Yes, high SWR affects digital reception way more than analog reception. Holl_ands has a long discussion of why some where in this forum.Would improving SWR necessarily improve reception gain figures?
Impedance varies with frequency. The SWR is the impedance mismatch from 300 ohms (which is a setting in the 4nec2 program). The further away from 300 ohms, the higher the SWR, ie the Impedance mismatch. Because Impedance consists of real and imaginary resistances, it will take time to figure out the best impedance value for this antenna. Then if you wanted to, build a custom coax balun for it to connect to a 75ohm coax downlead. (Most ham coax is 50 ohms).We are assuming that the raw feed point value is 300ohms. Maybe the 4:1 balun is not going to perfectly balance the impedance at any point.
SWR Mismatch Loss dBi ----- ----------------- 1.0 0 1.5 .18 2.0 .51 2.5 .88 3.0 1.25 3.5 1.60 4.0 1.94 4.5 2.25 5.0 2.55
No, no, SWR is a ratio. 1 to 1 is the best you can get. If your meter had a zero setting, then that would have been 1:1, which would correspond to 1.0 in our context. A lot of older equipment used general purpose meters, whose scales would run from 0 to 10, 25, 50, 100 etc.Two way radio SWR should be less than 1.0
Moving the feedpoint down to 5 inches above the bottom of the slot, the gain peak is 4.38 dbi 5.32 SWR at 488 mhz with a bumpy curve downward to 1.63 dbi 2.60 SWR at 698 mhz.I still have the antenna shown in post#8 mounted at my shop. It's still giving awsome performance. When I have some more time, I'll try moving the feed point down to the 1/3rd distance to see what happens.
You input the Characteristic Impedance as a global setting. It can be any figure so 4nec2 is fine for ham or wifi etc usage also. Since common TV cable is either 300 ohm twin lead or 75 ohm coax and the common TV baluns sold are 4:1, 300 ohm characteristic impedance is the figure to use for TV antennas. The 300 ohm standard was set in the 30's/early 40's. Many at that time wanted a 600 ohm standard for TV, as 300 ohm twin lead required higher tolerances to manufacture. 75 ohm twin lead was just impossible to make, heh.Does 4nec2 determine the actual feed point impedances or does 4nec2 assume that the antennas impedance is already designed at 300Ohms?
It looks like a 6:1 coax balun (450 ohms to 75 ohms) is the best compromise for use with the antenna in post #8.Then if you wanted to, build a custom coax balun for it to connect to a 75ohm coax downlead.
http://www.digitalhome.ca/forum/showpost.php?p=1026872&postcount=726I believe nec2 is capable of analyzing cylinders, but not sure about slotted cylinders.
Fig. 7-20 (page label 92, pdf page 96) of this pdf shows some effects of moving the feed point.I would think that making the slot to the designated dipole length and then dividing it in half by having the balun centered would make more sense than moving the balun off-center.
These two messages from the NEC-list archives may be instructive:I believe nec2 is capable of analyzing cylinders, but not sure about slotted cylinders.