W8AMZ 80M TO 6M OCF DIPOLE ANTENNA PERFORMANCE – ROUND 2.
Since my last postI have made some interesting discoveries, and remembered a few things about microwave engineering 101. In my post listing the performance of the W8AMZ 80M to 6M OCF dipole, I mentioned that the performance varies on installation. While this is true, I found that a few things mattered more than others. First, let me describe the new antenna installation.
The antenna is now approximately flat-topped; that is, one end of the antenna is no longer bent around a tree limb, heading downward (as described in round 1). It isn't extremely flat, and there is a slight skyward-Vee shape to it, since there is nothing supporting the antenna in the middle (at the relatively heavy balun). I tried several different tensions in the rope holding the antenna up, which yielded varying degrees of the depth of the "Vee" shape that is cupped skyward. The antenna performance (as viewed on a 3-30MHz scan of a network analyzer) didn't seem to change at all.
So varying the height and flatness of the antenna didn't seem matter.
Now the next part, however, gave me wildly varying performance: the length of the transmission line leading up to the antenna. I have one length of antenna heading down from the balun, to a UHF jack drilled through my wall. This is fairly long, at around 50 +/- 10 feet. This part of the setup didn't change. What did change: I varied the lengths of coax going from the UHF jack inside my man cave to the transceiver. I only needed probably 2 feet of coax, but I have 3 different lengths: coax 1 ,at about 20 feet; coax 2, at about 20 feet; and coax 3 at about 5 feet. Coax 1 and 2 weren't exactly the same length, but I am not sure how different they are.
I will take 40M as an example. By using, say coax 1, I got better than 2:1 across the entire 40M band, 7.000-7.300 MHz, with a roughly flat SWR across much of the band, about 1.6:1 or 1.7:1. Using coax 3, I got a more pronounced dip, and the SWR wasn't less than 2:1 across the whole band.
Another neat experiment: I put a UHF female T, with a male to male coupler going to the UHF jack in the wall. Coax 1 went from the through port of the female T to the transceiver, and coax 2 was attached to the T side of the female T, with nothing on the an (an "open stub"). With this setup, I got less than 2:1 from 28.275MHz to 29.700MHz; that is most of the 10M band! Using coax 1 only, I got less than 2:1 from 28.740MHz to 29.700Mhz. Using this "T" arrangement with an open stub, I got roughly 500KHz more bandwidth at less than 2:1 SWR!
This started to jog my memory about microwave engineering stuff; the length of the transmission line does have an effect on the return loss looking into the antenna. I am going to go look up smith charts, and post another update on this.