Hi topband lovers
The internet has become an example of bad practices resulting on wrong
conclusions. Opinion are drawing based on anecdotes results where the
antenna is the problem, not the lack of basic concepts that very few hams
understand them.
My first beverage was built with a kind of Faraday cage to isolate the
primary from the secondary. Metal box was a MUST to eliminate the common
mode noise from the feed line to be amplified by the preamplifier. The
project was published on Ham Radio Magazine on the 70?s.
Today with the internet you only see ?new stuff?. You see it everywhere and
because everybody has easy access to it, it?s become ?best practice?.
Any receiving antenna with the same RDF has the same performance,
directivity is the only way to reduce S/N ratio. But and here is the ?but?,
if don?t implement the hardware correctly you don?t see the result expected.
You build a flag and connect to the amplifier and expect to hear the
benefits of the RDF signal to noise improvement.. Simple as 1+1 =2.
Most of the people that already knows everything from the internet get 1+1
=5. Wow the antenna does not work, where the extra 3 is coming from. It is
not my fault, if it is from the web it is true?(not really)!
Any flag, beverage or any receiving antenna, even your TX antenna when used
to receive have the same issue when connected using a coaxial cable. The RX
antenna has only two terminals and feeds the preamp with only 2 currents.
One for each terminal.
The coaxial cable has 3 currents, two inside the cable and one outside the
cable. Ignoring the 3rd current is a basic mistake the will deteriorate the
RDF.
You can see almost every where on the internet a 3:1 BALUN using a binocular
core, most of them are balanced input and balanced output. Like the one you
see on the LOG web site , or W8JI site or several other places. All of them
uses a plastic box and a connector F , BNC, UHF or any other where the
internal side of the connector is directed connected to the two wires from
the BALUN secondary. Making a BALANCED to UNBALANCED connection.
Just few mm of open wire on the unbalanced side of the connector is long
enough to allow the 3rd current from the outside of the coaxial cable to get
inside the coaxial cable mixing it with the signal coming from the BALUN
primary.
If you open the shield the noise from outside the shield gets inside. The
two ends of the coaxial cable have this problem as well several pin 1
problems on the RX system,, open flame relays .. etc.
This point is the input point of the preamplifier! ,and the gain will
amplifier any signal at this end of the preamplifier feed line.
How bad it could be? It depends on the intensity of both currents, if the
signal from the BALUN is 30db stronger than the signal from the external
coaxial shield, the deterioration will be? -30 db.
If the coaxial runs for 100 ft and only grounded at the preamplifier input
and the preamplifier grounded on the radio input, you have almost a full
size ¼ wave vertical the intensity of the external current will be the same
as your TX antenna.
The amplifier will overload for sure because of Am BC energy will enter the
preamplifier at the plastic box we are talking about. Band pass filter is a
must on 160m.
The best way to avoid the 3rd current is a twisted pair feed line. The
twisted pair cancel the common mode current on each twist. You can run 200ft
of twisted pair without pick up any common mode noise, the twisted pair
must run 10 inches far from the ground or any metal conductor. That what
made the coaxial so popular, you can run dozen of coaxial cables together
through the window or that PVC pipe through the wall, but not two twisted
pairs.
The second way is to reduce the current from outside the cable to get
inside is to use a choke to attenuate the signal pick up by the coaxial
cable. The way you ground the cable and where to put the choke has different
results on the attenuation.
Just one example, my friend IIian was operating on Afghanistan as T6LG. He
was using an inverted V inside a US military base, he worked few Europeans
on the first few months on 160m, after I send him instructions to build a
flag using twisted pair 100 ohms striped from a Ethernet cable, Ilian worked
near 1200 stations on 160 in the last 45 days on the military base.
The flag is the same any shape you can make it. How it will perform depends
on the feed system.
When you get 1+1=5 , don?t blame the antenna, try to figure out where the
extra 3 is coming from.
73?s
JC
N4IS
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