160m mobile antenna project

Mike S Mather ZL2CC

This shows the development of a 160m mobile antenna for 1.855MHz use. I do not usually use computer design programmes and prefer the “trial and error” method. This 160m mobile antenna is built using those principles.

I decided for ease of construction and mobile strength to make the antenna ‘base loaded’ where the loading coil is at the base, rather than a ‘centre loaded’ antenna.

Initially, I cut a 50mm former to wind the coil approximately 500mm long of pressure pipe and used two end caps for mounting the fittings.

I wound 100mm of this with 1mm enamelled copper wire and then the rest with 0.5mm wire (as that is what I had in the ‘junk box’).

On each end I bolted a quick release male and female to suit my other mobile antennas system. I joined these fittings to a bolt through the coil former wall, so that I could adjust the amount of coil easily and to make a neater appearance. I then fitted the top capacitance whip and tested for resonance with a home brew antenna analyser. It was way down in the broadcast band so there was way too much inductance windings.

I removed a lot of the windings and tested as I went. I found that approximately 100mm of 1mm windings and 50mm of the smaller wire gave me a resonance in the 160m band. Therefore the former was way too long. So I cut this down to a manageable size and continued the testing.

During cutting the former I modified the top connection from the quick release to a fixed connection. Initially, I thought I would use my 10m band whip as the capacitance rod but decided a shorter rod and adjustable stainless steel whip would give better adjustment for any frequency changes later.

I carefully took off more windings until it became resonant at 1850 with the chosen top section. Although I was initially hoping to get resonance at 1.855MHz, the 1.850 frequency is used in Hawkes Bay and 1.855 is used in the Gisborne area and my mobile ATU can take care of the mismatch. Or, I could trim the top SS whip.

Essentially it was finished so I taped over the windings and spray painted the end caps to make it look good.

The top section is an old 2m 5/8 wave antenna with a longer piece of stainless whip in the top. I wanted as long a whip as possible to help with the efficiency. It will be very inefficient anyway as it is only 1.8m in total length. Not being an expert in using computer analysis I have no idea what the efficiency would be.

The antenna was tested one evening with a friend (ZL2RC) in Gisborne approximately 20km away and the results were good communications both ways. The test was with 100w from my mobile TS-50 rig. Next is to try a daytime test around the Gisborne area to see how it performs around hills and buildings. It will never be a DX antenna but it was an experiment to see how 160m mobile would perform and for fun. The efficiency will be very low.

The pictures show the stages of the development.

Original 500mm coil former

I initially thought I would need a lot of inductance for 160m. I do, but using enamelled wire takes up less space and the windings are closer together. In the past I have just used insulated ‘1mm’ electrical conduit wire.

Cut down to a manageable size and change of top fitting

The tape stops the windings springing off the former.

In this form it was tested and adjusted to resonance on the vehicle.

Tested before completion.

The whole thing testing

Testing on the vehicle before final painting.

Finished antenna in vehicle.

Taped and painted.

With a small hole in the bottom cap to let out any water.

A large shrink tube will hopefully complete the coil section if I can find one big enough.