Linked dipole

After my wife bought me a QMX¹ a little while ago – a very early birthday present – the time arrived to spend a little more time in getting a proper antenna. I already had an end-fed half-wave (EFHW) for 10-20, and a 17-foot (about 5m) vertical telescoping whip antenna with radials. This is not including a coil for 40m for the vertical, or the coil for 40m on the endfed.

As the QMX version I received was the mid-band version (60m, 40m, 30m, 20m, 17m and 15m), this would limit me to pretty much 20m, 17m, and 15m, of which I am only really familiar with 20m. All lower frequency bands would be impossible to use.

I had spent a little time researching antennas and finally found someone describing a linked endfed, which led me down a rabbit hole and deciding I’d make my own linked dipole. I designed my own balun, designed and printed the dipole center to fit snugly over my Spiderbeam mast and got the links provided by DC6GF.

Third iteration of the dipole center and is now completely flat. Prints in half the time as the original and is probably stronger.

Then I spent an entire sunday dropping and re-raising the mast to tune all the individual links. My bike stand was super useful for this.

After lowering the mast section-by-section once, I found out that I could simply lay the mast down – it is only supported on two sides. After that I could simply walk up to the links that I was tuning and shorten them appropriately.

Combining the links was done by simply stripping a tiny bit of the insulation and taping the ends together. I had already looked at getting a soldering iron in the field but this worked absolutely fine for setting it up.

The formula below really helped, just put in the frequency (in MHz) that you measure the lowest VSWR at and the frequency (in MHz) you want the lowest SWR at and then divide by four because you’re dealing with quarter-waves. Always keep some error margin, as the velocity factor of your wires will change the actual length needed. Cut off about 80% and try again.

(\frac{300}{f_{measured}} - \frac{300}{f_{desired}}) \div 4

For example, when trimming the dipole for 20m CW (say 14.050) and the SWR currently dips at 14.000 you can take off about 2 cm off each side of the dipole. If the measured dip is currently at 13.8, it’s more like 10 cm.

Or for 60m, the SWR dipped at about 4.900 MHz for me, when 5.355 is desired, this meant I could chop about 1.30m off each side (I started with 1m to be safe).

The SWR + Smith chart for all bands are below, stored by the NanoVNA onto the SD card (cool feature).

I just need to solder the banana plugs onto each end.