On Sat, Jan 29, 2005 at 12:15:36PM -0800, Jim Lux wrote:
> Looking at "off the shelf" multiband HF vertical antennas (i.e. R8, 6BTV,
> etc.)..
>
> Are there any significant(! (>1dB)) radiation efficiency differences
> between them that wouldn't be swamped by such installation dependent effects
> like grounding, nearby stuff, etc.
>
> They all look like trapped multiband vertical monopoles, as far as I can
> tell, and they're all about 25-30 feet long.
>
> Presumably there are mechanical differences in hardware, etc., but what I'm
> really looking for is any noticeable "power sucking loads" that are inherent
> in the design (sure, our vertical has a 2:1 SWR bandwidth of 200kHz on 80..
> we have a 10 dB pad in the 80m trap!)
>
There are certainly differences in the way they are fed. The 6BTV is
bottom fed with radials, while the R8 has a choke balun and matching
transformer (see <http://www.iol.ie/~bravo/r7_vertical.htm> for details
on the similar R7) with counterpoise elements which are much shorter
than a quarter-wave radial. The effective length of the R8 is somewhat
longer than a quarter wave on each band (I'm not sure if it is an
electrical half-wave, however).
The Hy-Gain AV-640 is similar to the R8, but uses top-loading coils and
capacity hats for decoupling instead of traps. Both of these have
separate vertical elements for the higher-frequency bands. Neither of
these is rated for 80 meters.
The K7LXC/N0AX report on verticals has some interesting comparisons of
several different models.
73, Bob N7XY
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