Making a 160m antenna for a small backyard

Antenna’s for top band can be really big. Having a small garden of 8 bij 8 meters I wanted to research if I could make a relatively small antenna capable of making local contacts on 160 meters. I decided to make a loaded vertical using a Spiderbeam fiberglass pole of 12 meters long with a ground system which will be nearly invisible.

Ground System
Loading Coil
Cable Connection
ground system
loading coil
cable connection

The ground system consist of a 2 meter copper tube which has been driven into the ground with a hammer drill to it’s full length. This tube is connected to 2 ground radials of 10 meter length hidden between the vegetation. They are all connected to the outside shield of the coaxial cable.

The loading coil is made of 0.8 mm enameled wire on a 80 mm PVC tube form. The coil has 44 closely wound windings and measures 180 μH at 1 Mhz.

From the center conductor of the coax cable, 2 meters of wire go up a 12 meter Spiderbeam pole, where the wire ia attached to the coil. The other side of the coil is connected to 10 meters of wire which goes all the way to the top. A short piece of RG-58 coaxial cable is coiled around a ferrite core to block any RF on the shield due to possible mismatch.

NanoVNA Measurement
first measurement

This antenna would resonate at 1.4 Mhz with an impedance of 69 Ohm, measured with a NanoVNA. I then entered the dimensions at giving me a coil inductance of 194 uH, close enough!

There are several ways of moving the resonance frequency to 1.8 Mhz. We could change the inductance of the loading coil by taking several windings off, but I decided to move the coil upward. This should give me better efficiency but it may also cause trouble because the bandwidth will decrease.

Playing around with some numbers on the above website I decided to move the coil 4.5 meters up (see drawing on the last page). This moved resonance to just above 1.8 Mhz. I then measured the VSWR with 20 Watts output. This vertical has a bandwidth of about 30 kHz

with a VSWR lower than 1:2.

VSWR measurement
Vertical in the
                evening sun
VSWR measurement
vertical in the evening sun

Some tips if you want to try an antenna like this:

Here is a drawing with sizes for this antenna:

Drawing with