Well, Jim, here is what N4IS stated (bolded words my emphasis) -
> > looks like he found the twisted pair to reduce the noise
> Yes, but there's a bit more to it. Twisted pair, by its nature,
> minimizes noise pickup. That rejection is maximized if the
> terminations at both ends of the line are balanced -- that is, each
> side of the line has equal impedance to ground -- and if there is
> minimal common mode coupling at each end
Hi guys
Jim is 100% right, balancing both ends is a MUST. But there is more. I would
like to share what happened few month ago with a WF project. Peter N8PR
built a beautiful WF with a polarization rotator. Peter can turn his WF
horizontal or vertical. Very nice project. We decided to check the phase
when the WF was about to go up. I measured the phase using a oscilloscope
driving the feed line input with 5W to measure the phase at the loops. First
at the 9:1 BALUN that feed the loops, the phase was correct and near 180
degree, but the amplitude was not the same between the two loops, let's say
10% different from each other. Then when I measured the phase at the
resistors, the phase was very different between them, near 50 degree and the
amplitude way off, like 50% difference. It was hard to understand why such
difference between the two loops and even harder to understand the phase
difference between the transformer and the resistor in the same loop. After
removing all BALUNS and measuring them at the bench with a VNA we found one
isolation BALUN inverted. Peter was feeding his WF with 75 ohm, and the
BALUN had 4 turns on the 50ohms side and 5 turns on 75 ohms, and it was
connected backward, 4 turns to the 75 ohms and 5 turns to the 50 ohm point,
where the two 100 ohms line are connected 180 degree. After connecting the
BALUN the way it should be, the phase measured at the transformer and the
resistor become equal in both loops, and the amplitude also was equal as
expected, the difference in phase or amplitude was not detectable anymore.
It was the first time I faced this situation. The FLAG antenna, or EWE, K9AY
, pennant and WF are actually a loaded loop. Adding one resistor inside the
loop the gain drops but you get a cardioid pattern, good front back and
good RDF. The signal when reach the first vertical wire is reflected to the
second wire, and the signal when reach the second wire the signal is
reflected to the first wire. When the signal reach the resistor it is
dissipated, and when the second signal reach the transformer it goes to the
feed line. The combination of these two currents give us the cardioid
pattern. In the WF there is a combination of two cardioids to get side nulls
and higher RDF. All this work fine "IF" there is no mismatch when the signal
current reach the resistor and the BALUN, any mismatch impedance inside the
loop will change the phase, and as a result, a deterioration in directivity.
The value of the resistor should be very close to the impedance at the
transformer, Example, for a FLAG feed with100 ohm line and 9:1 BALUM giving
900 ohms impedance, the resistor should be near 900 ohms to avoid SWR
inside the loops. This is necessary to keep the right phase to form a
cardioid pattern. The FLAG is no resonant an can be used from 1Mhz to 10
MHz, BUT it is very sensitive to impedance mismatch.
Before correcting the 50/75 BALUN the SWR measured with a MFJ was near
1;1,8. After correcting the BALUN it was 1:1,5. Measuring only the SWR does
not tell much about the antenna. The FLAG wants to work, like Luis once
said. However if there is a mismatch anywhere the results won't be good. It
is complicated to measure impedance on 160m, near AM signals around.
The FLAG or dual FLAG like the WF require a perfect match to provide the
expected performance, and any common mode current can compromise the RDF and
the overall performance.
Do It right at the first time, don't change anything in the project! Use
what was recommended.
Regards
JCarlos
N4IS
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Topband Reflector
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