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, 
>and 1530
>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.
Mauri I4JMY

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