Ground mounted Vs Elevated vertical

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DAVE CUTHBERT:
Four elevated radials provide no substantial improvement in feedline decoupling compared to two radials. A 1:1 choke can greatly reduce coax shield current.

Jerry Montgomery:
CV1BDO wrote:
"So my questions are:
a) The height of feeding point is critical or not?
b) Is there any need for the radials to be resonant for a given band (40m), or the antenna will perform better with as much more radials as I can install, even though some of them will be longer or shorter than resonance?"

The height of the feedpoint is not critical, but the radials should start at the feedpoint.  If the radials start below that, the resonant frequency and feedpoint impedance of the antenna will change.

If you have elevated radials and they are not resonant, most of the radial current will go to the radial(s) which have resonant frequencies closest to the operating frequency.  (The lowest impedance radial will get the most current.)  Shorter or longer radials may have insignificant contribution, depending on their exact length.  You could end up with effectively only one radial.  That will make the antenna not omni-directional, as well as contributing more loss (compared with using several resonant radials).  The antenna is likely to not be resonant anywhere close to the expected frequency, and the feedpoint impedance can be different from expected.  You will need a very good choke on the feedline because the feedline might receive most of the radial current if it happens to be resonant close to the operating frequency.

Jerry, K4SAV

Jason Hsu:
The advantages of ground radials over elevated radials:
1.  Ground radials don't require supports at the right locations.  Elevated radials do.
2.  Specific lengths are less important, because the ground detunes the radials anyway.  With elevated radials, you need to make sure that they are the right length.
3.  For a fan vertical or other multiband 1/4 wavelength monopole antenna, you know that the antenna is operating at 1/4 wavelength resonance.  For a multiband antenna, it's hard to make sure that only the 1/4 wavelength radials are the active ones.  (Longer radials could rob some of the antenna current from the 1/4 wavelength radials.  This would mean less low-angle radiaton and more high-angle radiation.  Since the upper bands don't support NVIS, you want only low angle radiation.)

Jason Hsu:
The main advantage of elevated radials over ground radials:

The greater the elevation above ground (in terms of wavelength), the less the ground loss (assuming the same number and length of radials).  A ground plane antenna (which, by definition, has to be a certain minimum height above ground) has no ground loss.  If you move the ground plane antenna to the ground, it becomes a ground-mounted vertical antenna with a just a few radials and some ground losses.  (Air is a lossless insulator but the ground is a lossy insulator.)  A vertical antenna with elevated radials is the intermediate state between a lossless ground plane antenna and a lossy ground-mounted antenna.

For that 20m-6m multiband antenna, I think you should make it a ground-mounted antenna simply to make sure you it behaves like a 1/4 wavelength vertical.  If the vertical element is sufficiently long, you'll have plenty of radiation resistance, and the antenna efficiency will be more tolerant of ground losses.  You can also add more radials to improve efficiency, and ground radials don't require the supports that elevated radials do.  The upper bands aren't that noisy, so you don't need to eke out every dB like you have to on the lower bands.

If you insist on using elevated antennas, I think you should use two fan verticals or fan dipoles - one for 6m/10m and one for 12m/15m/17m/20m.

Sotiris Katsimaglis:
Many thanks to all for your replies & comments. I forgot to mention that all radials will be at the same height (2m) with feeding point above ground.
As a starting point, I will install 4 resonant radials for each band (40m & 30m) and later I will add some more, not resonant, and check the difference. This vertical will operate on 80-30m and will be a home made Butternut HF2V.
I will make some tests on 40m during this weekend, during the A.VOLTA RTTY Contest, operating barefoot as SV1BDO/3. I will be very happy to see some DX calls in my log.

73, Sotiris

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