On Tue, 29 Aug 2006 07:43:15 GMT, "Lee"
wrote:

Hi All.....

I have just bought a Yaesu VR5000 receiver/scanner and while looking round
for a wideband antenna
came across the good old log periodic but what a price!!!, circa £200+......

I have a discone but it`s not a lot of use up around the 2gig area (or
anywhere else!!) so i thought i
might have a go at making my own, the magloops are working great as is the
QFH 137megs
weathersat ant, thanks to everyones assistance!.....

Is there any software around to design a log periodic to around
2400megs??.....i`ve found some but is there any better?..

Regards...

Lee......de G6ZSG....


Hi Lee,

Simply build a discone to the scale of 2GHz. The reason why the
present one might not work is that if it is designed for a very lower
base frequency, it is TOO BIG.

Discones can be as wide banded as over a 10:1 frequency range, but
this is for matching (certainly useful, but this does not necessarily
make it usable).

For instance, if you have a discone that has a low frequency design of
450 MHz and claims to go as high as 2GHz; then it may well match at
those frequency extremes. On the other hand, at 450 MHz it is looking
out at the horizon, BUT at 2GHz it is looking into the sky. The
higher the frequency, the more the most favored direction climbs up.
In other words it becomes deaf unless you want to hear an overhead
satellite. This may be the source of your disdain for your current
discone.

Build a Discone whose lowest frequency would be around 1.2 GHz.

How do you know what the lowest frequency is? Measure one of the
skirt elements (the skeletal part of the cone). It should be a
quarter wave of the lowest frequency of intended use. In this case,
1.2GHz, that would be 6 cm or so. The high end would then comfortably
range up to 2.4 GHz, match, and still look at the horizon.

Of course, the simple ground plane at 2GHz would work as well, unless
you really need widebandedness (and it would probably work even then).
Simply observe the simple geometry of keeping the element no longer
than 5/8ths wavelength at the HIGHEST frequency you plan to monitor.

If you still want a log periodic, simple scaling will always work.
However, remember that scaling must be applied to ALL physical
dimensions (this means element diameters too).

73's
Richard Clark, KB7QHC