144MHz propagation experiment – antenna

This article documents a station for propagation observations on 144MHz, in this case the antenna system part of the entire system.

Above, the antenna is a 4 element Yagi with Gamma match.

The feed line is about 12m of RG213 coax (sheathed with 13mm HDPE irrigation tube for bird resistance), and for measurement purposes its electrical length was determined by placing a short circuit on the load end and measuring input Z, adjusting the length of feed line ‘subtracted' to get a load end reactance close to 0Ω. When the equivalent length of the short circuit part was subtracted, the adjusted length was 12.23m.

Above is the VSWR plot at the load end of the feed line. The antenna is tuned a little low, VSWR=1.62 @ 144.2MHz. VSWR is a little higher than expected. The antenna is 51 years old, but refurbished 13 years ago in 2007, and warrants a review based on the observed VSWR. I have another identical antenna which I will rework and swap in if needed.

Above is the R,X plot at the load end of the feed line. Z=72+j19Ω @ 144.2MHz.

Under these conditions, loss in the coax is 1.04dB.

Gain of the antenna system is about 8.8dB, so with 2W of input power, EIRP is 15W.

Measurements were made with a Rigexpert AA-600 driven from Antscope (1) v4.2.57 which allows real time measurement with feed line de-embedded.