On 30/09/10 17:49, LA4RT Jon wrote:
writes:

I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication
is that while it may require a large number of buried radials (16)
before efficiency levels off a bit, similar efficiency can be obtained
with just three radials elevated more than 100mm above the soil.

N6LF made extensive measurements and essenstially confirmed this. He
wrote a 7 part series of articles for QEX. You can download them at
his site:
http://www.antennasbyn6lf.com/2009/1...periments.html

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Practicality. In most cases, you either want to be able to walk above
the radials, (i.e. bury them or leave them on the ground) or below
them. This means at least 2 - 2.5 m up, and there will be some
sagging. Essentially, your vertical just got that much shorter. But if
what remains is tall enough, it's a great choice. If a friendly farmer
lets you borrow a field in wintertime, stringing four elevated radials
is a lot less work than rolling out 32 on the ground.

Hi Jon,

Noted.

One of the designs I am exploring is an eighth wave vertical over
elevated quarter wave radials, three top guy ropes the the radial
straining posts, and the top section of the guys are top hat wires. The
whole thing is tuned low so that it is matched to 50 ohm line with a
shunt inductor. Modelled performance is within 0.5dB of a ground mounted
quarter wave with 32 buried radials. Elevating the radials with the
shortened vertical is not a big structural challenge, even to 2.5+m to
allow a bit of sag in the radial catenary. (I never said I wanted to rig
the radials at 100mm, just that above that, the efficiency was
relatively high and didn't vary much between 100mm and a few metres.)

But, such a design does not conform to the Rules of Thumb commonly
trotted out for low HF Marconis.

I will look at Rudy's articles, always an interesting read.

Thanks
Owen