Hi Bob!
I recommend impedance matching your Beverage for several reasons:
- The efficiency of a Beverage is rather poor. For an eight foot high
580 foot Beverage on 160M, the main lobe will be about 10 dB down from
a full size vertical. This would not be a problem in itself; however,
amateur transceivers are designed for good dynamic range with
transmitting antennas! With many amateur transceivers, the inherent 10 dB
loss in the Beverage puts weak signals too close to the internal noise
floor of the receiver. With my TS-850 on SSB, I prefer not to use a
preamplifier, but on CW is usually prefer to use my Advanced Receiver
Research preamp (an excellent pre-amp by the way...).
- If you do not match the Beverage to the transmission line, the power
transfer from the Beverage to the transmission line will be very poor.
This will make the efficiency of the system much worse the the 10 dB loss
mentioned above.
- Furthermore, inefficient power transfer from the Beverage to the
transmission line makes the system more susceptible to stray signals
coupled from the currents flowing on the outside of the transmission line.
Minimization of stray coupling from signals flowing on the outside of the
coaxial transnission line is a complete topic in itself...
- If you impedance match your Beverage, you can use common test
equipment -- such as the AEA Antenna Analyzer -- to "baseline" the VSWR
performance of the antenna and check it before the big contest or
DXpedition (what... you don't baseline the performance of all of your
antennas and check them on a regular basis ???)
Some Beverage users have been lucky enough to "throw them up" and get good
performance; however, most of the excellent performing antenna systems
on topband (and other bands too...) result from meticulous attention
to detail!
If live in an area where transformer saturation from nearby AM broadcast
stations is NOT a problem, an ideal transformer is the Mini-Circuits
Laboratory T9-1 transformer. It costs only $3.95 and is available from
stock, credit cards accepted, at 800-247-6343 (M-F 0800-1730 Eastern
Time). Minimum order is $25, so combine your order with your buddies!
If you live within several miles of an AM broadcast station, you will need
to build a transformer capable of operation in the presence of strong
signals without saturation.
73!
Frank
W3LPL
donovanf@sgate.com
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Sponsored by Akorn Access, Inc & KM9P
|