Posted by
Vic K2VCO on
Oct 27, 2006; 6:01pm
URL: http://elecraft.85.s1.nabble.com/Attic-Antenna-tp394788p394815.html
[hidden email] wrote:
> I would want a lot more info before spending any money.
>
> The BB7V is priced at US$400. It's a 22 foot vertical aluminum radiator
> with a nice mast clamp (mast not included) and a cylindrical feedpoint
> device.
>
> The big question is: What's in the feedpoint device?
Here's some more information (but no mention of what's in the magic
cylinder): <
http://www.hamradio.co.uk/pdf/Diamond/BB7V.pdf>
However, it is possible to see that the radiator is a continuous
conductor. Therefore, the counterpoise function must be provided by the
mast, coax shield, or lossy ground (depending on mounting). This means
the antenna's performance would be highly dependent on mounting, and it
would be prone to RFI and RF-in-the-shack issues.
The antenna supposedly works from 3-30 MHz with SWR less than 2:1. The
SWR curve shown is suspiciously similar to that of a resistively loaded
antenna. Such antennas can be useful in certain circumstances (B&W
makes several versions), but the efficiency can be quite low on
frequencies where the impedance of the radiating part is high or
reactive. The B&W antennas are well designed and relatively large; even
so, the efficiency is significantly below that of a dipole.
A 22-foot vertical without top loading will be inefficient on 7 MHz (it
will be a dummy load on 3.5), especially without a decent counterpoise.
Even if one only has room for a 22-foot vertical there are lots of
better choices.
I agree with Jim. Put up a dipole and spend the leftover $395 on beer.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco_______________________________________________
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