Some considerations about vertical antennas - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: October 05, 2023, 09:00:23 pm
content: Worth a careful read imVHohttps://practicalantennas.com/designs/verticals/5eights/the only thing he lacked is the effect of feedline (coax) and poles, but that can be fixed with some choke, the remainder seems to be sound, at least after my coarse checks, using NECagain worth reading imVHo

author: Co6aka
author_rank: Unknown
timestamp: October 06, 2023, 03:15:50 am
content: Another consideration is a 3/8-wavelength vertical, which unfortunately isn't often discussed and analyzed. Elevated base even works well with just a counterpoise, but a typical radial system is better for lower frequencies. Multi-banding is relatively simple with higher frequency 1/4-wavelength "stubs" attached to the main vertical at their 1/8-wavelength above the base point. Top loading with a coil and capacity hat is effective at reducing physical height. Feedpoint impedance is mythically "200 Ohms plus some inductance" but in actual practice it will vary quite a bit, depending upon the standard factors. A remote tuner/autotuner readily provides a perfect match if it includes a common mode choke.My first experience with the 3/8 was when the 30-meter band became available for use; easily worked the world with just 100W. Second iteration was for 40-meters with a stub for 30-meters, which worked just as well as the 30-meter monobander.

author: A.Z.
author_rank: Super Contributor
timestamp: October 07, 2023, 06:40:37 am
content: I've played in NEC with the 3/8, but then didn't go as far as building one <sigh>, anyhow the NEC model showed some pretty nice characteristics and is also easy to put togetherI started with the vertical radiator, defining it so that segment #1 was at bottom and setting its lenght to 3/8 wave, then added six horizontal 1/8 wave radials at bottom in the "classic" GP configuration, set NEC to 200 Ohm impedance (like using a 4:1 at feedpoint) and ran the model; the match wasn't good due to the reactive part of impedance, so I added a capacitor at the base of radiator (segment #1) and adjusted its value to "cancel" the reactance, so finally getting a good match to 200 Ohm, then it was just a matter of optimizing the radiator and radials lenghts to improve the matchThe resulting antenna model showed a pretty low takeoff lobe, a gain around 1.3dBi and good bandwidth; willing to build it, I think one will want to add a resistor in parallel with the capacitor to shunt DC and avoid issues with staticAnother interesting antenna is the 3/4 horizontal dipole, basically you have two 3/8 arms, with two capacitors at their inner ends connected to a 4:1 balun, the dipole shows quite some gainin particular if it's possible to raise it at 0.6 Lambda

author: Co6aka
author_rank: Unknown
timestamp: October 07, 2023, 01:59:15 pm
content: Yep, ya need the cap to cancel the inductance, or use a tuner. And a plate choke (what I used) or at least a high resistance high voltage resistor. A (what I thought was a) clever design feature on a 160/80/75-meter homebrew 3/8 vertical was using a spark gap as part of the series capacitance. The main structure/radiator of that antenna was basically a tower, with "stubs" for 80-meters and 75-meters.The caps I used for my creations were in the 5-10kV range. Under 100pF if merember correctly. A vacuum varicap with insulated tuning shaft might work nicely.