Balun on R7
cow...@isma8.monmouth.army.mil
Roland WF4P/AAR2AA
W VanHorne
: What is the collective opinion of using a balun on the Cushcraft R7?
: Roland WF4P/AAR2AA
Jim Hollenback
: What is the collective opinion of using a balun on the Cushcraft R7?
: Roland WF4P/AAR2AA
What problem do you think your curing? If the R7 box is at all like the R5
box, it already has about 7 or so wraps around a core.
73, Jim, WA6SDM
jho...@cup.hp.com
a3...@lehigh.edu
h.us (W VanHorne) writes:
>cow...@isma8.monmouth.ARmy.MIL wrote:
>: What is the collective opinion of using a balun on the Cushcraft R7?
>
>: Roland WF4P/AAR2AA
matching transformer input at the base ??
Dick K8WHA
>
Michael Cowart
: In article <47q61e$5...@acme.freenet.columbus.oh.us>, wvan...@freenet.columbus.o
: Dick K8WHA
: >
maybe I am missing something here, but doesn't the R7 have a built-in
balun?
Mike WA5CMI
W8JI Tom
>Why put a BALUN in when the antenna works off a counterpoise system with
a
>matching transformer input at the base ??
>
> Dick K8WHA
Because the counterpoise and matching transformer are both marginally
effective in that antenna.
It really needs the additional balun.
73 Tom
Jim Cummings
: 73 Tom
Would you be kind enough to explain why a balun is needed for that
antenna? The R7 is a half-wave antenna which, without the impedance
matching network at the base, theoretically is an open circuit. In real
like the impedance could be measured in terms of thousands of ohms. Thus
the impedance matching networks allows the amateur to use easily
available 50 ohm coaxial cable to match the antenna to the line. Why on
earth would anyone want to put a balun there? The inclusion of a balun
would probably introduce wierd circulating currents because the input to
the impedance matching network is expecting to looking at a 50 ohm line.
Thus, who could predict it indeed the antenna would work?
73 and live better digitally
Jim, VE3XJ
W8JI Tom
In article <48g0e5$1...@crc-news.doc.ca>, jcum...@dgim.doc.ca (Jim
Cummings) writes:
>Would you be kind enough to explain why a balun is needed for that
>antenna?
I'll try.
>The R7 is a half-wave antenna
Nope, it's shorter than that. It may be loaded to appear like a half wave,
but it isn't an actual half-wave on any band. It is an asymmetrical
vertical with a very small counterpoise.
>Thus which, without the impedance
>matching network at the base, theoretically is an open circuit. In real
>like the impedance could be measured in terms of thousands of ohms.
Probably not. If it were, they would have a hard time feeding it on multi
bands!!! The element is short. Plus the end impedance of a real half-wave
varies with L/D ratio of the element, and can range from a few hundred
ohms for a thick conductor to a few thousand ohms for a very thin wire.
There was a thread covering this, I think it was end feeding a 40 meter(?)
vertical.
>the impedance matching networks allows the amateur to use easily
>available 50 ohm coaxial cable to match the antenna to the line. Why on
>earth would anyone want to put a balun there?
Because the ground plane at the bottom is too small to establish a good
ground reference. Considerable voltage exists at it's junction to the
antenna element. Since the feedlines shield indirectly couples to that
point, unwanted parallel current will flow.
>The inclusion of a balun would probably introduce wierd circulating
currents because >the input to the impedance matching network is expecting
to looking at a 50 ohm >line.
Not at all. As a matter of fact, the exact opposite occurs. Without a
choke balun, unwanted parallel mode currents flow down the feedcable since
the ground plane is so very tiny in that antenna. The transformer at the
base of the R7 reduces the parallel currents, but does not entirely
eliminate them. The balun cures it while still presenting the correct 50
ohm source to the antenna.
I use choke baluns on all my antennas. During technical calls about RFI or
RF feedback, the first step is always to ask if that general type of
antenna is being used.
In most cases, I always recommend adding a choke balun. By the way, I
generally handle several severe RFI and RF feedback complaints a year when
R7's and similiar (cough) "ground independent" antennas are used with
amplifiers!
73 Tom
Ilkka Kontola
>You should NOT have to use a balun on an R7 or any other
>of this type antenna. It is an off-center fed vertical
>dipole fed with a matching unit. The matching unit
>converts the 200-300 ohm feedpoint to unbalanced 50 ohms.
A current mode balun (choke balun) which reduces common
mode current in the feed line should work well with
R7. As a matter of fact in the black matching box there
is one already. Rumor has it the built-in balun is not
very efficient ie the (common mode) impedance could be
higher.
A more traditional (transformer-like) voltage balun would
probably worsen the situation. Voltage baluns work well
only when load is very symmetric (balanced).
See ARRL Handbook for more a complete description of the
balun types (1992 version: pages 16-8 and 16-9)
My setup:
- R7 on a 2m mast atop a 6m high building
- a few meters of coiled RG-213 as a common mode choke
- a split tube ferrite core on the coiled cable in order
to reduce the Q of the choke
- two split tube ferrite cores a few meters apart on the
feed line
>73, Roger WB0DGF
73, Ilkka OH3NJC
--
Ilkka Kontola Amateur radio: oh3njc
Nokia Mobile Phones
Tampere, Finland
Roger A. Cox
current mode balun (RF choke). Measure the VSWR of the antenna
on several frequencies with just the supplied match box and a
length of coax attached. (Make sure you have 10-20 feet of extra
coax cable) Record this information. Coil the coax a few feet
away from the match box. ( 6 to 8 turns with a diameter of
approx. 12 inches) Again, measure the VSWR on the same
frequencies and record. To be sure, move the coil to a new
location 5 to 10 feet toward the transmitter and remeasure. If
the coax is "cold", the coil will have little or no effect. If
the VSWR changes, a current balun should be added. Make sure
that there are no other environmental factors that would change
the VSWR during the measurements, such as cars or people moving
around near the antenna, or wind moving the antenna around.
73, Roger WB0DGF