144MHz Antennas

The current 2m antenna is a Cushcraft A14410SN, temporarily extended to twelve elements for an extra dB of gain, part of the search for just enough ERP to get over that EME threshold... Once again I used NEC4WIN to analyse and sort of optimise the additional elements, and the results are below.

Polar plots of the 10 and 12-element versions of my antenna (click thumbs for larger image)

This temporary 144MHz antenna was designed using NEC4WIN trial software, and was intended to use up some junk from the garage, notably a few steel rods of about 1/4 wave at 2m. The programme allowed me to optimise the design for minimum cutting: as the rods were steel, I wanted to make as few cuts as possible. I started with the requirement that the reflector would just be two poles joined end to end, then adjusted the driven element for best mixture of gain, match and F/B ratio.

When the antenna was built, it's SWR was exactly as predicted by the software (1.3:1), and once attached to the mast there was no need for any further tuning. NEC plots for this antenna are below.

I think the 2 element has proved itself very worthy of the mast space during the good tropo conditions. When it was first put on to the mast the rotator wasn't yet ready, so I had to chose a fixed beam heading, and prevailing conditions suggested 190 degrees. First QSO with it was to F9IE (IN86), followed by EA1YO (IN73), at 1058km, and GU6EFB (IN89), all with 20-25W from the IC202 + linear. Up to September 8th, this antenna has accounted for 9 countries and 36 locators on 2m.

The photo below shows the mast at half height. The two nested Delta loops are for 6 and 4m.

144MHz, 2 ele antenna radiation angle - antenna pointing right on screen

144MHz, 2 ele antenna azimuth - antenna pointing up on screen

For a start, as far as the 144MHz operation is concerned, the antenna is actually a 48MHz quad, operating on the third harmonic. Its 2:1 bandwidth at 6m is from 47 to 53MHz. That it also gave a very good match at 144MHz was bit of a surprise at first. Luckily, I had just downloaded an evaluation copy of NEC4WIN antenna modelling software, and thought that would be an excellent question to evaluate it with.

The NEC plots confirmed my own impressions that the antenna was both very broadband at fundamental, and also resonant at 144MHz, exhibiting four main lobes approximately 30 degrees out from the ends of the antenna.

48MHz radiation angle - antenna pointing left / right on screen

48MHz azimuth of 12 degree lobe - antenna pointing up / down on screen

At 48MHz the pattern is typical of all single quad elements, with its major lobe being bi-drectional. Gain is calculated at 8.59dBi, quite respectable for a bit of wire and some 75R coax to match it! Elevation angle is a function of height above ground, and this antenna will eventually be moved up to 12m, which will drop its 12 degree lobe down to 6 degrees.

144MHz radiation angle - antenna pointing into screen

The situation on 2m is much more confused! Surprisingly there is a lobe with very low radiation angle, at 4 degrees, and despite most of the RF going upwards, this lobe still represents a gain of 5dBi. The azimuth plot below confirms the four major lobes I had found in practice.

144MHz azimuth of 4 degree lobe - antenna pointing up / down on screen

As the 4 degree lobe is radiated within 30 or so degrees of the antenna's end, it seems likely that the polarisation would be close to vertical or slanted. The opposite slant should be obtained by turning the antenna to an adjacent lobe. Not an ideal antenna for DX working on 144MHz, more suited to satellites maybe, but it does radiate and presents a nicely matched load at the transmitter due to the 75R matching line.