Using the HP 8753B Vector Network Analyzer I took a look at what my Classic 33 Yagi and 3-band fan dipole looks like from the shack. The Classic 33 is my original that dad got used back in the 70's It was refurbished once and installed at this QTH in 1989. The antenna has not been touched since. The feedline is Belden 9913 which was installed in 1989. Getting concerned that this air core coax may have water in it after all these years. For today's test the outside conditions are dry and have been for a couple of days. The Fan Dipole I put up last year. It consits of three dipoles feed from a common point. The dipoles are for 40, 30, and 17 meters. Dipole spacing and length of the elements are from a military document (Army I think) I found online. This document was written back in the 50's or 60's if I recall. It is installed as a sloper off of the tower favoring the west. I used the old 9913 coax that feed the 40-meter inverted V that this antenna replaced.
Looking at the Classic 33 first
I don't have a way to download the screen shot of the VNA so I used my Canon 30D camera to take a picture of the screen. I see I need to work on the focus. I used a tripod in front of the VNA but it was easily moved out of postion. If I am going to continue to use the camera I need to build a fixture that attaches to cart that holds the VNA.
The first screen shot is a Smith Chart view on 20 meters.
I was playing with the buttons on the VNA so now on to the Return Loss and SWR plots.
The SWR on the Classic 33 is not very broad on 20 meters. The yagi elements were installed favoring the voice part of the band. So the low end of the CW band suffers. SWR is 3.2:1 at 14.0 MHz, 1.16:1 at 14.2 MHz, and 1.6:1 at 14.3 MHz
15 Meters is better with the SWR ranging from 1.75:1 at 21.0 MHz to 2.0:1 at 21.450. Best SWR at 1.43:1 is at 21.160 MHz
10 meters is a wide band. The Classic 33 was intended to be used in the lower part of the band below 29 MHz. Installed for the SSB part of the band SWR ranges from 1.87:1 at 28.0, 1.17:1 at 28.3 MHz, 1.57:1 at 28.5 MHz, and 2.28:1 at 29.0 MHz. This image shows the return loss as well as the SWR in ().
The results of the tests look as I remember them when the antenna was installed at this QTH. After I made these tests I disconnected the yagi and placed a short on the end of the feedline. Using a return loss measurement and calculating the length of the coax line I came up with the following:
Length of line = 156 Feet
Loss measured at 50 MHz = 2.4 DB
9913 has a loss at 50 MHz of 0.9 DB/100 Foot according to one datasheet I found online. At 156 feet the loss per this specification is 2.8 DB. So this loss is within spec give or take my measurement error.
My notes from the installation back in 1989 indicates the disance from my connector panel in the shack to the connector at top of the tower is 136 feet. All the measurements I have made using the VNA or an MFJ analyzer show the length of my line to be 156 feet. Not sure why I have a discrepancy of 20 feet. I plan on changing out this feedline so I will measure what I pull out and see if my VNA or my notes are correct.
Now the Fan Dipole
Looking at the Fan Dipole return loss from 5 to 55 MHz you can see the three bands. Also a good return loss is noted around the 6 meter band at 49.5 MHz just a little shy of our allocation.
Looking at 40 meters the SWR ranges from 1.34:1 at 7.0 MHz to 1.8:1 at the top of the band, 7.3 MHz.
Moving to the 30 meter band. SWR is highest at the lower end of the band. 1.46:1 at 10.1 dropping to 1.20:1 at 10.180
On 17 meters the lowest SWR of 1.03:1 is below the band at 17.740 MHz. For the lower band edge of 18.068 the SWR is 1.7:1 rising to 1.8:1 at the top of the band. If I get ambitious I may try to shorten the 17 meter dipole a bit to pull the lower SWR inside of the band.
