Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KO7PAS, Jun 4, 2021.
I had thought about mounting it vertical. I don't really have another way to do it.
A vertical dipole can work very well. One tricky thing is that (assuming you are feeding it in the middle) you should try to keep the feedline at a right angle to the dipole - this can be challenging construction-wise. You also ideally should try to keep the bottom end out of reach of humans and animals as the ends have high voltage, and being too close to the ground could detune thing.
Good timing for getting on 10m
If you are going to configure this as a vertical dipole, I recommend that you use a copper or aluminum tube or even steel pipe for the bottom half of the element and pass the coax through this bottom tube.
Your shield should be connected at the top of this lower tubing (it is at the midpoint of the antenna now).
This will eliminate Transmission Line (TL) / coaxial interactions with the radiating pattern that you will get if you run the coax horizontally at the mid-point.
You will need to have some kind of dual mounting bracket assembly ( 2 muffler clamps from an auto store and 2 long bolts) to an isolated, non-conductive (wooden closet pole, bamboo, fiberglass) which will mount the bottom half of the bottom tube. Here is a link to a representative version of what I'm describing: http://www.alpharubicon.com/elect/vertdipoleniet.htm
This kind of antenna has been around for a very long time.
A variant of this is what WWV uses and also I recall many decades ago seeing a mobile version that was developed by Motorola for the then 'new' fiberglass Chevy Corvette bodies that did not provide a metallic ground plane / counterpoise for a simple vertical.
That's a great idea! I hadn't thought of doing that!
an old CB radio base station antenna makes an excellent vertical. Years ago, when I started, I used the old Antron-99 with great results on 10m. it has tuning rings, to dip the SWR, I set mine to 28.400.
Those are still popular and on the market, commercially: Usually called "co-axial" vertical antennas. The ones made by Kreco look like this:
The "skirt" which forms the bottom half of the radiating element allows the TL to feed right through it, and it's best to isolate the coax jacket from the inside of the skirt tube, as at the bottom (1/4-WL down from the central feedpoint) there can be quite a big voltage differential between the coax shield and the end of the skirt tubing. But they perform well.
Thanks Steve for providing this. I couldn't find a really good picture yesterday on the web when I responded to the OP.
Ground plane antenna is sort of the same thing. Just a spread out skirt. I never had much luck with 'skirt' or 'coxial' antennas, but I never tried hard either. I always seemed to have problems with CM because the SWR would be all over the place. Also realize if you use a pipe for the bottom, it's VF is most likely different than a wire and will need to be adjusted appropriately. And yes, the voltage at the bottom of the pipe can be quite high, piercing the coax insulation.
I've seen people just use CM chokes at the half wave point, but I never had any luck with them either, again I didn't try hard, it's just too easy to make a horizontal dipole.
As mentioned in my prior post to the OP, these have been around for a long time. If WWV uses them and Motorola made them for auto use, I'm sure that they do work. I don't understand how Common Mode (CM) and SWR problems are presented if the antenna was designed and built correctly......obviously, the impedance will be about 75 ohms and matching to a 50 ohm TX will be the only concern....but that is a given.
Motorola and WWV use specific materials and dimensions. Even the Moto antenna's I've had use of, used a specific type coax and specific lengths and specific equipment for specific purposes. The average 'ham' will not do that. There are many variables in hamdom.
I'm just giving my anecdotal evidence to warn of issues, which as usual is probably irrelevant.
Of course, too, it could work perfectly fine.