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HF Vertical ~ radials question

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I was considering making a 40 meter vertical portable antenna (using my tripod I got at Habitat for Humanity last year) for upcoming hurricane drill so I was reading up on HF verticals.  I understand from my research that the radials should be the same length as the radiating vertical part of the antenna.  Thats fine for something store-bought where all the parts are already pre-measured and cut to specs.  But, I'm starting from scratch.

See, I expect my antenna to be about 16 feet tall (give or take) and I wanted to make a slide-up-and-down section for tuning it.  That worked great for my mobile antenna on my truck where it was grounded to the frame and I could just loosen the allen screw and slide the little tuning section up and down to lengthen or shorten it and get the SWR down. 

But on this particular antenna, if what I'm told is true, that would mean I'd have to assemble it to see the SWR, make an up or down adjustment to the vertical, then trim all the radials, then test it again, repeat, repeat, repeat.   Seems to me that would be a lot of work to trim 30 radials each time I tested it.  Then what if I need to lengthen it a tad?  I have to go add wire to 30 radials?

So my question is does the length of the radials matter that much or do they need to be the same length as the vertical, as I've read, sort of like a dipole needs to have both legs the same length for a good SWR?   Why I question is that the 2 meter/440 mobile vertical antenna on my truck uses the truck body as the ground plane.  But my truck is a lot wider/longer than the 4 foot or so vertical mobile antenna.  So the "radial" for that antenna isn't the same legth as the antenna height.

Thanks for you responses.  I'm new to vertical HF antennas.

73 de Anthony, KD3Y

Edited by KD3Y
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Anthony, I'll give you a much longer response, possibly later today, 'cause I'm rushing out the door, but I did want to point you to a neat vertical antenna which makes great use of radials. I don't expect you to buy it considering how good you are at making things, and considering I love building my own antennas, but this one has some ideas in it that may be of use. There are also a couple of videos.


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Hi again.

My follow up answer may sound lazy to you (it is) but if you are interested in antennas, you need to buy the ARRL Antenna Book. This truly is a comprehensive volume of all things antennas, including grounding. Chapter 3 (of my 24th edition) is all about grounding.

Now, before picking out the points that I think are most important, let me begin with my tired old speech about complexity. Amateur radio is an exercise in compromise. Anything we do has so many variables. Whether it be grounding, transmission, lines, or antenna design, it's never as simple as a single variable. I could also add that modern day antennas, paired with modern day feedlines and modern day transceivers allow us to overcome many of the shortcomings of a mediocre installation. Obviously, a bad installation is a bad installation, but if an antenna is properly designed, relatively well tuned, and installed not too close to a building or to man-made interference, then it will more than likely operate acceptably well.

Now back to the question about length. First of all, good grounding assumes a flat ground. Does it make a difference considering what I just said above? Actually it does. Many have tested what an inclined plane does to a signal and it's very noticeable. As for the number and length of radials, this is where it gets super duper complex (or not). The complex version is too long to type here but suffice it to say that because of ground conductivity, because of wire thickness, because of height above ground (or below) that the number and length of radials is incalculably variable.

The conventional wisdom among amateurs is definitely that the radial should be the length of the height of the antenna. Another bit of conventional wisdom is that the shorter the antenna, the more wires you need. This actually makes life simpler because the, as you can imagine, cutting dozens of wires that are the length of a half wave 80m antenna will be expensive and a PITA.

A third bit of conventional wisdom, and btw, all of these 'conventional wisdoms' are backed by actual numbers and years of trial and error. So, the third bit is that given the choice between length and quantity of radials, length is better.

A very important element in radial design is to be equal! If you have something in the way on one side, forget it... You're better off having all the radials shorter but the same length.

As to the number of radials, based on a 1937 study, broadcast towers were deemed effective when they had 120 radials!!!!!! Not a typo. My advice to all amateurs has been the following. I like the number 16. I can calculate my way higher or lower, but 16 has always worked for me, both in practice and mathematically. But if you want absolute precision, I will take the time to calculate the exact/approximate number and length 😉

Not sure if this has been helpful or useless...

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Thanks Jim,

Very helpful.  No, I wont' ask you to take time to do all the exact calculations.
I do appreciate the point of all radials being equal.  That would've thrown me if I had something in the way, I would've just cut that radial shorter than the rest.   My 2/440 vertical has only 4 radials about 10 inches long (Diamond X50).  So I assume the more radials the more effective the antenna.   I'll have to delve into that book.  So many questions I have.

"Anything we do has so many variables. Whether it be grounding, transmission, lines, or antenna design, it's never as simple as a single variable."
Yeah I'm used to that concept.  Same way with my banjo.  EVERYTHING changes the tone.  The type of wood the neck is made of, the type of wood the resonator is made of, the ambient temperature, the humidity, the brand of strings, the type of head used, the tension of the head, the tension on the truss rod, the material and style of the bridge, the type of metal used for the frets.  No two banjos sound the same and something as simple as changing the type of strings can make it sound like a totally different instrument.   ("There ain't no sound like Bogue Sound"...http://www.boguesoundgrass.com)
I reckon the laws of frequency physics don't change regardless of where they're found.

I'll give it a shot after a little more research and let you now how my HF vertical turns out.

Anthony, KD3Y

Edited by KD3Y
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I absolutely love the banjo metaphor. I play classical guitar (a little 🙄) and know what you mean. With your permission, I will use the metaphor going forward.

Yes, increase the number of radials, especially in the VHF/UHF range.

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