Topband: Beverage height experiment results
Thu, 23 Nov 2000 10:57:41 +0100
>The drop in gain between
>the 2 foot and 10 foot was a few S-units (whatever that means in dB) as
>compared to about the same drop between the 10 foot and the
>antenna. For the first time, I was actually starting to think I need a
>preamp. I am wondering if the 10 foot Beverage was working more as a
>wire close to the ground?
>I also did some BCB tests for rejection off the side on 1510, 1520,
>kHz. The 2 foot Beverage was perhaps an S-unit better in this respect.
On weak signals, the antenna efficiency still plays a role and it has
to be found, of course when it's possible, an optimal compromise
between directivity and available signals at antenna terminals.
Under a certain level of antenna efficiency most of the theorical
advantages of a very low wire vanish and the system approaches a sort
DF antenna rather than beeing a real DX tool for amateur applications.
If there's no other choice and the antenna has to be that, it's worth
anyway to try, but when a beverage can exist the use of a preamplifier
generally doesn't solve anything but adds problems instead of curing.
When one feels the need of a preamp it's the sign that actual receiver
NF is not enough to satisfy requirements. Any active device is never
linear as an antenna is and adds noise to the system. (its inherent
noise plus the noises generated by its unlinearity when many signals
are applyed) If the required gain is moderate and the applyed signals
are reasonably high not to require the lowest NF (i.e. when
compensating a transmission line loss) a reasonable active device to
use can exist. When required gain is noticeable and NF has to be quite
low (i.e very unefficient antenna) the success is at great risk.
When the antenna is not narrow band (small tuned loop) but broad band
as a beverage or wires, troubles skyrocket.
If the low wire reported drop, 12 dB under a beverage at canonical
height is really that, it's very possible (nearly sure) that
practically speaking the low beverage with a preamp won't let you hear
what the higher does, in spite of a better directivity.
If the reported drop is much less than 12 dB (i.e. a fake reading of an
S meter in a linear reading with a logaritmic scale) there is a margin
where the use of a preamplifier could instead help.
By experience, anyway, I always found better not to use any active
device when I can enhance inherent antenna efficiency and reduce losses.
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