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

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.

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 https://www.wireantennas.co.uk/antenna-loading-coil-calculator 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

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

• You need an antenna analyzer when building it. I have used a cheap NanoVNA bought on Ebay. You can find them for around 50 Euro.

• Make sure you make the diameter of the coil more than 1.5 times the length. It will make the Q factor of the coil larger and the antenna loss smaller.

• Don’t wrap the antenna wire around the Spiderbeam pole, try to keep the wire free. Also, don’t put the pole through the coil. Because the bandwidth of this antenna is so small any disturbance near the vertical wire or coil will have an effect on the VSWR.

• Try to keep the vertical free from surrounding antenna’s. I discovered a rise in VSWR with a doublet antenna at a few meters distance.

• Solder all your connections and don’t use screws or shrink terminals. They will cause trouble over time due to oxidation.

• This antenna is a compromise and it will never match a full size vertical with a good ground system. But it will be good enough for making nearby QSOs.

Here is a drawing with sizes for this antenna: