a bit longer radiator, so trimming may be needed to optimize for SSB.

5. It is normal to cut radiators long, so that they can be trimmed back for preferred use. It is much easier to trim shorter than to trim longer.

73 de Brian K3ES