AUT – MobileOne M40-1 40m helical

This article describes an Aerial Under Test (AUT) that features in some of my experiments and write ups and is subject of some current experiments. It is a MobileOne M40-1 helically loaded vertical for 40m installed in the car roof. It is in the style of the popular US antenna, the Hamstick, but this is a little longer and the results are not directly applicable.

I hasten to add that this configuration is not suited to travelling, it is just a rather ideal mounting of a helically loaded whip without the questions that arise from the effects of roof racks, bumper mounts etc.

The M40-1 is fitted in the centre of the station wagon roof, the roof is 1.5m above ground and the antenna is 1.5m long including a 200mm unloaded tip (tip of the antenna is highlighted with a pink dot). (The setting is not the test site.)

It is naive to think that the car body does not carry currents that contribute to radiation, though that is popularly believed by many if not most hams.

Above is a sweep of the antenna impedance at the feed point about resonance. The response is typical of antennas of this type of antenna where R is fairly constant with frequency near resonance, and X changes relatively quickly, and Rt at resonance is 21.0Ω.

We can estimate Q from the graph as Q=fr/(2*R)*dX/df=7.063/(2*21.0)*(44.0–38.6)/(7.115=7.015)=138.9 which is a moderately high Q antenna, about 10 times that of a n ordinary half wave dipole. If it were matched with a lossless broadband matching network, VSWR(2.62) bandwidth would be 7063/139=51kHz, and the VSWR(1.5) bandwidth would be 41% of that or 21kHz… quite narrow.

The feed line for this test is 7.5m of RG58.

Above is a calculation of feed line loss from measured input impedance, loss=0.42db, transmission line efficiency is 91% so power to the antenna will be 91% of that measured at the transmitter.