-----Original Message-----
From: topband-bounces@contesting.com [mailto:topband-bounces@contesting.com]
On Behalf Of Victor A. Kean, Jr.
Sent: Saturday, November 08, 2008 5:57 PM
To: topband@contesting.com
Subject: Re: Topband: reversible Beverage v none reversible Beverage
On Saturday 08 November 2008 12:59:57 pm Neil Carr G0JHC wrote:
> I have a east/west reversible Beverage, (reflection transforms etc) it
works
> well East, but poor to the west.
>
> What's the trade offs with a reversible Beverage, compare with a dedicated
> one in same direction? Any?
I have 6 "center-fed" 2-wire, 2-port Beverages which have reflection
transformers (or the equivalent) at both ends. They work very well
in terms of directivity and deliver plenty of signal.
I would suggest that the trade-off of a 2-port Beverage versus 2 distinct
single port Beverages is psychological: one always wonders if the
reflection end works well enough. Well, it doesn't really matter.
First important fact: after about 2 or 3 wavelengths, the Beverage begins
to lose as much signal as it picks up. Therefore, regardless of the
quality of the reflection, the reflected signal is usually sufficiently
attenuated by the time it reaches the "wrong" end that the directivity is
not noticeably affected. Note that this applies to longish (1000 foot on
160) Beverages over "normal" ground. Shorter Beverages or "poor" ground
probably decreases the attenuation which would allow some of the "wrong
direction" signal to appear.
Second important fact: its hard to prevent a reflection. Reflection
transformers try hard to guarantee that all of the available common-mode
(Beverage-mode traveling-wave) signal is converted to differential-mode
(transmission line mode) signal. However, the common-mode to differential
mode conversion takes place to some degree unless the 2-wire balance is
perfect. Thus, no matter how crappy the reflection transformer is, you'll
always get some signal.
In my experience, I've had the ground wire break off the reflection
transformer, which causes the "other-way" port to go silent while the
"normal way" port remains unaffected. This indicates good balance. I've
also found that I get the same results with reflection transformers, and
deliberate imbalance brought about by tying one wire to ground and letting
the other float. So, I've stopped using reflection transformers.
You didn't say whether your problem was lack of directivity or lack of
signal output. For lack of directivity, look for common mode signal
ingress at the antenna end of your feedline. Try removing the reflection
transformer and just tie on wire to ground and let the other float. For
lack of signal, look for bad coax, bad transformers, bad connection, all
the usual culprits. Report back here!
Victor, K1LT
Actually, I have noticed a decrease in signal strength with a reversible
2-wire Beverage. I use the DX Engineering equipment and my NE-SW Beverage
wires are 500 feet long and stand about 8 feet up. The wires are 14 gauge
insulated stranded wire and are a foot apart. The NE direction plays very
well, but the SW direction is somewhat lower in signal strength as evidenced
that I have to turn the pre-amp up a bit to equal the NE signal.
I'm not sure why this is. There is a 75 ohm F-connector plug on the feed
transformer as I'm feeding with a single RG-6 run and switching with DC
voltage as per DXE specs. The connections are all solid and good ground on
both ends. I use 4" wide copper flashing that I laid in a post hole with a
wire connection pop riveted to the flashing. This has worked FB for the last
few years.
73, Jamie
WB4YDL
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