Shortened dipole for 40 meters

The question comes up on the tower talk mailing list about using a single 30m dipole (actually 15m long) for 40 meters with a matching network. Is the gain significantly less? Is the matching network a problem.  4nec2 answers these questions.  The models were lossless (for the antenna, not for the matching network), and modeled as 10 meter and 7.5 meter long dipoles fed in the center, with the element radius of 1 cm.

Full size is 2.08 dBi, Shorty is 1.75 dBi, mostly because of the slightly wider main lobe for the shortened antenna.


Here's the gains of the full size dipole vs the 75% length dipole (with the matching network):

Gain of 75% sized dipole for 40m

Gain of full size dipole for 40m

Now let's look at the impedance of the short antenna and the required matching network

Impedance of 75% dipole around 7 MHz:
Capacitive, about 30 ohms and -350j to -250j ohms

Impedance with the low pass T feed, tuned for 7.15MHz

4nec2 has a neat feature where it will calculate the values for a matching network.  I picked the low pass T, since I knew I'd need some significant series L to compensate the antenna's overall capacitive reactance.  An L network would also work, as would a tapped inductor.

So the series L (about 7.5 uH) compensates for the capacitive reactance (-300j) and brings the resonance to 7.15 MHz.  Here's the VSWR plot for the matched antenna.  Not too bad, the 2:1 bandwidth is about 350 kHz, the 1.5:1 BW is about 180 kHz.

Let's compare that to the VSWR of the original 40m antenna. which has a wider 2:1 bandwidth, 460 kHz, but a narrower 1.5: 1 BW of abotu 150 kHz (because the best match is only 1.4:1 due to the original antenna being a 72 ohm impedance)